TRANSACTIONS OF THE SECTIONS. 47 



On Electrljieatloi}. Bi/ Thomas T. P. Bri-ce Warren. 



Wlien au insulated wire or cable is connected to a battery, and the deflection 

 noted on a galvauometer, tlie first rush of current into the cable is due to tlie 

 electrostatic capacity ol'the insulator. Battery-contact beiuji' still maintained, the de- 

 flection falls verj- rapidly at first, and gradually becomes reduced for some time after. 

 The ratio between the deflections for equal periods of contact is independent of the 

 length, and is greater or less according to the specific resistance of the dielectric. 

 The ratio is unaltered under diflerent electromotive forces, so long as constancy is 

 maintained during the time of observation, and the deflection itself the same at 

 the end of the first period of contact. But when, with diflerent electromotive 

 forces, the deflections at the end of the first period of contact are not the same, we 

 may obtain the deflections which should be given on prolonged contact, if we know 

 the deflection for a corresponding period by any electromotive force, since the 

 deflections for the first period of contact will have to one another the same ratio 

 which the deflections at any other period of contact have ; thus, if with a given 

 electromotive force we obtain at the end of the first minute's contact a deflection 

 of 84, which at the end of the second minute is reduced to 7G, and with a difi"erent 

 electromotive force we have a deflection of 70 at the end of the first minute's 

 contact, the deflection at the end of the second minute will have the same ratio to 

 70 which 84 has to 70. Under diflerent temperatures the resistances corresponding 

 to one, two, three, &c. minutes' contact follow the same law of variation ; thus if 

 R = >• X const, represent the resistance after one minute's contact, then 



r, r', r", »•"', r" are the resistances determined after 1, 2, 3, 4, n minutes' contact 

 respectively, and R, R', R' ', R", R" the required resistances for the same differences 

 of temperature t, and at the end of 1, 2, 3, 4, n minutes' contact. If at any tempe- 

 rature T we obtain a deflection CI after one minute's contact which at the end of 

 the second minute falls to tj, we may calculate what the deflection should be at the 

 end of the second minute for any other temperature by knowing only the deflection 

 after the first minute at this temperature. Let G and f/ be the deflections after one 

 and two minutes' contact at a given temperature, and G' the deflection at the end 

 of the first minute at any other temperature, then G : G' : : r/ : [/ ; (/ will be the de- 

 flection at the end of the second minute at this temperature. By calculating in this 

 way the value of [/ and comparing it with the actual reading, much more reliance 

 can be placed on the value of a test than can be done by correcting for temperature 

 in the usual wa}'. We are thus quite independent of temperature for knowing 

 whether a core or cable has received the slightest injury in manufacture. G and// 

 may readily be obtained by testing a core at a fixed temperature, as 75° F., as is 

 now done. Coils having the same dimensions have rarely the same ratio in their 

 resistances on prolonged contact with a battery; but when several coils are joined 

 together the ratio between the deflections for any two successive durations of con- 

 tact may be obtained from the reciprocals of the deflections of the several coils. In 

 reducing tests of insulation by discharge to measures of resistance, it is impossible 

 to obtain but approximations in the ordinary way of making the tests. The best 

 way is to charge the cable or core for one minute, and leave it free for another 

 minute, and then note the discharge, recharge the core, and take the instantaneous 

 discharge. By this method we know exactly the amount of electrification which 

 has been given to a core, but by taking the instantaneous discharge first, e^-en 

 although contact with a battery is made for one minute, we cannot say how much 

 electrification is retained in the core. When a core is thus connected to a batterv 

 for one minute, and afterwards removed, electrification still takes place, but of 

 course not precisely as if connected to a battery; for the insulator, instead of being 

 acted upon by a constant charge, is afi'ected by the variable charge consequent upon 

 leakage ; but when the core is held free for one minute it is very easy to ascertain 

 how much effect the electrification has added in reducing the loss. " The amount 



