PATTERSON] SELF RECORDING ELECTROMETER 



Table II. 



17 



There is thus a decided gain in the sensitiveness by using two 

 vanes; but, as before mentioned, the results are only indicative of 

 what might be expected in the finished instrument, and cannot be 

 used for comparative purposes. It may not be out of place here to 

 mention that on testing the openness of scale with one vane and with 

 two vanes in the finished instrument, the former gave a deflection of 

 11 volts per mm. under the same conditions that the latter gave 5" 2 

 volts per mm.; showing that the double vaned needle gives double 

 the openness of scale. 



As a result of the preliminary tests the electrometer. was constructed 

 with a needle having two vanes 4|" in diameter, and a boom 6^" long. 



Description of the Instrument. 



Plate I, fig. 1 shows a photograph of the needle system with the 

 boom attached. The vanes of the needle are made of aluminium 

 0'085 mm. thick and in order to give them rigidity the edges are 

 slightly turned up as sh(>wn in the photograph. 



In order to render the instrument fairly dead beat it is neces- 

 sary to have a strong damping attachment, but at the same time it 

 was felt that a system that would do away with the usual dashpot 

 arrangement of a platinum vane swinging in a jar of acid, with the 

 surface tension effects on the needle and the usual troubles incident 

 to this method, was much to be desired. This was accomplished 

 satisfactorily by the Cambridge Scientific Instrument Co. by attaching 

 to the rod of the needle a very light circular vane which swings be- 

 tween the poles of two strong permanent magnets as shown in plate 

 1, fig. 1. These magnets can be adjusted separately to suit the position 

 of the vane. 



The method of attaching the boom is shown diagramatically in 

 fig. I. The rod B has a hook on each end to take the suspension and 

 the needle respectively. This rod is insulated by the quartz tube 

 to which are attached the sleeves to support the pointer and its counter- 



