Testing of Various Insulators for Telegraph Lines — Discussion

[Trade Journal]

Publication: Journal of the Society of Telegraph Engineers

London, England
vol. 7, no. 23, p. 174-197


The CHAIRMAN: We will now resume the discussion on Mr. Gavey's Paper on Insulators for Aerial Telegraph Lines, read at the last Meeting, and I will call upon Mr. C. V. Walker to favour us with his remarks.

Mr. C. V. WALKER: I have but few remarks to make upon this instructive paper. The part I have taken in insulators is very small —very limited. For four or five years during the early history of the South Eastern telegraph lines, which were erected as far back as 1845 by Mr. (now Sir W. Fothergill) Cooke, the insulators were barrel-shaped, of brown stone-ware, pierced with a tubular hole, and with which telegraph engineers are even now more or less familiar. I soon found that the insulation in those days was very unsatisfactory between London and Dover. We had, for instance, no gutta-percha-covered wires in the many tunnels, most of which were very wet. The insulation employed therein was even the same as with open wires, namely, No. 8 B. W.G., suspended upon the barrel-shaped insulators, clipped on oak arms that were fastened to the tunnel by wall-eyes in the usual way. The step which I took under these circumstances was but one solitary step in advance, and was taken not with a view of introducing a perfect insulator, which would have required other and heavier charges. My form of insulator, the double cone, was but an expansion of the Cooke barrel, and the only alteration on the poles that was required for its introduction was an alteration or enlargement of the clips for attaching the cones to the arms. I would not venture further. We still largely, or for the most part, use this double cone, and get from it very respectable insulation. Ours is not a long telegraph line; the greatest length was a little over 100 miles, since reduced, from London to Deal, where the time-ball is successfully dropped daily at 1 p.m., along this insulation.(1) The great feature about insulators, especially on Railways, that lead, as in our case, from London, and go some few miles through its smoke-laden atmosphere, is, that they should have the least possible attraction for smoke and soot. I have always a gang, or more than one gang, of men at work washing insulators, and no sooner do they get to the end of a length than they have almost to begin it again. There is, in fact, in the London district, a perpetual washing of insulators going on, clearing them from the soot and dust that gets attached to them. I have no doubt there are gentlemen present who will have more to tell you of their experience with the, in some respects, complicated forms of insulators which have come into use on the long lines they maintain, and on which the delicate operations of modern days with regard to duplex and quadruplex telegraphy have also been introduced.

 

Double-Cone Insulator.
Double-Cone Insulator.

 

Mr. LATIMER CLARK: I will venture to add a few remarks to those which Mr. Preece and Mr. Walker have made on the subject of the history of insulators, especially as regards those of the Electric Telegraph Company. The earliest form of insulator I have seen was the original quill used on the Great Western line; and I am happy to see with us to night Mr. Greener, who has had a longer experience in these matters than any one. I believe he was connected with the erection of the original telegraph on the Great Western line, and subsequently with that on the Blackwall Railway, and I hope to hear some remarks from him on the subject.

The next form I saw was the earthenware cone which Mr. Walker has termed the barrel-insulator, and which was much used by Cooke and Wheatstone. At the date of my first connection with telegraphs, in the year 1850, I saw at Liverpool some insulators which were made by Mr. Hatcher, which I mention, because they had a cup turned up within exactly like the interior of the Johnson and Phillips' insulator. I do not quite know the motive for its being thus turned up'; it was probably either to increase the conducting surface or to keep out rain, but the idea struck me as being good. There were only a few miles of these near the Liverpool Tunnel, and if they are still in existence it would be interesting to preserve a specimen of them.

Mr. Edwin Clark, as Mr. Preece stated the other evening, became the engineer of the Electric Telegraph Company at this period, and he introduced the insulator which was known as the dew-cap insulator, consisting of a stem of brown earthenware, protected by a cap of zinc, suspended from oak arms by an iron bolt. That worked wonderfully at the time, and we supposed that all difficulties were overcome, but at length it began to share the fate of all insulators, and to show signs of weakness, owing to the accumulation of smoke and dirt.

