On the Detection of bad Insulators on Telegraph Lines. 103 



the fluid does not ever coincide exactly with the axis of the crust, 

 and he obtains what amounts to the following formula, 



C* ,d.a>s , r- f da f5 e'. 3 C a ,<fo« , 



Jo P ~~dJ~ 1 P ^~ +a Jo P ^ 



y-dar da+a )/^ar da )/~dj- da 



which differs from mine by one term in the first denominator. 

 This, it will be seen, equals unity when p' is constant. 



I am, 



Yours faithfully, 



John H. Pratt. 

 Calcutta, June 9, 1871. 



XIII. On a Practical Method for detecting bad Insulators on 

 Telegraph Lines. By Louis Schwendler, Esq.* 



ONE of the many practical measures, and certainly not one 

 of the least important, introduced during the last few 

 years with the view of increasing the efficiency of the telegraph 

 department, is the establishment of a scientific system of testing 

 all materials and instruments employed on the line. Many 

 practical results have already been obtained therefrom ; but it is 

 not the object of the present communication to enter into the 

 details of this most interesting subject ; I will only point out 

 one important fact that has been established : — 



A great many lines in India contain electrically defective insu- 

 lators — some to such an extent as to lower the insulation to a 

 degree which is fatal to the direct and regular working of long lines. 



How such insulators could creep in, notwithstanding the 

 care taken in England to secure efficient telegraph stores for 

 India, is a question with which I cannot deal at present, but 

 which may perhaps form the subject of a future paper when 

 more data have been collected f. 



The very fact that electrically defective insulators, showing 

 nothing externally, do exist and are distributed over lines of such 



* From the Proceedings of the Asiatic Society of Bengal for March 1871. 

 Communicated by the Author. 



t The cause for the low insulation of insulators seems to be the porous 

 state of some porcelain, through which a minute quantity of water diffuses 

 itself in time. When an imperfect insulator is heated, it always becomes 

 perfect ; but immersed a sufficiently long time in water it becomes again 

 imperfect. 



The leakage seems to be invariably in that part of a porcelain which is 

 cemented in the iron hood. 



