620 Mr. Q. Moody. Hobbs on the Relation between P.D. 



parallel witb tbe screw was attached to the carriage and the 

 latter held in place upon the ways by three spring-clips. 

 This device obviated all drifting contact and made the zero 

 setting very definite. 



Fig. 2. 



It was found necessary, in order to avoid vibration and 

 similar effects, to attach the support of the spherical electrode 

 to the interferometer-bed itself, insulating it of course so 

 that the circuit would be complete only by the contact of the 

 surfaces. This electrode was also screwed on a taper shaft 

 which turned in a perfectly-ground sleeve, so set that the sur- 

 face could be rotated on its axis at an angle of 45 degrees from 

 the outside of the box. It is easily seen that by this arrange- 

 ment the points of contact of both surfaces could be shifted 

 after each discharge, and fifteen or twenty discharges taken 

 before removing the box. This box was made of brass and 

 carried on its sides the shafts of the two gears (G 3 and Gr 3 ) 

 by which the rotation of the plane electrode and the main 

 screw (L) of the interferometer was effected. It also carried 

 a socket through which the shaft (H) for turning the 

 spherical surface could be run after the box had been sealed. 

 The joints of the three shafts were made air-tight by pieces 

 of rubber tubing which allowed sufficient rotation for the 



