Publication: Electrical World
New York, NY, United States
Insulating Overhead Wires.
BY DAVID BROOKS.
In 1876 the French government obtained insulators from nearly all the nationalities of Europe, for the purpose of galvanometrical tests in comparison with their own, with which they were not satisfied. These tests or trials, were conducted by the commission de perfectionnement, of which M. Gaugain was chairman. The result of six months trials was the adoption of the Prussian insulator. Fig. 6 shows a section of this insulator and its support or bracket. Fig. 7 shows the French insulator then in use. Fig. 8 shows the present French insulator.
It will be seen that they gave the insulator the insulating properties, so far as form is concerned, but held on to their bracket and the two epaulets or shoulders which disfigured the old insulator. They were of no use, and rendered the insulator much more difficult and expensive to make. The iron bracket or support was fastened to the pole by three wood screws.
The Prussian bracket is made all in one piece, and screws directly into the pole. It is very strong, of wrought iron, with three sides — triangular shape — with a knife-edge projecting upward. This is to prevent the drops of rain from dashing upward into the sheltered portion of the insulator.
The commission took notice of this feature, and made their support of sufficient length and uprightness to accomplish the same purpose.
Among the insulators exposed for trial in the yard attached to the storehouse and factory of the government were ten of the familiar glass-and-bracket, obtained from the Western Union Company, and ten of the Varley inverts, much used in England.
I was curious to learn the respective merits of these insulators and was shown the record. Lowest in the list was Varley's, and next in order was the American glass-and-bracket. Mr. Varley stated in his report on the Western Union Telegraph lines, written in 1867, that his was higher than the French in the proportion of eleven to one, but this was in comparison with the old French insulator, which was defective in form and porous. The body was so thick that it was impossible to bake them in the kiln sufficiently to completely vitrify the body. The new French insulator is made of splendid china and is a perfect insulator as regards porosity.
While in Paris I procured a dozen of the new French insulators and as many of the Prussian, and also two galvanometers from Ruhmkorff, such as were used in the test of insulators by the commission. On my return to London I procured a dozen of Varley's inverts, a Thomson mirror galvanometer, resistance coils, and everything necessary to make the insulator business a study. The French Government furnished me with specimen insulators of other nationalities. The Russian and German were modifications of the Prussian, but not with so much protected surface; and their fastenings were not so well devised to keep the inner surface dry in rain. This feature I have made a study.
In December, 1867, I began these tests in the yard of my factory. For the first three months I could get no deflection at all on the French. When on the glass on cross-arms I could get fifty thousand divisions or more. The earthenware inverts did not stand as high as the glass and pin. Mr. Culley in every edition of his book recommends the insulator to be so placed that the outer surface can be washed in rain. In the eighth edition he says: "The insulator should be fixed above, not under, the arm which carries it, for when placed underneath the portion which is covered by the arm does not get washed by rain."
Experimenting on this feature, I used to turn the English cross-arms and insulators upside down and found by this changed position they tested fully three times better than with the glass and pin in their natural position; but when both were in natural positions the glass and pin stood the highest, from the fact that the pin was longer and farther removed from the spattering of drops of rain.
We placed the English insulator with the iron bolts or supports in the cross-arm as low as the shoulder would admit them, and in the same manner with the glass and pin, the latter of which has an enlargement above the cross-arm giving the pin additional strength, but the greater length of the pin carried the glass three times as high as the English insulator. Turning the cross-arms over did not improve the glass and pin; the glass being so far removed from the cross-arm, the arm above was no protection, but in the English insulator it was a protection, especially to the inner cup, as may be seen from the representation of this insulator in Culley's book — or in Prescott's.
Curious to know what good was derived from washing the outer surface of the insulator by rain, I wound a wire round the bottom of the outside cup of a single insulator and wound another wire in the groove to see what resistance or insulation was obtained by the outer surface during these ablutions when insulation is most needed.
A perfectly clean Varley insulator tested in this manner does not show one-tenth of a megohm resistance. The long style of glass — or the one that has grooves for tie wire near the top of the insulator — shows even less resistance, although the glass does hot present over half the conducting surface.
