626 ELECTRICAL MEASUREMENTS 



.' 2m (ii), = ^#/2 

 (i'i)< = -^7 = -JT = K times the scale reading: 



K is a constant of the instrument. This gives the instantaneous 

 value of the current at a particular point on the wave, and the 

 form is traced point by point as in the contact method. 



E.m.f. Waves. Electromotive force waves are obtained by 

 placing the primary of a suitable air-core transformer in series 

 with a high non-reactive resistance, which is across the line. If 

 the resistance is so high that the circuit is practically non- 

 reactive, a determination of the form of the current wave in it 

 gives the wave form for the potential difference applied to its 

 terminals. As* the resistance must be large, the method is not 

 applicable to low voltages. 



With this arrangement all the measuring apparatus is entirely 

 separated from the primary circuit. This may be advantageous 

 if the voltage is high. 



The transformer should have a variable ratio. If the m utual 

 inductance is not known, the apparatus may be calibrated by 

 use of a sinusoidal current. The maximum ordinate may then 

 be calculated from the measured value of the current, and com- 

 pared with the reading of the galvanometer, 7 2 . 



Or, a wave may be plotted, using the readings of the galvan- 

 ometer, its root-mean-square computed, and this compared with 

 the same quantity determined by a standard alternating-current 

 ammeter. 



Flux Waves. Referring to Fig. 390 the flux which threads 

 the secondary is at every instant proportional to the current in 

 the primary, the wave form of which has been determined. Thus 

 the form of this flux curve has also been found. The total flux 

 through the secondary at any instant will be (?i)/m. 



This suggests that in case the form of a flux wave is desired, 

 it is only necessary to replace the secondary of Fig. 390 by a coil 

 of N turns wound around the core, through which the flux passes. 



For the case considered above the total flux linkages at any 

 instant, t, is given by 



,., 7 2 

 (*0 = -47- 



