120 TREATISE ON ALTERNATING CURRENTS. 



current at the moment of switching the alternator on to the trans- 

 former, which may occur if the residual magnetism in the core 

 happens to be in the same direction as the magnetizing force due 

 to the first current wave. In this case the E.M.F. of self-induc- 

 tion of the primary would at first be comparatively small, and 

 unless the precaution is taken of inserting a starting resistance the 

 first rush of current, although not likely to injure the transformer, 

 may be large enough to blow any fuses in the circuit or damage 

 the current-measuring instruments. Before taking any readings, 

 the starting resistance is, of course, cut out. 



In the primary circuit must be placed a wattmeter, W (see 

 Fig. 40), the fine-wire coil of which is connected across the 

 primary terminals of the transformer, T, while the thick coil is 

 placed in series with the primary of T, and carries the main 

 current from the alternator D. The secondary circuit of the 



transformer is closed 

 through an ammeter, A, 

 and a non-inductive 

 variable resistance, E, 

 and a voltmeter, V t is 

 placed across the se- 

 condary terminals. S is 

 the starting resistance 

 in the primary circuit. 



The primary current will not, in general, coincide in phase 

 with the primary PD ; hence the necessity of using a wattmeter 

 to measure the input. 



The secondary current will coincide in phase with the secondary 

 terminal PD, since the load is non-inductive, so that the pro- 

 duct of the ammeter and voltmeter readings gives the output in 

 watts. 



Let the reading of the wattmeter when the transformer is at the 

 given load = w watts, and let the readings of the ammeter and 

 voltmeter be * amperes and v volts respectively. 

 The efficiency, TJ, is then given by 



iv 



11 = w 



By altering the resistance in the secondary external circuit 

 the efficiency at any load may be determined. 