The Prussian insulator tried in this manner is very much less than the glass, and the French chinaware still less than the Prussian; and this is explained on the principle of the affinity or non-affinity of these different materials for moisture. The earthenware has what is known as a salt glaze, which has an affinity for moisture in the same manner as oil has an affinity for steel or glass. A drop of oil upon a pane of glass of ordinary size, laid in a horizontal position — if the glass is perfectly dry — will soon cover the whole surface, and in this manner a few drops of rain will soon spread all over the entire surface of the earthenware, and the earthenware surface is the first to show conduction. The moisture on the porcelain does not flow so rapidly, but remains upon the surface in isolated bodies. On the French china the moisture gathers in thicker isolated bodies, and still later begins to show conduction. On the earthenware the water runs freely and in thin body, and begins to drop from the bottom of the insulator sooner than on the others. When the globules of water become so broad upon the surface of the porcelain as to connect with each other, there is greater body of water for conducting the current; so when the surface of those materials most repellant of moisture is covered with water they conduct more than those less repellant. Ebonite, more repellant than china, and last to show conduction, drops lower when conduction takes place. These experiments are made with perfectly clean surfaces; but if the surfaces of the insulator are covered with smoke or carbon, it makes but little difference of what material the insulator is made. Clariat says, in speaking of the affinity of liquids for solids, "If the liquid has a greater affinity for the solid than the liquid has for itself, the solid will be wetted. If the liquid has a greater affinity for itself than it has for the solid, the solid will not be wetted."
Reflecting upon this law, I am reminded how nature covers the coatings of fish with a slime that they can the more easily move through the water without friction. Upon the same principle the bottoms of yachts and scull-boats are besmeared with soap to enable them the more easily to slip through the water.
Of all substances, paraffin is the most repellant of moisture. Testing the same outer surfaces of the Prussian insulator in rain, after the insulator had been dipped in paraffin, no deflection in the galvanometer could be obtained until it had been exposed four months. The water gathers itself in spherical bodies and drops off or rolls over the surface in the form of shot pellets. Exposed to the sun, dust and smoke, the paraffin in time lost this property.
In the Brooks insulator we placed this paraffin in the ceiling or upper portion of the protected inside surface, where it is least exposed to currents of air carrying dust and other impurities. The Brooks insulators show no leakage in rain until the inner portion is obstructed by insects, and then not till after an exposure of from four to six years.
The ingredients of the cement drives insects away for that period, but by removing the deposits in the shape of cocoons and webs, and rinsing the insulators in paraffine oil, their original insulating properties are restored. Insects never harbor inside of the common glass. They are never troublesome except when the insulator is made of an opaque substance, which hides them from the light. Taking advantage of this circumstance, I have shortened the shield of the cross-arm insulator so far as it is unnecessary to give the insulator strength. That portion below the cross-arm freely admits the light.
As to cement, Mr. Culley says sulphur splits the insulators, and I have referred to that fact in the fore part of this article. Sulphur, mixed with ten per cent. hard pitch, does not absorb moisture, and thereby expand. The pitch gives it adhesiveness, and the composition is an excellent insulator. Plaster of Paris and Portland cement much used are conductors, and thereby inferior as insulating cement.
The pitch and sulphur should be continuously stirred while in a melted state; on being worked otherwise, the sulphur, which is the heavier, will go to the bottom of the vessel.
The above is almost entirely in reference to the influence of rain and fog upon insulators, and the manner in which the effect can be obviated. There is no earthly reason why a line of telegraph should not work as well in hard rain as in a clear day. When it comes to absolute insulation, there is less escape in rain than in a clear, hot day, because all insulating substances are affected by temperature. The ordinary glass and pin is of a hundred times higher resistance in a clear, cold day of winter than in a clear, hot day of summer; yet there is no difficulty with the insulation in the hot day of summer, because the leakage, if the glass insulators are perfect, is too little to be shown in ordinary working.
The difference is best shown by the time the line will hold a charge.
I shall conclude this article with a few remarks on the economy and efficiency of good insulation. When you go to England, you are impressed with the large size of their conductors and the necessary weight and strain upon the poles. In England they used a No. 4 wire for what we would call ordinary circuits, say from one to two hundred miles. They use a battery for each wire and earth wire upon the poles and cross-arms to carry the escape to the ground and not pass to the other conductors and cause confusion of signals. As soon as you cross the channel, for circuits of equal length, you will find the French or Germans using a wire between ten and eleven gauge, or a wire less than one third the size and cost. Five circuits are worked from one battery, and they have no trouble in wet weather. They scarcely know when there is rain by its effect upon the insulation. There is no need of such heavy conductors, earth wires attached to the posts, or a battery for each circuit, unless the insulation is extremely low.
