Disjunction- currents betiveen Poles of different shapes. 25 



tides of mercury in their separation from the surface. To in- 

 vestigate this the following series of experiments were made. 



Experiment 25. — The conical brass point previously used was 

 placed vertically over a porcelain dish which was filled with mer- 

 cury. The mercury was connected by a conducting-wire with 

 the point e, and the sharp point with the point c (see figure, 

 p. 15). From this the following results were: — 



Mercury Mercury Mercury 



positive. negative. positive. 



Mean deflections . 37*2 43-0 40-1 



Here, then, the deflection was greater when the mercury was 

 negative ; in the foregoing experiments the reverse was the case. 

 Hence the electromotive force of disjunction must have been 

 smaller when sparks were formed between mercury poles than 

 between brass poles. This is confirmed by the following two 

 series of experiments. 



Experiment 26. — The conical brass point was removed and 

 replaced by a brass knob 17 millims. in diameter, which, as 

 special experiments showed, acted about the same as a disk of 

 the above magnitude. There were thus obtained in two inde- 

 pendent series of observations : — 



Mercury 

 negative. 



Mean deflections. -J ^o.^ 



Hence the deflections became considerably smaller when the posi- 

 tive discharge traversed the spark from the mercury surface to 

 the brass knob. 



It might be urged against the above experiments, that the 

 mercury does not retain a level surface while the spark passes, 

 but rises in the form of a peak towards the opposite pole. The 

 smaller deflection when the positive discharge goes from the 

 mercurial surface to the knob would, on this supposition, have 

 been caused by the discharge taking place from the peak to the 

 knob. But there was no such elevation of the surface perceptible. 

 Moreover the deflection, even assuming that a point was formed 

 of the same extent as the brass one, could not have been so small 

 as was observed when the discharge took place from the mercury 

 to the knob, unless we also assume that the mercury itself coope- 

 rated in making the deflection smaller. These experiments merely 

 show that the electromotive force of disjunction between mercury 

 poles is less than that between brass poles ; the true ratio be- 

 tween these forces cannot be ascertained by them. In order to 

 examine what takes place when both poles consist of mercury, 

 the following method was adopted, 



Mercury 



positive. 



359 



Mercury 



negative. 



51-9 



16-9 



29*2 



