66 



Bar us — Displacement Interferometer, 



verge on their front elevation from aa to the top tt of the 

 cylinder, while in their side elevation they are rigorously 

 parallel and equally long. They may be lengthened or short- 

 ened by turning the close fitting rollers aa, so that the cylin- 

 der hh may be placed accurately concentric to the cylinders 

 ih. The latter may be mounted with a cylinder of wood 

 passed quite through them, which is withdrawn after they are 



Fig. 1. 



IF 



Fig. 1. Cylindrical electrometer, vertical section. Front view. 



fastened. The clamp screws 11 and those at the end of a are 

 convenient for charging, the wires passing out of the light tin 

 plate cover GCDD through hard rubber cylinders at Z. The 

 top DD may be lifted off. "When there is no danger of 

 induction, DD may be removed. The ellipses of the inter- 

 ferometer, in a room free from draft, are quite adequately 

 stationary even without DD, a surprising result. 



