THE TRANSFORMER. 137 



owing to the fact that at the comparatively low saturation em- 

 ployed in the cores of transformers, both induction and leakage 

 field increase in almost direct proportion to the magnetising 

 current. 



Fig. 63 should be compared with Fig. 61, as both curves 

 were taken on the same transformer. Full load corresponds 

 to 20 amperes in Fig. 61, since the current is measured on the 

 low-pressure winding, whereas in Fig. 63 the current is 

 measured on the high-pressure winding, and full load is 10 

 amperes. 



Employing the formula for drop in the windings in the 

 same way as on page 132, k = ratio of transformation now 



T T 



becomes ~ instead of -= ! . 

 J- 1 -1 2 



Thus we have in this case the following expression for the 

 drop at full load instead of that given on page 132 : 

 Drop - 10 [-685 + (-183 x 1-935 2 )] 

 = 10 (-685 + .685) 

 = 13-7 volts. 



From the curve Fig. 63, the drop at 10 amps, is seen to be 

 16-6 volts, i.e., 2. 9 volts greater than the calculated drop, 

 exactly as in the case of the previous experiment. 



It is to be noticed that the voltage drop when transforming 

 up and when transforming down is not the same. 



The ratio of the resistances of the two windings is generally 

 approximately that of the square of the number of turns, 

 as in the present case. 



The same method may be carried out, using two windings 

 on the same transformer, instead of two separate transformers. 



Measurement of Ratio of Transformation. The following 

 is a simple method of comparing the number of windings of 

 two coils of a transformer not requiring a supply of alternating 

 current. It may be used to determine the ratio of trans- 

 formation of a transformer. 



The two windings to be compared are connected in 

 parallel to a couple of cells through variable resistances of 

 known value ; for instance, through two arms of a post- 

 office Wheatstone's bridge. A galvanometer or milli-volt- 

 meter is connected to another winding. The resistances 

 are then so adjusted that no deflection of the galvanometer 

 is produced on closing the switch in the battery circuit. The 

 r^tio of the windings will then be equal to the ratio of the 