I then introduced a similar form of insulator, but instead of the zinc cap I made the bell of the insulator as well as the stem of solid earthenware in one piece. That again seemed to answer wonderfully, and we thought again that we had arrived at finality. That form of insulator was also made of glass in a solid piece. Instead of having an iron bolt cemented into it, which often caused the glass to split, a good many were made with a screw stem cast on the glass, which screwed into a corresponding screw thread in the wooden arm. About that period I became myself the engineer of the Electric Telegraph Company, and very soon turned my attention to insulation, which in this climate is always a troublesome question. Mr. Gavey is good enough to say in his paper that my double-bell insulator, or invert, as it is called, has proved the parent of most of the forms which have since come so extensively into use. Now, the fact is that my invert had itself a parent. The first idea was derived by me from Brett and Littles' insulator, a few of which were once in use on the Whitehaven and Furness Railway. It was a single bell, not a double bell, with an iron stem of the usual form bolted on to the post, and that is the real parent, I consider, of all those inverted form of insulators now so extensively used. Taking that as a model, I began to cast about me to see how it could be improved, and I am happy to say that even hi those days we were following much the same guiding principles as those described by Mr. Gavey in his paper. We were aware that the increased length of conducting surface gave a greater resistance; we were aware also that it was important to diminish the diameter of the insulator. This insulator was discussed before all the officers of the Company before it was finally decided upon. The points we had in view were: first, to have a small bearing surface for the wire. At that period I had learnt from Mr. Walker that the insulator which he had described—that is, the double cone, narrow in the centre, with the two cones opening outwards—was doing extremely good work, and on considering it thoroughly I became convinced that its merits could not be entirely due to its form, for the two cones gave two paths of escape for the electric current instead of one. I was more inclined to attribute its merits to the extremely small surface of the wire which touched the insulator. Previous to that time we were in the habit of binding the wire to the insulator by surrounding it with wire, whereby a large conducting surface was brought into contact with the wet and dirt.

In Mr. Walker's insulator the wire only rested upon a small spot, therefore one of the points I determined to introduce in the new insulator was that of having a small bearing surface; the top of the insulator was therefore formed of two hooks crossing each other which prevented the wire from falling out, and the wire rested on a blunt bridge between them.

The next point I tried to obtain was a long distance for the electricity to travel. Pondering over the methods of effecting this, the idea of a double bell gradually presented itself to me. My first thought was to have an insulator exactly like Mr. Andrew's and some others, in which the internal bell was united in solid contact with the iron rod by cement; but I saw that by making the inner bell independent of the iron rod and not in contact with it I could increase the length of surface for the conduction of the electric current. I was so pleased with the idea that when I patented this insulator on behalf of the Electric Telegraph Company I did not even describe or mention the Andrew's form, as I considered it so inferior to the double bell. 1 further increased the distance the current had to travel by corrugating both the inside and the outside surfaces of the inner bell, and there was another supposed advantage in that. We imagined that the electricity would not travel so easily over the sharp edge of the corrugation formed by each ring, and that each sharp edge would dry quickly and would form a sort of stop to the electricity. I still further added to the distance the current had to travel by coating the iron pin of the insulator with porcelain enamel or with shellac, and by all these means I gave the electric current a distance of some 16 or 18 inches to travel before it could reach the iron arm. Another point I wished to gain was to make an insulation which could be removed, cleaned, or changed with great facility. I therefore employed cast malleable iron arms carrying a conical socket, and by means of the double porcelain hook it was easy to disengage the wire from the insulator and replace it with a clean one. My idea was, that near London and other smoky towns every insulator should be changed once every three months, or as often as required, but that system was never earned out. Those were days of great economy; and our directors thought the one greatest merit of all was to keep down the expenditure; they had constant returns before them, and thought if they saved a shilling or two in each man's wages, and dispensed with a little labour here and a little there, they were doing good service. I think in this instance they were mistaken, and in fact they gradually abandoned the system.

I had noticed that Mr. Walker employed white porcelain, and after testing it I came to the conclusion that it was a very promising material for our purpose. I endeavoured to get some made in England, and you can scarcely conceive the difficulty there was in obtaining them. Their cost was at first from 4s. to 4s. 6d. each, and even at this price out of numberless samples the majority turned out to be mere china clay. I accordingly visited every pottery district in England, and at last at Coalport and at Worcester I found porcelain makers able to manufacture them of good quality, and by degrees the price was brought within limits that we could afford to pay. From that time these insulators came more and more generally into use both in England and in foreign countries, and wherever I go in the most out-of-the-way parts of the world I am often gratified by seeing the double-bell insulators in use. Mr. Varley subsequently effected the improvement of making the insulator in two pieces, each of which could be tested separately, and this is the form now in general use.

The next original step in advance is, I think, the one taken by Messrs. Johnson and Phillips. It is too early to do more than form an opinion, but I think there is something promising in it, and the idea is new. I have myself tried the experiment of oiling the inside of the insulator, and with some temporary benefit, but the oil became sticky, and caused the dirt and soot to adhere so firmly that it could not be cleaned.

