230 PROCEEDINGS OF THE AMERICAN ACADEMY 



the Geissler's tube, the introduction of a condenser gave a marked 

 increase in the light, much more so than in air, but no decrease in the 

 deflection of the galvanometer. 



An attempt was made by Vierodt's method to measure the increase 

 and decrease of light consequent upon the introduction and withdrawal 

 of tlie condenser ; but it was found that the intensity was not sufficient 

 to obtain any accurate results. 



It will thus be seen that the effect on the galvanometer was the 

 same when any one of the gases was used ; and also that, when the 

 Geissler's tubes were in the circuit, the condenser might be introduced 

 or withdrawn with no visible result in the deflection of the galva- 

 nometer. 



With an air resistance the spark was a small one with two bright 

 points and a dark centre. This sparl^ gave a larger deflection of the 

 galvanometer, but very little light. When the condenser was intro- 

 duced into the circuit, the character of the spark was changed at once 

 to a larger one of even intensity. The light was greatly increased, 

 and the deflection of the galvanometer correspondingly decreased. 

 But with the Geissler's tubes in the circuit, the only difference which 

 could be observed between the sparks when the condenser was intro- 

 duced or withdrawn was in light. The galvanometer continued to 

 give the same deflection in both ca^es. 



A Geissler tube, therefore, affords a test for the presence and action 

 of a condenser in the secondary circuit of an induction coil when a 

 galvanometer fails to do so. 



