182 THE TRANSFORMER. 



It will be seen that in an auto-transformer the current 

 flowing in the primary circuit passes directly into the secondary 

 circuit, and hence the amount of current to be generated in 

 the secondary is less to this extent. This, together with the 

 fact that the number of windings through which the currents 

 flow is less than in an ordinary transformer, makes the losses 

 in an auto -transformer less than in one of the ordinary type. 



With high-tension currents there must be no connection 

 between high and low tension circuits, and this prevents 

 the use of auto-transformers in such cases. 



Tests of auto -transformers are carried out as described 

 for the usual types of transformers given above. 



In order to familiarise himself with the action of an auto- 

 transformer, the student should perform the following 

 exercise on any low -tension transformer which may be 



available : 



Exercise. (1) Supply the primary winding of a low- 

 tension transformer with alternating current, and 

 measure carefully the voltages of both windings. 



(2) Connect primary and secondary windings in 

 series, and supply the outer terminals with current. 

 Allow each winding in turn to supply current to a 

 secondary circuit, and note the" current and voltage in each 

 portion of the circuit. 



(3) Reverse the connections between primary and 

 secondary, and repeat the readings as before. 



(4) Supply the primary circuit with current, and take 

 current from the extreme terminals with the two windings 

 connected first in series and then in opposition. Take 

 readings of current and voltage in each portion as before. 



(5) Repeat these readings with the secondary winding 

 connected to the supply. 



In each case the student should indicate his connections 

 as shown in Fig. 84, and note in each diagram the current 

 and voltage in each section in the manner there shown. 



It is instructive to calculate the copper losses in each 

 form of connection from the current and resistance of the 

 windings, and to compare those with the copper losses to 

 be expected from the usual type of transformer, if the resis- 

 tance of the windings were increased in the same proportion 

 as their number, in order to give the same ratio of trans- 

 formation. 



