HIGH VOLTAGE ELECTROSTATIC INSTRUMENTS 67 



used. By means of a transformer, the unknown p.d. may be trans- 

 formed down to a fraction of its original value, and this fraction may 

 be calculated with fair accuracy (provided the transformer is of 

 suitable design) from the number of turns in the primary and 

 secondary coils ( 49). This method is also limited by the fact that 

 a suitably designed transformer for this purpose is both bulky and 

 costly when the voltages to be dealt with are very high. Another 

 method, originally suggested by Ayrton, subsequently by Peukert, 

 and recently reinvented by Marchant and Worrall, consists in sub- 

 dividing the p.d. by means of condensers. This method has been 

 used to some extent, and is cheaper than the other two. It is, how- 

 ever, by no means free from difficulties, chief among which are 

 leakage and variation of capacity with p.d. or frequency, or variation 

 of capacity due to accidental external circumstances capable of 

 affecting the capacity. 



Any one of the methods described may be employed satisfactorily 

 so long as the voltage is not excessive. But for very high voltages 

 it is better to use specially constructed instruments. A good deal of 

 attention has recently been devoted to the construction of such 

 instruments, and in what follows we shall briefly describe a few 

 typical forms. 



35- High -voltage Electrostatic Instruments 



In Fig. 47 is shown Lord Kelvin's Volt Balance. This instrument 

 is portable, and is suitable for pressures up to 30,000 volts (r.m.s.). 

 The fixed plate is insulated by a thick disc constructed of mica. 

 Above it is suspended an aluminium pan, which forms the movable 

 system. The vibrations are damped by a thin aluminium vane 

 arranged to swing between the poles of a damping magnet as shown 

 in the figure. The bulk of the dielectric between the fixed and 

 movable portions of the instrument is air. 



Fig. 48 shows an electrostatic voltmeter designed by Prof. A. 

 Grau,* and capable of reading up to 100,000 volts. The general 

 construction of the instrument will be easily understood from the 

 sketch. C is a fixed sheet-metal cylinder supported by a glass plate. 

 The movable system consists of a hollow metal cylinder B provided 

 with hemispherical ends, and weighted (so as to maintain it accu- 

 rately vertical) by means of a lead weight L. From this weight 

 there projects downwards a glass guide-rod which passes between 

 three guide-pulleys (only one of which is shown) arranged at angles 



* EkJttrotechntiche Zeilschri/t, vol. xxvi. p. 269 (1905). 



