Theorii of thf Quadrant Electrometer. 

 Hence the force becomes 



249 



ai appears to be about 10"^^;, 



and r/„ 



10- 



So that as long as the potentials of the quadrants are kept 

 small, no practical difference results. 



It is important to point out that although diminishing the 

 air-gap raises the potential of mnximum sensibility, it does 

 not follow that higher sensibility will be obtained by reducing 

 the air-gap. This depends on a-^. The Ibrm of curye for ^23 

 is shown. 



C23 



e-.o 



O^jr 



Diminishing the air-gap will moye the vvhole curye down^ 

 and B probably more than A. ^i, which is the angle made 

 by the tangent at A^ probably increases numerically as the 

 air-gap is reduced^but whereas h^i^n^^io zero as the air-gap 

 is reduced, a-^ tends to a finite limit. I therefore think it 

 probable that higher sensibility will be attained by reducing 

 the air-gap. 



We haye now to consider Ayrton and Sumpner^s results 

 with the White electrometer bifilar suspension . They found a 

 maximum about 1200 volts_, but after falling a little the sensi- 

 bility curye rises as the potential of the needle increases. They 

 show most satisfactorily that the effect is due to lack of 

 symmetry, and particularly to lack of symmetry of the guard- 

 tube. It is not, howeyer, clear that this effect is the same as 