I have no suggestion to make at the present time that would be of practical use, except to call attention to the merits of the enamelled iron stems which I used in my first insulators. We found the enamel was at that time not put on sufficiently well to ensure perfect insulation, and in consequence its use was abandoned both in England and on the Continent. But it is worthy of consideration whether it would not be possible now to obtain a more perfect enamel coating if attention were given to the subject, and whether an iron rod, coated with thick enamel or enamel and porcelain, in two coatings of considerable length, would not make a very excellent insulator. It would offer but a small surface for the current to pass over and one that would get cleaned by being constantly washed by the rain. It would require some kind of solid head to support the wire, in order that it might not be too easily destroyed by lightning. In conclusion I will only remark that it would be a good thing if some of these old forms of insulators could be preserved. To that matter our friend Mr. Walker would probably be able to help us, as could also other gentlemen present connected with the different telegraph systems of the country. A collection of the early insulators would form a most interesting record of what the telegraph has been in the past.

The CHAIRMAN: On the last occasion of our Meeting we had not the advantage of Mr. Gavey's presence, and if any Member desires to ask any questions or to have any further information I am sure Mr. Gavey will be happy to respond at once.

Mr. WALKER: I have by me a series of insulators, commencing with the quill, which I will call No. 1, and a piece of felt, which is No. 2—I am not sure which is No. 1—and I have a series of some of the old insulators which I shall be pleased to place in the museum of the Society if they are deemed of sufficient value for your acceptance.

The CHAIRMAN: I may mention that at the Loan Exhibition at South Kensington there is a pole which has upon it every known form of insulator that could be obtained. The quill is not there— the felt is—and if Mr. Walker will leave us the quill in his will we shall be pleased to attach it to this specimen pole at South Kensington. The suggestion of Mr. Latimer Clark is worth the consideration of the meeting, and I am sure we should be glad to start a specimen pole on our own account. There were some remarks at the last meeting left unfinished by Messrs. Johnson and Phillips, and perhaps, as several specimens of insulators have been brought here by those gentlemen, one or both of them may have a few more words to say on the subject.

Mr. PHILLIP: We have brought some insulators here to-night, and their dirty condition will give some idea of the severe conditions under which they have been tested. On the last occasion I referred to twenty insulators, which have been up for fourteen months, and have never fallen below an absolute resistance of between 1,000 and 2,000 megohms. On the top arm on the pole before you there is one of these insulators; below there are four others which were fixed immediately over a small iron smoke-stack ten months ago, the right hand one only being charged with oil. To-day the weather was very dry, and I thought the best way to try them was to get a jet of steam underneath the insulators, and having maintained this for about half-an-hour I could not get the slightest perceptible loss on the one which had been charged with oil, while the others were very bad indeed. There was a small wooden roof beneath these insulators which on Monday last caught fire, and the wooden arm was charred. I had for a long time quite made up my mind that the oil must be dried up. We did not attach much importance to this trial since it seemed so outrageously severe. I was therefore both astonished and delighted at my yesterday's result, since it has an important bearing upon the time the oil will last in hot countries, and it also shows how independent we are of the porcelain becoming dirty. On examination the oil was found to be quite fluid and clear.

I am glad to learn that Mr. Gavey is here to-night, and I would like to call his attention to the values he obtained on our No. 5 insulator. At the last meeting I pointed out that on November the 9th, in fine weather, 664.16 megohms were obtained, that on the 12th it fell to 221.428 in slight rain, and on the 13th, weather damp, it was as low as 27.125, while on the 14th, in thick fog, it rose to 7,350 megohms. Again on the 17th, during fog, it rose to 8252.941 megohms, having been down to 2,480 megohms in the interval. Now it is certainly curious that values obtained during a fog should be higher than those obtained in fine weather, and I shall be glad to hear any explanation Mr. Gavey may have to give us on this point. With reference to that most remarkable insulator, No. 6 in the table, I see in one instance an absolute resistance of 132440. megohms was obtained after the insulators had been up fifteen days, during most of which it had rained or been foggy.

Multiplying this marvellous result by ten to obtain the insulation of one insulator, we get 1.32 million megohms, a value which I imagine must approach very nearly to the resistance of the porcelain itself.

I was very interested with what Mr. Latimer Clark said in reference to his having previously seen an insulator with its lower edge turned up inside in the same way as our No. 5 insulator (Fig. 21), a section of which is shown in the drawing, and I can quite confirm what he said as to its being a good form, quite apart from the oil, and as compared with ordinary insulators, but they nevertheless soon give out. I put up twenty of this form without oil some time ago, and for several weeks they gave splendid results, but now they are little better than the ordinary ones. The drawing marked No. 17 (Fig. 10) is the other form which we sent to Mr. Gavey, an iron sheathed insulator, and the right hand drawing No. 20 is a smaller and cheaper form of No. 5 (Fig. 21). No. 5 has rather a large oil chamber, but there is no doubt that No. 20 would last a very long time without requiring renewal of the oil.

Mr. W. C. JOHNSON then explained the specimens of the No. 5 pattern of the fluid insulators which had been exposed for ten months, on a level with and about fifteen inches from the top of an iron funnel. This was the chimney of a furnace which had been in constant action during the time stated.

