1891.] Quadrant Electrometers. 55 



up to a certain point, and then diminished again as the potential of 

 the needle was still further increased. 



From experiments made by varying the distance between the 

 quadrants, we found that when the distance between the quadrants 

 was small, the sensibility first increased as the potential of the needle 

 was raised, then it diminished, and finally it increased again for a 

 still further increase of the potential of the needle. The curve, there- 

 fore, connecting sensibility with potential of the needle was inva- 

 riably of an oj shape for a small distance between the quadrants. 



As the distance between the quadrants was increased, the sensi- 

 bility curve flattened, becoming practically straight when the distance 

 separating the quadrants was 3'9 mm. For a greater distance than 

 this between the quadrants, the sensibility increased more rapidly 

 than the potential of the needle. 



The various curves accompanying the paper show that this 

 quadrant electrometer may be adjusted so that the variation of 

 sensibility with the potential of the needle may be made to follow 

 one or other of three distinct laws. If the quadrants be near together, 

 there are certain limits between which the potential of the needle 

 may vary without producing more than a small change in the 

 deflection corresponding with a fixed P.D. between the quadrants ; 

 for example, when the quadrants were about 2|- mm. apart, and the 

 fibres near together at the top, the deflection produced by a P.D. of 

 1'45 volts between the quadrants only varied about 11 per cent, when 

 the potential of the needle varied from 896 to 3586 volts, that is, by 

 2690 volts. When the fibres were far apart at the top, it was when the 

 quadrants were about 1 mm. apart, as seen in sheet III, that a similar 

 flatness was obtained in the curve connecting deflection with potential 

 of the needle. In this case the deflection of the needle was practically 

 quite constant when its potential varied between 2152 and 3227 volts, 

 and even when the potential of the needle was increased from 1434 

 to 3407 volts, that is, by nearly 2000 volts, the deflection did not 

 increase by as much as 9 per cent. This arrangement of the quad- 

 rants gives bat a comparatively small sensibility, but, where great 

 sensibility is not required, it would be a convenient one to employ, 

 as leakage of the Leyden jar, or loss of potential of the needle due to 

 the rapid absorption that occurs when the jar is first charged, would 

 only slightly affect the deflection for a fixed P.D. between the 

 quadrants. 



When the quadrants were at about 3'9 mm. apart, the deflection for 

 a given P.D. between the quadrants was almost directly proportional 

 to the potential of the needle. This then would be the arrangement 

 to employ when the electrometer is used with alternating P.Ds. 

 And lastly, when the quadrants were 4 mm. or more apart, the 

 deflection increased much more rapidly than the potential of the 



