72 Proceedings of the Asiatic Society. | Maron, 
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 insulators ; 
some to such an extent as to lower the insulation to a degree which 1s 
Jatal to the direct and regular working of long lines. 
Why 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 fuéwre paper, when more data have 
been collected.* 
The very fact that electrically defective insulators, showing no- 
thing externally, do exist and are distributed over lines of such 
vast extent, has created the necessity of having a reliable method 
by which such insulators can be detected, and other perfect ones 
substituted with the least possible expense. 
It is clear that such a method, if practicable, must be very sim- 
ple, and the instruments used portable and handy.t 
After some searching in this direction, the following method was 
found to answer the purpose most satisfactorily. 
The principle of the method is to produce magneto-electric 
currents through the resistance of the insulator under test, and to 
measure these currents by the effect they have on the body of the 
tester. 
* 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 heating an imperfect insulator, it becomes always perfect, 
but immerging it a sufficiently long time in water, it becomes again imperfect. 
The leakage seems to be invariably in this part of a porcelain which is 
cemented in the iron hood. 
+ To use a deflection method is out of the question, because the still compara- 
tively high resistance of insulators, which have to bedetected, would necessitate 
a high electromotive force, and a very delicate Galvanometer, which arrange- 
ments could not be made easily portable, as it is required when the tester pro- 
ceeds along a line. 