He said all the specimens are now covered with carbon about one-sixteenth of an inch thick, and are just as they were when removed from the funnel this morning.

Of these two on the left-hand side of the arm, one still has oil left in the annular space, and the other is the one which was not charged with oil. The latter is quite loose on its pin—the heat from the furnace was so intense that it has melted away all the sulphur cement which secured it.

In the former, the oil still remains, and, electrically, the insulator is as perfect as when fixed at first; and I must mention that they were found to be in this condition before the accidental destruction of the building beneath by fire.

The next ones on the arm are an ordinary form of Andrews' insulator, put up to test comparatively with the others, and a No. 5 pattern fluid insulator. The latter was not charged with oil; both of these gave the low result when tested mentioned by Mr. Phillips.

The CHAIRMAN: What is the condition of the oil in the one in which the cement is destroyed?

Mr. W. C. JOHNSON: The cup had not been charged with oil, but it has a similar position on the arm to that one in which the oil remains. Flame was frequently emitted from the chimney-top, and I should have expected to have found the porcelain cracked under these conditions.

On another part of the pole is shown one of those which was fixed under ordinary conditions in January, 1877, and removed this morning; also some of the iron-clad forms. In these the oil-cup is capable of sliding down the pin, for the purpose of examining the condition of the oil or for cleaning. The cup is retained in working position by means of a binder of thick lead wire. A ring of india-rubber was at first employed, but was found to deteriorate rapidly when exposed to the atmosphere. But an objection has been raised to the employment of the lead wire in India, as it is probable that it might be stolen by the natives to cut up for shot. But that cannot be regarded as an objection that would hold good in England.

No. 13 is one of the larger forms of fluid insulators. The large iron hood has a large female thread cast in it which screws on the porcelain sheath and well protects it. It is similar in size and appearance to the brown-ware Varley form so extensively adopted in the postal telegraph lines.

Mr. GREENER: I am very sorry I was not able to hear the paper read. This is one of the most interesting subjects, so far as my own feelings go, viz., the insulation of telegraph wires, and that I suppose is because I have been connected with telegraphs for so many years. I started very early in telegraph work—in Mr. Fothergill Cooke's days—and, as far as insulators go, Mr. Latimer Clark said I had something to do with the first telegraph on the Great Western line. That was not so. My career began first on the Blackwall line, and we did not have much overland line there, only a little bit between Poplar and Blackwall of about 300 yards, and about sixteen wires. In those early days there was a bit of a ring for the wires to rest upon, fixed by staples. The next thing was a little cone. The next thing I remember in the shape of an insulator was Mr. Walker's double cone, which he has stuck to throughout, and therefore I suppose he must find them answer; and then after that there were certain alterations. Mr. Hatcher had one or two when he was engineer of the old Telegraph Company. Then Mr. Edwin Clark made his earthenware insulator with a zinc cover, and then came the solid earthenware.

The next great improvement was, I believe, Mr. Latimer Clark's invert. That was the next great step as regards insulators, and with that insulator I have had some experience on long lines, that being a porcelain insulator with double cup. I had to do with a line of 2,000 miles, from Constantinople to Bagdad, consisting of two wires, one being insulated with Mr. Latimer Clark's invert, and the other with Siemens' insulator with a single cup. The insulation on the Clark's porcelain insulator was always very very much higher than the other, but it was seldom we could get any distance upon it on account of the weakness of the insulator; it was not strong enough, and got broken. The single cup line was much lower in insulation in bad weather, but we could always manage to get through comfortably, showing that insulators want mechanical strength as well. as electric qualities. This was in 1854, when Mr. Latimer Clark brought out that invert, and insulators of that description have been made from that time to the present. I have the honour to be inspecting engineer of telegraph stores for India. We have had many forms in India of double-cup insulators, but all have been very much the same thing as Mr. Latimer Clark's invert. We have had different shapes, but they have been no great improvement.

