460 



Mr. Morris Owen on 



(3) Experimental Difficulties. 



One of the chief difficulties in these experiments was that 

 of maintaining a uniform pressure between the specimen and 

 the wheel during the rubbing. If the ebonite rod supporting 

 the specimen did not fit very tightly into the socket in the 

 lever, or if the contact with the wheel was too near the front 

 edge of the specimen, the rubbing became irregular, probably 

 owing to the sliding motion breaking into a series of impacts. 

 This occurred especially with the higher velocities of the 

 wheel. It was invariably accompanied by a jarring sound 

 during the rubbing, and followed by an abnormally small 

 deflexion of the electrometer. Several kinds of mountings 

 for the specimens were tried in order to overcome this diffi- 

 culty; the most satisfactory, especially for the lower velocities, 

 was one in which the specimen consisted of a rectangular 

 plate, about 4 cms. long, attached at its upper end to the 



Fig. 1. 



SuLPHUf? 



Wheel 



£30N/TE ftOO 



SP£C/A*f£/V 



sulphur support from which it projected downwards, the 

 wheel rubbing downwards near the lower end of the specimen 

 (see fig. 1). The only difficulty experienced with this ar- 

 rangement was that the specimen was apt to become detached 

 from the sulphur support under the greatest pressure when 



