1 II 



THE TRANSFORMER. 



where C l = current measured. 



.R, = resistance of winding. 



The whole of the power spent in magnetisation is actually 

 converted into heat in the core, and consequently plays an 

 important part in the heating of the transformer. 



IN Fig. 66 are shown the results of an experiment carried 

 out upon a small arc lamp transformer designed for a 

 secondary voltage of 50. The thick line curve I. shows the 

 readings of the no-load current actually observed. Curve III. 

 is the curve of iron loss obtained by dividing the watts by 

 the voltage of the supply (the copper losses were too small 

 to be taken into account). Curve II. shows the magnetising 

 current calculated in the manner just explained. 



10 



20 



40 



50 



30 



Volts. 



FIG. 66. NO-LOAD CUBBENTS OP A TBANSFOBMEB. 

 I. No-load current. 

 II. Magnetising current. 

 III. Iron loss current. 



It will be noticed that the curves I. and II. do riot appear 

 to pass through zero, although obviously the voltage and 

 current must become zero together. The explanation is 

 that the curve would bend downwards at the lower end, as 

 shown by a thin dotted line, although the instruments 

 available would not read sufficiently low to enable readings 

 to be taken at this part of the curve. This bend is owing to 

 the fact (referred to under Experiment VI.) that at very 

 low saturation the permeability of iron is lower than at 



