264 Prof. Thomson, A Method of Comparing the [Feb. 11, 



In this there were two coils, A and B, in the primary circuit; in 

 one of these the substance to be examined was placed, and in the 

 other a bulb containing gas at a low pressure ; when the jars were 

 discharged the alternating currents produced in the primary induced 

 a luminous discharge through the rarefied gas in the bulb, and 

 the brightness of the discharge served as an indication of the 



Fig. 2. 



strength of the currents flowing through the primary circuit. 

 The substances to be compared were introduced in spherical 

 vessels of constant size inside the coil A, and the diminution their 

 introduction produced in the discharge B observed; when two 

 substances produce the same diminution they have the same 

 specific resistance. The more easily the discharge passes through 

 the bulb B the more sensitive is this method ; for this reason it is 

 advisable to take some trouble in the preparation of this bulb ; 

 the most sensitive bulbs I obtained were made by adding a little 

 bromine vapour to air at a very low pressure. With these bulbs 

 when a sphere about 3 inches in diameter filled with a mixture of 

 1 c.c. of H^SO, to a litre of water was introduced into A the 

 diminution in the brilliancy of the discharge was quite marked ; 

 if the sphere were filled with distilled water no effect was pro- 

 duced. This arrangement serves very well to illustrate the laws 

 expressed by equation (6). Suppose that we introduce into A a 

 secondary circuit made of a good conductor, say a portion of a 

 copper cylinder which nearly fits the coil. When this cylinder is 

 introduced the discharge through B is brighter than when it is 

 absent. This follows at once from equation (6). The brilliancy 



of the discharge, /, is measured by the value of I ( ^^tt^ ) dt. Now 



we can get from equation (6) the value of I when the secondary 

 circuit is absent by putting either ilf == 0, or /Sf = oo , in the right 

 hand of the equation ; doing this we find 



^ ^G' LR ^^^• 



When the secondary circuit is present, then if 8 is very small. 



