256 ELECTRICAL MEASUREMENTS 



working parts of the instrument are insulated from the outside 

 or protecting case. This removes the possibility of incurring 

 shocks when the instrument is touched. 



To increase the forces acting on the movable systems of 

 electrostatic voltmeters, up to about 1,000 volts, Lord Kelvin 

 devised the multicellular instrument, an example of which is 

 shown in Fig. 140. 



A torsion wire suspension is used, and the increase of the 

 deflecting force, which is in proportion to the number of cells, 

 is sufficient so that a pointer and scale may be used for reading 

 the deflections. The instrument, however, is not portable in 

 the ordinary sense. In the voltmeter shown in Fig. 140 the 

 damping is effected by a disc which turns in a viscous oil con- 

 tained in a little glass vessel at the bottom of the instrument. 



Two vertical plates are connected to the movable system 

 and screen it from the action of the set of quadrants near which 

 they are placed. 



Use of a False Zero Reading. It is sometimes desirable to 

 measure a small direct-current voltage without drawing any 

 current. The use of the electrostatic voltmeter suggests itself 

 but the normal curve of the reflecting form of this instrument is 

 very nearly a parabola, as is shown by Fig. 138, so that a low 

 potential difference gives a very small deflection which cannot 

 be read with accuracy. The difficulty may sometimes be over- 

 come by superposing the P.D. to be measured on a fixed and higher 

 voltage. For instance, if the instrument gives a scale reading of 

 50 cm. with 50 volts, a potential difference of 2 volts applied 

 directly to the instrument will give a deflection of 0.08 cm. 

 However, if it be superposed on a P.D. of 50 volts the increase in 

 deflection will be 4.08 cm. As the upper part of the calibration 

 curve is nearly straight, the deflections from the false zero are 

 practically proportional to the voltage. 



Condenser Multipliers for Extending the Range of Electro- 

 static Voltmeters. The range of an electrostatic voltmeter may 

 be extended by means of condenser multipliers as indicated in 

 Figs. 141 and 142. 



A condenser of the proper capacity may be joined in series with 



the voltmeter. Then V = TV M - where 7 2 is the reading 



CM 



