Mr Lahy, A string electrometer. 



Ill 



give a larger sensitiveness for a given tension, or as is to be expected 

 theoretically the sensitiveness for a given fibre tension depends 

 on Vjd?, where V is the P.D. and d the distance between the plates. 

 This could not be fully proved as the tension of the fibre did not 

 remain sufficiently constant. With the plates at 3"3 mm. apart 

 the deflection of the fibre as its potential changed became markedly 

 different. With the fibre tight its deflection was still proportional 

 to its potential (the straight line of fig. 5), but when slackened the 



•15 -1 -05 



Potential of fibre in volts 



Fig. 5. 



position, which the fibre took up for different potentials, is repre- 

 sented by the curve AB (fig. 5); on lowering the tension still 

 further the fibre had two stable positions and apparently an 

 unstable one not experimentally realisable. In other words, if the 

 fibre is slackened the sensitiveness increases, but for a decreasing 

 range of voltages : theoretically we pass through infinite sensitive- 

 ness to instability. This property of the instrument, which sets 

 a superior limit to its useful sensitiveness, is best grasped by 

 examining fig. 5. 



In Mr C. T. R. Wilson's* tilted electroscope, as is well known, 

 this property is used to obtain a high sensitiveness, the adjust- 

 ments (plate potential, tilt and length of gold leaf) are altered 



* Proc. Camb. Phil. Soc. xii. p. 135, 1903. 



