Mr. T. T. P. B. Warren on Electrification. 443 



g, 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 g be the deflections after one and two minutes' 

 contact respectively at a given temperature, and G' the deflection 

 at the end of the first minute at any other temperature, then 

 G : G f : : g : g' ; g' will be the deflection at the end of the second 

 minute at this temperature. 



By calculating in this way the value of g' } 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 way. We are thus quite independent of temperature 

 for knowing whether a cable or core has received the slightest 

 injury in manufacture. 



G and g may readily be obtained by testing a core at a fixed 

 temperature, as 75° F., which 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 de- 

 flections for any two successive durations of contact may be ob- 

 tained 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 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 dis- 

 charge first, even although contact with the 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 battery for one minute and 

 afterwards removed, electrification still takes place, but, of course, 

 not precisely as if connected to a battery ; for the insulator, in- 

 stead of being acted upon by a constant charge, is affected 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 had in reducing the loss. 



The amount of electrification retained at any given interval is 

 proportional to the quantity of charge remaining at that time. 

 The longer battery-contact is maintained, the slower will a core 

 or cable lose its charge, and conversely. 



In a cable which has been charged by contact with a battery 

 for one minute and afterwards held free for one minute, the elec- 

 trification will be the same as if, instead of being held free, it had 

 been left connected to a battery having the last tension, thus : — 



2G2 