I think the time is come when, if I was an inventor, I should look about me to get a good insulator. I hope some of our young men will do so. I think really this insulator of Johnson and Phillips' is something in the right direction. I don't know whether it is the thing, but it is something out of the track trodden in for these number of years, and when it was first brought to my notice I said I did not believe in it—simply, I suppose, because I had been used to the old double-cup for so many years that I had no thought of anything else. I was invited to their works to see these insulators tested. I went there with Mr. Schwendler about last May, and we tested them after they had been up only about three months, with oil in them. It was very dry weather at the time, and we sent up a man to pour water upon them. There were about 20 of these double-cup insulators put up. These insulators were not affected in the least, whilst others went down to nothing. I felt then there must be something in these insulators, but I did not see how they could be used practically with the oil. I thought the oil would get out if it was upset, but it did not. I suggested to them that they should try experiments with regard to the cooperation of the oil, and they did so; and after some months I went again and I found the insulators in the same condition, and tested as before, whilst the other insulators were, as before, low. I had one taken down, and I tried to get the oil out, but could not. It stuck to the sides of the cup, and when turned back it went into the trough again. I have taken several tests of these insulators to see how they went on, and for some time past I have felt I should like to come here and say something about them. Yesterday I got Mr. Graham, my assistant, to go to Charlton to test these insulators. The last time I did so the 20 insulators gave a resistance of 14,000 megohms. Yesterday was a fine day, and Mr. Graham found it was no use testing them, as the one was almost as good as the other. Mr. Graham went again this morning early, when there was a little fog and snow. The resistance of the 20 insulators of No. 5 form was then 2,756.8 megohms—absolute resistance of 20 insulators, which is very fair I think, giving a resistance of 55,137 megohms per insulator. There were four ordinary insulators tested at the same time, and the absolute resistance per insulator was 19'483 megohms, being 19.483 against 55,137 megohms, and that, after being up for ] 4 months. I believe the insulators had not been touched in any way. I went down myself again and saw these black gentlemen. I thought if there was any insulation about these fellows we must have greatly improved, and I had one that had got oil in it tested, also one without oil, and there was no deflection to be had upon them, whilst the ordinary insulators gave dead earth. After Mr. Graham came away, Mr. Phillips put on a jet of steam, and the one with oil in it remained the same, and the one without oil was bad, showing, I think, that the oil is doing its duty very well. I may say I have great faith in this insulator, and am glad to say that Major Champain has ordered 100 miles of them to be put up along a sea-coast line in India, which will be a good practical test for them, because if they will answer on the Mekran coast they will answer anywhere. At the present time it is necessary on that coast to have men constantly cleaning the insulators.

As to the different kinds of insulators and the porcelain they are made of I have had the testing of a large number, and there is a great difference in the insulating properties of porcelain. Of course, my experience in that respect is not worth much, because my tests have been made when the porcelain was quite clean and dried with heat to do away with surface leakage, and I found that the English ware will give perhaps two millions of megohms to three millions; but the highest results I have obtained are really from Schomburg's insulators, and they gave, when clean and dry, from 10 to 12 million megohms. In a country like this I believe one would be as good as another in a fortnight.

There is one thing on reading the tables given in the paper, which is most surprising to me, and I would like to know what porcelain this insulator is made of, being so high, compared with all others, —that is the first on the list, No. 6. I have seen that form and examined it, and I have it in my mind with other insulators, and there must be something very peculiar about it. It must be excellent porcelain for the insulation to be so high. May I ask how long these insulators have been up?

Mr. GAVEY: They were put up for the special occasion and were tested at once. Most of the other insulators have been erected and tested at various times previously. These, No. 6,1 had just erected, and they had not been tested previously. The day after they were erected the tests commenced. All the others had been up and simply washed and cleansed before the testing was commenced.

Mr. GREENER: Then these were new insulators, and had not been up previously?

Mr. GAVEY: Yes. The time varied from three to twelve months. No. 6 were absolutely new insulators.

Mr. GREENER: I quite understand that now. I would say, I do hope we shall now go on and see if we cannot improve our insulation. I am glad to see Mr. Graves here. I think it would be a good thing to test insulators for a length of half a dozen or a dozen miles. If we could get Mr. Graves, when the Society think they have a good insulator, to put up half a dozen miles on a Post Office line, that would be a good practical test. I do believe in practice. Theory is a very good thing, but I believe more in the other.

The CHAIRMAN: I may state that Mr. Graves has already erected experimental lines of insulators to a much greater extent than that. He has put up lines insulated with eight different insulators selected from those which had given the best results in testing, including that of Johnson and Phillips, the others being selected from a great number experimented upon by Mr. Gavey at Bristol. These eight wires extend from Wesbourne Place to Uxbridge, 22 miles, and it has been a great misfortune that this month of March and February have been so dry that it has prevented the possibility of testing these wires under adverse circumstances and bringing the results before the Society, but no doubt we shall eventually be able to show excellent results from the testing of these experimental lines.

Mr. GRAVES: When rival inventors and rival professors submit their inventions and ideas to the Post Office it becomes my duty to consider them, and therefore I am somewhat chary in expressing in public the opinions I have formed in private. Last year various inventions in the matter of insulators were brought under our notice, and after discussion with Mr. Preece and others a determination was come to make experiments with various insulators upon a selected section of line from London to Uxbridge via the canal, partly on account of the facilities of access and partly as presenting favourable circumstances for testing the qualities of the various insulators. Starting with some of the old forms of insulators, we went on up to the latest introduction of Johnson and Phillips, whilst at the same time the opportunity was afforded of comparing the merits of porcelain with those of brown earthenware. The arrangements for the experiments are complete, but, as Mr. Preece said, we have, as yet, obtained no results owing to the dryness of the atmosphere. I would mention one thing. We have heard in connection with the comparison of different insulators that it is unfair to that class which gives the highest resistance to weight them with shackles or terminal poles, and we are advised that we ought to take out of circuit portions of underground line that may intervene, and take the true value of the insulation, uninfluenced by any such conditions. Now that may be very well in theory, but it is impossible in practice to carry out a long line without intervening lengths of underground line, and I say at once that the insulator which gives the highest results under the circumstances existing, would be the one that should carry the preference, because it is not always that the highest electrical qualities give the best practical results combined with the greatest economy of construction.

