328 ELECTRICAL MEASUREMENTS 



Any losses in the insulation or oil used in the transformer 

 are included in the measurement, so the oil should be specially 

 dried. The condensers CC may be oil-immersed. Perfectly 

 dry oil must be used so that the dielectric may be as free from 

 losses as possible. 



With the arrangement as just described the potential difference 

 to give a full-scale deflection is only a few hundred volts. To 

 adapt the arrangement to high-voltage work, the voltage v 

 must be applied to the electrodes, T\, by means of a con- 

 denser multiplier, as shown in Fig. 192. 



The voltage vi which is dependent on the current is obtained 

 as before. The plates of the condenser multiplier are at a, b, c, 

 d, and by adjusting the position of the plates a and d, the multi- 

 plying power may be varied as desired. Air condensers are used. 

 In the original apparatus the "plates"- a and d were long sheet- 

 metal cylinders with hemispherical ends. They were hung by 

 insulating cords so that they could be readily raised or lowered. 

 By this means the range of the instrument could be increased 

 to 250,000 volts. 



POWER MEASUREMENT IN POLYPHASE CIRCUITS 



Blondel* s Theorem. 7 If energy be supplied to any system 

 of conductors through n wires, the total power in the system is 

 given by the algebraic sum of the readings of n wattmeters, so 

 arranged that each of the n wires contains one current coil, the 

 corresponding potential coil being connected between that wire 

 and some point on the system which is common to all the poten- 

 tial circuits. If this common point is on one of the n wires and 

 coincides with the point of attachment of the potential lead to 

 that wire, the measurement may be effected by the use of n 1 

 wattmeters. 



The receiving and generating circuits may be arranged in any 

 desired manner and no assumption is made as to the way in which 

 the e.m.fs. and currents vary. 



To prove the theorem, denote by the subscripts 1, 2, 3, . . . 

 n the different supply wires, by v\, v^ . . . v n , the instan- 

 taneous potentials of the points on the various wires which form 

 the terminals of the absorbing device, and by i t if , . . i n , the 



