A Short -Period Electrometer. 239 



millimetres in area. In front of the strip and connected to 

 the terminal by a thin wire is a thin plate of copper (not 

 shown in the figure) bent so that its edge faces the strip and 

 is less than 1 millimetre from it. A gap in this plate allows 



Fi<r. 1. 



-1JT 



P, P, ebonite pillars ; R, It, glass rods ; S, strip : M, mirror ; L, spiral 

 spring ; K, ebonite sheath containing attracting plate ; X, movable 

 platform. 



a beam of light to pass to and from the mirror. Behind the 

 strip is another thin plate of copper imbedded in a sheath of 

 ebonite, K, and also with its edge facing the strip. The 

 whole is mounted on an ebonite support, and placed in an 

 ebonite vessel provided with a small window and filled with 

 a transparent, insulating oil of suitable viscosity and of fairly 

 high dielectric strength. It is also advantageous that the 

 oil should have a high specific inductive capacity*. 



A small platform, N, of ebonite tipped with cork can be 

 raised by a screw until it comes into contact wkh the lower 

 edge of the mirror. A horizontal adjustment of the platform 

 then allows the mirror to be tilted so that the reflected ray 

 can be made horizontal, or can be given any desired small 

 elevation. 



The phosphor-bronze strip is thus between two plates to 

 one of which it is conneoted. If the plates are charged to a 

 difference of potential, the strip is repelled by one plate and 

 attracted by the other. The mirror then, since one of its 

 edges is fixed, is deflected through a small angle proportional 

 to the square of the difference of potential of the plates, as 

 in the idiostatic use of the quadrant electrometer. A beam 

 of light, proceeding froin a small circular aperture illumi- 

 nated by an arc lamp and condenser, passes through a convex 



* The oil found most suitable was a mixture of about equal volumes 



of castor-oil and Singers machine-oil. \Mhen freshly mixed it is slightly 



yellow, bat the colour deepens after a few weeks, and the oil must then 

 be replaced by a fresh mixture. 