Mr. GREENER: With regard to shackles, underground wires, and terminal posts, I think you may have the finest insulator that can be produced in point of resistance, but there is always something that will break down your lines as long as they are lines. Mr. Preece will bear me out that there are always bad weak places in a line, and except those are kept good it is no use to have good insulators; and I do not see that it is a fair test for insulators unless those faults are kept out of the circuit, because, be as careful as you may, if there are faults in the line those faults may unjustly be attributed to the insulators. Many years ago Mr. Preece had some lines under his charge, and under his instructions I had to go from station to station and pick out the bad places. We could not work through more than ten or twenty miles. They were altered, and then all was right. I think, if it is possible to test insulators clear of those things, it would be a better test.

Mr. GRAVES: You are quite right in one respect; if you are testing the electrical qualities of an insulator, the more you are free from disturbing elements the better; but if you want to determine the merits of an insulator for practical purposes the existing conditions attaching to all telegraph lines must be borne in mind. While it is desirable to give attention to the subject of insulation in the direction in which we are going, it is equally desirable, in fact more important, that attention should be given to the subject of terminals and the connection of underground wires with overground. We are more let down by that than anything else, and it requires the greatest care. We find deterioration comes on gradually, and it cannot always be detected in time to prevent the evil before it occurs. With respect to insulation, it is necessary to give attention to the terminals from one end of the circuit to the other.

Mr. JOHNSON: As to the terminals of the line and the shackles, they were points that Mr. Greener brought to our attention at an early stage of our labours in fluid insulation: and I think Mr. Greener will admit that they were well met by Mr. Phillips as to the connection of land lines and terminals. The terminals can be connecting underground wires with aerial lines was produced by Mr. Phillips, which I believe will be put into practice in the test to be made on the Uxbridge Canal.

Mr. R. K. GRAY: I think the question of Johnson and Phillips's insulator as against the others may be very simply considered. The whole thing lies in considering whether the insulation of the surface of oil is greater than the insulation of the surface of the porcelain of the insulator. In making the test it appears to me quite sufficient to test the insulating properties of the oil, as compared with the porcelain, quite separately from the testing of any other insulator. Looking at the results of tests of other insulators as against those of Johnson and Phillips, it appears that no person has determined the value of the porcelain surface as compared with the surface of the oil. It would be easy to arrive at a conclusion whether the oil is better than the porcelain.

Mr. WILLOUGHBY SMITH: I think something like eighteen months ago, when Mr. Phillips introduced this insulator to me, I suggested that if he would like to place two or three of them in my grounds he could do so, and they might remain there. It is just to say I examined them yesterday. He had put up two kinds of insulators; one was what is called the invert. I suppose the oil had got down the side, and it very much resembled what is called a " Catch-'em-alive 0," being thickly encrusted with different kinds of insects. In the form we see here to-night the insulator is as clean as possible, and there are no signs of oil having left the cup. I should say if these were tested they would be equal to what they were when they were first filled fifteen or sixteen months ago. I am not much interested in aerial insulation, my hobby being, as you know, subterranean work, which you will have to come to in the end. I will take the first opportunity to test the insulators in my grounds, but I have not done so yet. They are hung amongst the trees, where they have been for the last sixteen months. When I first took the place there was the voice of the nightingale and the smell of the rose; but now a gasworks has been placed there, and I think you could hardly have a worse place to fix the insulators in; but I am pleased to find that the atmosphere of Charlton is more impure than where I reside.

Mr. JOHNSON wished to mention that the form of insulator to which Mr. Willoughby Smith referred as offering a tempting bait for flies had been experimented upon by Mr. Graves amongst the first, but that was sent to Birmingham, and that form had since been abandoned, and, therefore, need not be taken into consideration in the discussion.

Mr. JOBSON (responding to the Chairman's invitation) said: I have had no practical experience of the testing of insulators. We are manufacturers only, and not scientists, for which I am thankful, for we must all agree that tests of this sort must be in a great degree fallacious, as practice alone can bring out the real merits of a thing.

