THE TRANSFORMER. 181 



When the secondary of an ordinary separately wound 

 transformer is loaded, the current in the windings com- 

 posing it is in the direction of the induced (or " back ") 

 electromotive force and opposite to the direction of the 

 voltage applied to the primary winding. Thus the load 

 current acts as a demagnetising current on the core, and in 

 order to maintain the magnetic flux constant, the current 

 in the primary windings increases in direct proportion to 

 the amount of current supplied by the secondary. Also, the 

 ratio of the primary current to the secondary current must 

 be inversely proportional to the oiumber of turns in the 

 two windings (neglecting the no-load current), so that the 

 total magnetising effect on the core may remain constant. 



The ratio of primary to secondary windings of a trans- 

 former whose transformation ratio is 5 to 1 is, however, 

 4 to 1 when the connections have been modified in the 

 manner under discussion. 



A demagnetising load current of four amperes in the 

 secondary winding will consequently correspond to a 

 magnetising current of one ampere in the primary turns ; 

 but this is directed in the same direction as regards the 

 conductor leading from the junction of the two windings, and 

 as shown in Fig. 84, the currents in the primary and secondary 

 unite to supply a load current of five amperes for each ampere 

 in the primary coil. Hence, the ratio of currents in the 

 auto-transformer is inversely proportional to the voltages 

 of the circuits and the general behaviour of transformers 

 of both classes is the same as regards the circuits to which 

 they are connected. 



The method of connecting the windings of an auto- 

 transformer is usually the one described above, and the 

 transformer is then used for transforming doumwards, in 

 cases where the ratio of voltages is comparatively low. 

 Obviously the supply might be taken to the winding termed 

 the secondary above, and the end connections might give 

 out current, in which case the transformer would transform 

 upwards. It would also be possible to connect the windings 

 so that their voltages are in opposition instead of in series. 

 In this case the terminal voltage will be the difference 

 between the voltages of the two sections, instead of their 

 sum. For experimental purposes, auto -transformers are 

 often wound so as to give a large range of voltages by different 

 connections of the windings. 



