132 THE TRANSFORMER. 



Two curves should be plotted from the results of this 

 experiment ; the first one is ratio of transformation, which 

 should be plotted vertically upon a base representing 

 secondary current. 



Plot also a curve, comparing secondary voltage and out- 

 put current, plotting current horizontally and voltage 

 vertically. Draw through the no-load voltage a horizontal 

 line. The ordinate between this horizontal line and the 

 curve of voltage gives the value of the drop corresponding 

 to any load. 



The curve shown in Fig. 61 gives the results of a test 

 made in this way on a small 1 kw. transformer with a nominal 

 voltage ratio of 100 : 50. The primary voltage was kept 

 constant at 100. In this case a single curve represents both 

 secondary voltage and ratio - of secondary to primary 

 voltage on the two scales given at the s'de of the Fig. 61. 



A careful measurement of the resistance of the windings 

 of this transformer made while they were still warm gave the 

 following results : 



r l = -685 ohm. 

 r 2 = -183 ohm. 



248 



The ratio of the number of turns was y^ = 1 935 . 



ljtj 



Inserting these quantities in the formula just given the 

 following values for the copper drop in the secondary 

 voltage are obtained. 



Drop --C, jjr + 



At full load of 20 amps. O> = 20. 



.-. Drop = 2off|||, + -183] = 20 (-183 + -183) 



= 7-3 volts. 



Referring to Fig. 61 the actual loss of voltage between 

 no load and full load is seen to be 51 -4 41 -2= 10-2 volts. 



Probably magnetic leakage will account for the greater 

 part of the difference between the 10'2 observed and the 

 calculated drop of 7-3 volts. The magnetic leakage 

 increases, like the drop in the conductors, with the load on 

 'the transformer. The value, 10-2-7-3 =2-9 volts, will 

 not, however, represent exactly the value of the magnetic 



