158 



Dr. C. B. Eadcliffe on Electroscopic [May 31, 



Now what has to be done in preparing for an experiment is to get the 

 gold leaves in that second position of divergence into which they pass when 

 the discharging rod is applied to the cap of the charged electroscope. 

 First of all, the gold leaves are made to diverge to a given degree by 

 charging the electroscope in the manner which has been described ; then 

 the conducting rod is placed in position and the gold leaves are made to 

 take up the full degree of divergence by so doing ; and when this is done 

 the electroscopes are ready for use. 



The electroscopes being **set" in this manner, the experiment which 

 has to be performed consists in bringing the body, whose electrical con- 

 dition has to be examined, to the cap of each electroscope in turn, and in 

 noting the movements of the gold leaves. The experiment is simple, and 

 the results are these. When the body is electrified positively, it causes 

 increased divergence of the gold leaves in the electroscope in which these 

 leaves are electrified with positive electricity ; and vice versa, when the 

 body is electrified negatively, it causes increased divergence of the gold 

 leaves in the electroscope in which these leaves are electrified with negative 

 electricity, and diminished divergence in the electroscope in which these 

 leaves are electrified with positive electricity. These are, as will be easily 

 understood, the movements of the gold leaves which must take place under 

 these circumstances. Moreover the charge of electricity in the electroscope 

 reacts upon the body which is brought to the cap of the instrument, and 

 produces, in a way which is intelligible enough, a certain small amount of 

 increased divergence of the gold leaves of both electroscopes. Now this 

 slightly increased divergence of the gold leaves in both electroscopes is of 

 little or no moment when bodies electrified with comparatively large 

 amounts -of free electricity are made to act upon the caps of the instruments, 

 but it is of great moment when these bodies are electrified with minute 

 amounts of free electricity ; for in this case the movements of the gold 

 leaves arising from the action of the free electricity will be exaggerated or 

 masked according as they happen to be in the same direction or in the 

 opposite direction to the movements produced by the reaction of the 

 charge in the electroscopes. Thus, if the degree of increased divergence 

 in the gold leaves of both electroscopes arising from the reaction of the 

 charge in the instruments be = 2, and if the alteration in the divergence 

 of the gold leaves produced by the action of free electricity be of the same 

 value, that is =2, the result of this latter action will be, not increased 

 divergence of the gold leaves =2 in one electroscope, and diminished 

 divergence of these leaves =2 in the other instrument, but increased 

 divergence =4, in the electroscope in which it causes increased divergence 

 (for in this case it is the action of the free electricity plus that of the 

 reaction of the charge in the instrument, 2 + 2 = 4), and no alteration of 

 divergence in the electroscope in which it causes diminished divergence 

 (for in this case it is the converging action of the free electricity minus that 

 of the diverging action of the charge in the instrument, that is, 2 — 2=0), 



