312 Dr. Sumpner and Mr. Phillips on a 



There is really another factor the value of which would not 

 he quite constant if the induction density in the gap varied 

 with the position of the moving coil. But in the present 

 instrument this factor is essentially constant owing partly to 

 the shape of gap adopted, and partly to the fact that for a 

 reflecting instrument the movement of the coil is very slight. 

 Numerous tests have shown that for any given choice of coils 

 and condenser, the scale deflexion is strictly proportional to 

 the square of the applied voltage quite up to the limits of the 

 scale used (300 mm. each side of zero for a scale distance of 

 1 metre). 



The numbers denoted by m and n may each be chosen 

 either 200, 2000, or 4000, while the capacity K may be 

 given widely different values. It is thus clear that the 

 instrument can be used as a voltmeter for a large number of 

 ranges. It will be sufficient to indicate two of these. From 

 a number of tests made under various conditions, the value of 

 expression (3) when V is measured in volt3 and K in micro- 

 farads is found to be 1*6 x 10 -4 for a scale deflexion of 

 200 millimetres. It follows that this deflexion can be 

 obtained 



for 200 volts if ?n = 4O0O,?i = 200, and K = 32 x 10- 4 m.f, 

 or for 20 millivolts if m = 200, rc = 4000, and K=4'0 m.f. 



The deflexion is independent of frequency and wave-form 

 if the field winding to which the voltage is applied has a 

 resistance negligible in comparison with its impedance. This 

 will always be the case if the frequencies used are high. But 

 if the frequency is low and the mass of copper used in the 

 magnetizing coil is small, the resistance of this coil will 

 become comparable with its impedance. The deflexion will 

 then be dependent on frequency, though for a given frequency 

 it will still be a measure of the product K V 2 . Thus if a coil 

 of two turns be wound round the core of the magnet and be 

 used as the exciting winding (ra = 2), and if the moving coil 

 be used with the same condenser and field winding as in the 

 second case above (n = 4000, K=4), a deflexion of 200 mm. 

 will correspond with a reactive voltage in the two-turn coil 

 of only 0'2 of a millivolt. But it will be necessary to apply 

 a much greater voltage than this to cause the magnetizing 

 current to flow through the resistance of the winding. The 

 instrument will still act as a voltmeter for constant frequency 

 circuits, but its indications will be sensitive to change of 

 frequency. 



With the instrument as actually wound, the effect of 



