468 



Induction- Spark between Magnetic Poles. 



portional to V . It was supposed necessary that the potential 

 of the quadrants attached to the short rod, which was at a 

 great distance from the electrometer, should be proportional to 

 V . The opposite quadrants were at potential zero, being con- 

 nected with the earth ; and since when the deflections are 

 small they are proportional to the difference of potential of the 

 two poles of the instrument, it was supposed that the deflection 

 of the electrometer-needle would be a relative measure of the 

 potential of the plate. 



A great many observations were taken with this apparatus ; 

 and the results agreed with the former ones, not only qualita- 

 tively, but very nearly quantitatively. I select the following 

 series of observations to show this. The difference is in this 

 case not so widely apparent, owing to the extreme weakness of 

 the current used — it being at this time, in order to get small 

 deflections, the weakest used in the whole of my work. 



Primary broken 



Primary broken 



outside. 



inside. 



75 



95 



10 



9 



73 



9-4 



7 



9-5 



8-2 



8-5 



8-2 



9-2 



10-6 



95 



9-5 



8-6 



5-2 



8-4 



10 



9-7 



8-8 



8-5 



8 



9-4 



65 



10-5 



9-4 



10-9 



7 



8-1 



9-2 



10 



5-3 



8-7 



10-5 



12 



8-3 



7-8 



8-5 



8-4 



9-8 



10-6 



99 



8-6 



Mean 84 



9-3 



These were taken in sets of ten, with wonderfully close 

 means, notwithstanding the apparently great probable error. 



Another series, with a stronger current, gave as the ratio of 

 the results 1*364. 



The results of the investigation are as follows : — 



1. By breaking the primary circuit of an induction-coil in 

 a magnetic field, the length of spark produced by the secondary 



