DETERMINATION OF WAVE FORM 625 



commutator, D. The commutator is driven from the dynamo 

 shaft or by a synchronous motor and is so constructed that the 

 connections to the galvanometer are reversed at each half wave. 

 As shown in the figure, the commutator is suitable for a two-pole 

 machine; if there are more poles, it is merely necessary to increase 

 the number of segments correspondingly. 



The distance between the brushes is 180 electrical degrees. 

 They are mounted in a holder which can be moved concentrically 

 with the shaft so that they may be set at any point on the wave. 

 Their position can be read from a graduated circle. It is con- 

 venient to have the arrangement such that the brushes can 

 readily be moved in a succession of equal steps. Thin metallic 

 brushes which will not short-circuit the commutator should be 

 used. The apparatus must be well-insulated ip prevent the 

 entrance of stray currents. This form of commutator is applic- 

 able only when the two halves of the wave are alike (except 

 for algebraic sign), that is, when only odd harmonics are present. 



The galvanometer should be of the moving-coil type and one 

 which correctly integrates a transient current. 



When the commutator is in action the galvanometer experi- 

 ences a deflection which is proportional to the average value of 

 the current during a half cycle. The reading of the instrument, 

 7 2 , is given by 



Let R = resistance of secondary circuit of transformer. 

 m = mutual inductance of transformer. 



/ = frequency. 



2 = instantaneous induced e.m.f. in secondary. 



I'-. = reading of voltmeter. 



i i = instantaneous current in primary. 



ii = instantaneous current in secondary. 



7 2 = average value of current in secondarj 

 Then . dii 



m 



40 



rr T T 



(><+! p + f p + 5 T 



I di\ = I fydt = R \ izdt o 



J, jt J> 