In the fifth edition of Mr. Culley's book, he states that, "taking a circuit better situated as regards insulation, between Belfast and Dublin (being out of the way of smoke), weather at Dublin damp, at Belfast dull, the insulation of a No. 4 wire is 91,900 ohms per mile." Taking such a line in this country, say one hundred miles in length, the insulation resistance of the entire line would be only 919 ohms, while the resistance of the wire itself is 550 ohms, and on a No. 7 wire it would be about 1,100. It will be seen that the insulators in the case of No. 7 wire would not show as much resistance as that of the wire itself. I do not believe it possible to work a wire by the American closed circuit system* under such conditions, yet Mr. Culley says in the margin under the head of remarks, "working well." He gives another instance of a No. 4 wire not so favorably located, giving an insulation resistance of 70,000 ohms per mile on a very wet day. Taking a No. 6 wire one hundred miles the insulators would not show as great a resistance as the wire itself.
The Pennsylvania Railroad Company have a special wire for reporting trains on each of their divisions of the main line. Each division is over a hundred miles in length, the middle division one hundred and thirty-five miles. On this division or circuit there are thirty-five relays, averaging one hundred and fifty ohms each, and the total resistance of the circuit is greater than that of a No. 4 wire a thousand miles long, or say, from New York to Chicago. Yet they have no difficulty in working this circuit on the American closed plan in the roost humid or wet weather, but with ordinary insulation it would be impossible.
In 1867 Mr. Varley came to this country and made an examination and report of the condition of the Western Union lines. He states; "I tested nearly all the relays in 145 Broadway on January 23, 1868. They varied from 69 ohms to 1,205, The 69 ohm is where it should be on a long line, No. 2 to Chicago." One would naturally suppose that that statement contained a typographical error had he not soon followed it by another equally misleading, to wit: "By Ohm's laws the strength of a current is proportional to electromotive force divided by resistance." A 69-ohm relay is entirely out of place on a circuit of nearly a thousand miles. You could use a sounder of 5 ohms resistance instead of that relay and increase the strength of the circuit, but you get little or no magnetic effect. If the wire on the 69-ohm relay was one-half the size you would have double the length and twice as many turns or convolutions on the helices, making a slight allowance for the covering of wire. Doubling the length of the wire would fourfold its resistance. It would then have 276 ohms resistance. The total resistance of the circuit would be increased 209 ohms. If the length of line was 300 miles of No. 8 wire, at 13 ohms per mile, the total resistance of wire, instrument and battery would be in the neighborhood of 7,000 ohms. Adding the resistance, 209 ohms, necessary in changing the relay to one that gives twice as much magnetic effect, we increase the total resistance of the line about 3 1/3 per cent. By making the spools in the relay longer, we can still to great advantage increase the magnetic effect by using a still finer wire giving a greater number of turns; and carrying out this principle, the relays used in Germany, upon ordinary lengths of line, have over a thousand ohms resistance." But Mr. Varley went upon the principle that to make a line work in bad weather, you must use low-resistance relays and large wires or heavy conductors. To work a line with imperfect or extremely low insulation, low-resistance conductors and very strong batteries for each are necessary, while with proper insulation one-quarter of the battery power will work five such lengths of line of three times the resistance. It is done in Germany and France.
Carrying out this principle recommended by Mr. Varley, the Western Union Company have unnecessarily spent millions of dollars which could have been saved. The Prussian insulator and bracket cost about twenty-five cents; just as good porcelain is made in this country, both at Trenton, N. J., and Greenpoint, N. Y. Whether those potteries can mold them in the exact Prussian form, I am unable to say. I am told by intelligent operators that there are now three times the number of wires strung through the country that are necessary to do all the business when the weather is fine; but during hard rain business is very much delayed by defective insulation.
The greatest amount of moisture in the atmosphere is during rain and fog, with the ground covered with melting snow. Such a condition of affairs occurred in Philadelphia on Jan. 28, and in the Eastern States on the 29th. The New York Herald of Jan. 30, referring to the condition of the overhead wires, says: "Communication between Europe and this country would have been entirely interrupted for many hours yesterday, had it not been for the Mackay-Bennett cable via Coney Island." A similar condition of affairs existed between New York and Chicago, April 5. The Philadelphia Evening Telegraph of April 6, under the heading of a "Wide-spread Storm," says: "The only wire working between Chicago and New York this morning is the one carrying the Western Union stock quotations. At the Western Union, Mutual Union, United Lines and Baltimore & Ohio, messages for the East were not received, except 'subject to delay,' which meant delivery to-day, to-morrow, or next day. Not one of the private wires, for which something like $400,000 per annum is paid by the brokers, was working."
* Which Mr. Culley says seems adapted only for well insulated lines of few instruments.
|Keywords:||General : David Brooks : Washington Pottery : Union Porcelain Works|
|Researcher notes:||It can be inferred that David Brooks is referencing Washington Pottery and Union Porcelain Works as the potteries capable of manufacturing porcelain insulators.|
|Supplemental information:||Article: 9744|
|Date completed:||May 9, 2009 by: Bob Stahr;|