Discussion of Brooks' letter about insulators in India

INDIAN AND AMERICAN TELEGRAPHS.

In No. VII. of the Journal of the Society of Telegraph Engineers, I observe on page 115 a letter written by Mr. D. Brooks, dated Philadelphia, August 21st, 1874. As this letter contains some criticisms of parts of my paper on "Some Points in Connection with the Indian Telegraphs," it is necessary for me to enter again into the matter. In my paper, the question, What would produce the insertion of one percent of bad insulators of a certain resistance on a line of a definite length? was investigated mathematically, and the result arrived at was, that in the case under consideration the consumption of battery material would be increased 60 per cent., and the received or effective current diminished 20 per cent. Mr. Brooks objects to this, informing us that, in addition to the battery material employed in producing the line-current, battery material is also used up when the poles are insulated. As, however, this local action is in no way connected with the good or bad insulators, it had absolutely nothing to do with the question I was considering, which concerned itself solely with the battery material legitimately employed in producing the line-current in the different cases considered.

Farther on, Mr. Brooks asks, "Are the benefits derived from the accurate quantitative testing of new insulations at the stores in Calcutta, Bombay, and Madras, commensurate with the care bestowed?" and quotes what I stated to be the average insulation of the lines in India to prove that the answer is in the negative. The average insulation given is that of the lines constructed in India during the last quarter of a century, whereas, the quantitative testing of the insulators was only regularly established in 1871, in consequence of the insulation of some of the lines being much lower than it would be if all the insulators were good. Mr. Brooks refers to some lines in America that have as much as a hundred millions per mile in rain. Lines constructed of Schonberg porcelain insulators (many thousands of which, not some thousand, as printed in the Journal) I passed for the Indian Government at the end of 1872, should have (as can be deduced from the results of the tests, page 189 of my paper) an insulation of two hundred thousand millions per mile, if only the rims or edges of the porcelain cups were dry, the remainder of the cups being perfectly wet outside and inside, as in the testing. Even diminishing this result a hundred per cent. to allow for damp on the edges also of the cups, we should still have with Schonberg porcelain an insulation during rain twenty times as high as that of the best lines in the United States. As, however, I did not expect that Schonberg insulators solely would be used in the construction of any one line in India, but that they would probably be mixed with the cheaper insulators, either of Messrs. Pinder Bourne, or of Madame Defuisseaux, I anticipated in my paper a very much lower probable insulation, one in fact nearly as low as that of the very best American lines.

With reference to signalling, Mr. Brooks condemns the acknowledging tap given in India, by the receiving signaller, alleging as his objection that in order that this acknowledgment may be given the pen must be dropped at the end of each word. The acknowledgment is of course given with the left hand, and does not, therefore, materially interfere with the speed of working; consequently Mr. Brooks's inability to see " how two good operators could, by the Indian method, get more than twenty messages of twenty words each per hour over a line," will now be removed, and he will understand how that fifty such messages hourly is no uncommon number.

Mr. Brooks next refers to Duplex Telegraphy. For full particulars of what has been done in India under this head, I would refer to the papers published by Mr. Schwendler, in the "Journal of the Asiatic Society of Bengal," vol. xliii. Part II. 1874.

The most serious question, however, referred to by Mr. Brooks is the subject of the proper resistance for a relay. As he quotes a report of Mr. Varley's on American closed circuit working, and proposes applying the rules therein given to open circuit working, it is not surprising that he arrives at the conclusion that " doctors disagree, and that there is no subject upon which there are such a diversity of opinions as upon the proper resistance of a relay." From this I conclude, therefore, that the mathematical analysis of the subject cannot be generally known, and in consequence I append an investigation, in which I have proved:—

I. That, with the system of working employed in India, high resistance relays are the best for long lines.

II. That, with the system of working employed in the United States, rather low resistance relays must be used for even long lines.

III. That, in consequence, closed circuit working is a far more expensive system than open circuit working.

IV. That the American system of using one-line battery only for several lines is far more troublesome and expensive than the employment of several line batteries.