Mr. GAVEY: I do not know that I have occasion to reply to many of the observations made by members on the paper which is before the Society, but there are one or two points to which I would refer. In the first place, Mr. Phillips raised a question as to certain discrepancies in the tests recorded of the insulators numbered 21. These I can explain. I must confess that I was at first disappointed on comparing the results I obtained with those published by him, and, as I had some reason to think that damp had invaded the interior of these insulators before they were filled with oil, I had them taken down on the third day, exposed to a temperature sufficient to drive away every particle of damp, and then re-erected; after which the resistance rose in a remarkable manner. I think that is a simple .explanation of the results. Subsequently this insulator dropped down again. The explanation of that I cannot give, unless it be that the oil was somewhat defective. I used rosin oil, but it was not subjected to any special test to ascertain its purity or electrical resistance, and it is possible, inasmuch as my results do not bear out those of Mr. Phillips, the fault lay with the oil. With reference to the question raised by Mr. Gray as to the relative resistance of the glaze and of the oil, I may observe, that, in this special application of Johnson and Phillips's, it is not so much a question of the specific electrical resistance of each substance, as of its affinity for moisture (assuming, of course, that both are good insulators) that has to be considered. Of course, if you take a piece of good porcelain, and by suitable precautions prevent all deposit of moisture on the surface of the glaze, you can scarcely obtain any evidences of direct conduction by means of an ordinary Thompson's reflecting galvanometer; but, if you take any series of insulators and expose them to the atmosphere of this country on the driest day, you can always get a certain degree of deflection, which shows that there is a film of conducting moisture existing along the surface. I have tried it on a bright sunny day, and have obtained a definite reading on ten insulators. On placing these before a fire for an hour or two, so as to dissipate the surface moisture, this deflection has wholly disappeared. Oil having no affinity for moisture, we may assume that there would be practically no deposit of moisture on its surface, and it would therefore be free itself from this evil of surface conduction, and would likewise protect that portion of the porcelain insulator to which it was applied.

On the question of the extraordinary resistance given by the No. 6 insulator, I confess no one could be more surprised than I was when I obtained those results. I owe an apology for placing before the Society a paper in which I did not give some explanation of them, but the paper itself came on earlier than I anticipated, and further, just as I had concluded the series of tests recorded, certain departmental changes involved the removal of myself and the whole of my staff and apparatus from Bristol to Cardiff. Two months elapsed before I was prepared to resume my experiments in the latter place, and finally, as Mr. Preece has mentioned, we have had such extraordinary dry weather for the last six weeks, that little of any value could be achieved. Still I have had some results, which, perhaps, may tend in a certain measure to explain those I obtained before.

I may say my first impression was that the india-rubber ring on the insulator, which is interposed between the cast-iron bolt and the female screw, took up the role of the oil in Johnson and Phillips's insulator. That is, it was far away from the direct action of the elements, and having little affinity for the moisture of the atmosphere it remained comparatively dry under all circumstances, either of rain, fog, or other atmospheric changes. Naturally the first thing I did was to test a series of insulators with the india-rubber ring, and another series without; not, by-the-bye, the insulators which had given these results in Bristol, but another set of the same description. I may say at once the former results were neither obtained nor reached in any degree, and I will give a reading of one test made on the 28th of February, with a long insulator, with a washer, similar to No. 6. That gave a resistance of 22 megohms, -while the insulator without a washer gave 20.9. A series of short insulators of the form hitherto used by the Post Office, fitted with screws and washers, gave 5 megohms. A short insulator fitted with screws, but without the india-rubber washer, gave 25 megohms. The ordinary porcelain insulator of the same description, but with bolt fitted on in the old manner, gave 2.2 megohms. Mr. Varley's insulator gave 3.4 megohms. Later on I had the insulators that had given such extraordinary results at Bristol re-erected in Cardiff, on the pole containing those now referred to. Unfortunately, since they have been up we have had no rain, or rather none in the day-time, when an observer was on the spot for testing; but on the 21st of March, it having rained heavily in the night, a series of tests was taken in the morning, and I may say at once the insulator that gave such extraordinary results before now dropped to the same level as the last series of insulators, which I erected to endeavour to trace the cause of the former results. In other words, the long insulator, with washers, gave 1,250 megohms; without washers 1,041. The short insulator, with washer, gave also 1,041; without washer 1,550. The ordinary porcelain insulator, of the Post Office form, gave 833; Mr. Varley's 833; and No. C gave 1,550.

Mr. GRAVES: When you speak of results, were these tests of a single insulator or an average of several insulators?

