66 



ALTERNATING CURRENTS 



electromagnetic damping is employed. When a p.d. is established 

 between the movable and fixed portions, the former are sucked into 

 the spaces between the latter. The controlling couple is furnished 

 by gravity, but in instruments for low voltages spring control is used. 

 The main feature of the instrument is the cylindrical shape of the 



FIXED PLATES 



MOVABLE PLATES 



DAMPING MAGNET 



DAMPING VANE 



FIG. 46. Ayrton-Mather Voltmeter. 



plates, as a result of which the stress at every point of the movable 

 plate acts at the same radius, so that for a given area of plate a much 

 larger deflecting couple is obtained than with a flat-plate instrument, 

 in which the stress acts at varying radii, and so gives rise to a much 

 smaller couple. 



34. Methods of extending Range of Low- 

 reading Electrostatic Voltmeter 



It is frequently convenient to be able to extend the comparatively 

 narrow range of an electrostatic voltmeter, and various methods of 

 doing this are available. The most satisfactory of these is the sub- 

 division of the unknown p.d. by means of a resistance. Such 

 resistances, known as "multipliers," are supplied by the makers of 

 electrostatic voltmeters, and by means of them the range of an 

 instrument may be greatly extended. 



This extension may be carried on indefinitely, as it is simply a 

 matter of providing suitable resistances. But for very high voltages 

 the cost of the resistance becomes prohibitive, and the resistance 

 itself becomes bulky. Other methods of adapting a low-reading 

 voltmeter to the measurement of high voltages have therefore been 



