1895.] Conductivities of Badly Conducting Substances. 263 



with brightly polished knobs, between which sparks pass discharg- 

 ing the jars and starting alternating currents in the primary 

 circuit ; D and E are two coils wound on ebonite cylinders and 

 made as nearly equal as possible in every respect. 



G and H are two equal coils placed in series, and so arranged 

 that the electromotive force due to the induction between G and D 

 and between H and E acted in the opposite direction round the 

 coils, the ends of the coils were connected to the spark gap KL, 

 and the equality of the electromotive forces round the two coils 

 was tested by the absence of sparks in the spark gap. One of the 

 coils, G, was provided with an arrangement by means of which 

 portions of it could be short-circuited so as to enable the induction 

 between it and the primary coil to be slightly altered, this served 

 as a fine adjustment, the object of which was to make the induc- 

 tion between G and D equal to that between H and E. I found 

 it however impossible to make the adjustment so accurately that 

 there was a complete absence of sparks across the spark gap, the 

 most I could do was to adjust the coils so that the length of the 

 spark was a minimum. When this had been done a vessel 

 containing the substance whose resistance was to be investigated 

 was placed inside one of the primary coils, say I), the effect of this 

 was to increase the length of the spark gap ; a similar vessel 

 containing a solution of an electrolyte was then placed inside the 

 other primary coil, E, and the strength of the solution changed 

 until the spark length was reduced to its former value ; when this 

 was the case the specific resistance of the substance in D was the 

 same as that of the electrolyte in ^ : so that the resistance of the 

 substance in D could be compared with that of an electrolyte of 

 known strength. After many trials with this method I aban- 

 doned it in favour of the following, which I found much more 

 sensitive. 