Mr. GAVEY: Seven insulators were put up together and tested together. To some extent these further results would appear to show that the extraordinary resistance first recorded was mainly due to the influence of the india-rubber ring, and the fact that the original resistances were not reached in any later instance is readily explained by considering that in the course of three or four months the india-rubber, by exposure to the atmosphere, became deteriorated. The insulators No. G, giving the original figures, were erected as soon as they reached me from the factory. When re- erected they had been exposed for some months to the atmosphere— a sufficient length of time for the india-rubber to fail electrically. Similarly with the other insulators of like pattern, tested in Cardiff, the bolts, with their india-rubber washers, had for some months been exposed to the atmosphere in the store depot, so that they failed to have any effect on the total resistance of the insulators. Allowing, however, for this temporary action of the india- rubber washers, the fact remains that certain of the screwed form of insulators gave a much higher average result than those fitted with bolts in the ordinary manner. On closely examining some of the No. 6 screwed insulators, I found a portion of the inner cup, for a length of about one inch downwards from the point on which the washer presses, free from glaze. Now we all know the value of a good glaze on an insulator, but we likewise know its disadvantages. We are aware what a very strong affinity atmospheric moisture has for glass. If you put a glass vessel of any description in an atmosphere which contains any moisture, you will get moisture condensed on the surface, and if you put a highly-glazed insulator in any atmosphere in this country you will always get a visible reading on a delicate galvanometer, showing that there is a deposit of moisture on the surface. Now, glaze on a porcelain insulator is simply glass, and with a highly-glazed porcelain insulator you have, as far as condensed moisture is concerned, very much the same results as you would have with a glass insulator; and I need not point out how difficult it is to keep glass from condensing moisture on its surface.

It strikes me it is very possible that part of the high resistance recorded in this insulator is due to the unglazed portion of the inside of the insulator. In saying this, I must not be thought to advocate the use of unglazed porcelain for insulators. We all know the utility of the glaze on certain portions; but I think we should obtain better, and more and more uniform, results, by maintaining that portion of the insulator which is removed from the direct action of the elements unglazed, or by forming the interior of the insulator of some substance which has so little affinity for moisture that unless moisture in the shape of rain has direct access to that portion of the insulator it will maintain to a great extent an unchanged surface. Messrs. Johnson and Phillips have endeavoured to maintain that unchanged surface, and, as I have said, great credit is due to them for having introduced an entirely new form of insulator, and made a step very far in advance of all others previously introduced. Possibly, practical experience may raise an objection to that form which we do not see now. That is a question for the future, but there may be means of obtaining the same results in a more economical manner, or perhaps in a manner which may better meet practical requirements; but, although they have made a great step in advance, I do not think it should prevent others from endeavouring to arrive at the same results in another manner.

The CHAIRMAN: In according our thanks to Mr. Gavey for the paper he has brought before us, I am quite sure I express the sentiments of all present in saying we appreciate very highly his efforts, not only for the valuable paper itself but also for the extremely valuable discussion it has elicited. Mr. Gavey did not go much into the history of the matter, but he has been the means of bringing before the Society, in an extremely interesting manner, some of those most interesting historical facts associated with the insulation of telegraph lines. We have heard one of the principal inventors in that field to-night, Mr. Latimer Clark. We have also heard what Mr. Walker has said, and I am bound to say I can confirm, to a great extent, what Mr. Walker has said with regard to his cone insulator. Lines with Mr. Walker's cones have come under my superintendence. I have had to test those lines, and have been astonished to find that wires insulated with Mr. Walker's cones have tested as well as those carried by the higher classes of insulators. I think we must all agree, from the remarks we have heard, that the true standard of insulation is not so much the resistance of the insulators themselves as the resistance of those numerous points of leakage—faults in fact—which occur at the mouth of every tunnel, at every bridge, and at every connection of underground with aerial lines. In a country like this it is impossible to carry a wire twenty miles without terminals at two or three points. It is to these weak points we should now look. Mr. Greener called attention to the fact that some years ago we had under our charge a very bad line indeed. As he has said, we could scarcely work through-twenty miles, and the first thing we did was to attack those weak points. We cut out every terminal, and the result was that a better working line does not now exist. The insulators were the same, but the weak points were removed.

There is one point connected with insulation which has not been much dwelt upon—that is, the influence which climate has upon it. We have spoken almost exclusively of our experiences in England. Mr. Greener alluded to his experience abroad, but the paper itself, which will be circulated all over the world, will be read by anxious minds, who will compare the condition of insulation in their countries with the condition of insulation in England, and they will look at the fact that their insulation far exceeds that which we experience in England. I was struck with this during my tour in America. There I found the lines tested most beautifully, whilst the insulators, without exception, were the most execrable I ever saw. I am certain if any line out of London were insulated with American insulators you could not possibly work; yet they are able to work through thousands of miles. I had a letter from Mr. Hamilton, a leading American electrician, who was in this country a little while ago, stating that nothing struck him so much in England as our wretched insulation. He found that the best wires out of London tested worse than the worse lines in America—forgetting, it may be, the fact that the climate of the two countries is very different. In fact, the climate of America is so dry that you do not