536 Royal Society : — 



required ; but, ou the other hand, high tension in the primary was 

 found necessary. In many of the experiments accordingly from 

 10 to 20 of the smallest Leclanche cells usually made were em- 

 ployed with the small, and from 20 to 50 with the large coil. 



But these were afterwards replaced by a double fluid battery 

 suggested by my assistant, Mr. P. Ward, and described at the 

 end of this communication. 



For some time the experiments were conducted with the platinum 

 spring resting on the wheel; and the effects were varied by 

 altering either the pressure of the spring or the velocity of the 

 wheel ; but the gradual abrasion of the platinum through friction 

 proved to be a fruitful source of irregular results. This irregularity 

 of action, at all times difficult to compensate, and sometimes insuper- 

 able, was fortunately removed by a simple although delicate ad- 

 justment. It was, in fact, found that actual metallic contact 

 between the spring and wheel was not necessary, provided that 

 a layer or cushion of conducting material were interposed. Such 

 a layer was formed by a thin film of liquid drawn out by a thread 

 leading from a reservoir and resting on the wheel. Various fluids 

 were tried ; but the simplest, and on the whole the best, proved 

 to be dilute sulphuric acid, in the proportion of 1 drop of acid 

 to 6 drams of water. Generally speaking the better conductor 

 the fluid is, the better are the obtained results but, owing to the 

 insulating slots being very narrow in this instance, a comparatively 

 weak mixture of acid and water was necessary. In one wheel, 

 where the insulating slots are | in. wide, a mixture 36 times as 

 strong may as advantageously be used. The spring, which under 

 these circumstances became unnecessary, was replaced by a point, 

 the adjustment of whose distance from the wheel was simpler and 

 more accurate. This arrangement gave excellent results, eA'en when 

 the number of currents per second was reduced in some cases to 

 250 ; added to which the unpleasant and disturbing noise of the 

 friction was entirely avoided. 



Wheels having different numbers of teeth were also constructed, 

 and (what was perhaps of more importance) having teeth of different 

 breadths, so as to give with the same velocity of rotation contacts 

 of different duration. The breadth of the ebonite plugs, or length 

 of interruption of the current, was immaterial, so long as the current 

 was efficiently broken. It did not appear, however, that with 

 the same tube more could be obtained with wheels having different 

 numbers of teeth, than with the same wheel at different speeds. 

 But it was found that for different tubes different wheels occasionally 

 gave better results. 



With the contact-breaker here described effects similar to those 

 produced by the rapidly vibrating break were obtained. The striae 

 were formed in a regular manner, and advanced or receded, or 

 remained at rest, in a column usually unbroken, so long as the 

 velocity of the wheel was maintained without change ; and even in 

 the longer tubes, where the stria?, of the double discharge, advanced 

 or receded towards both ends at the same time, and appeared some- 



