ii4 ALTERNATING CURRENTS 



OB and OC are wound on the same core, forming a single continuous 

 primary BC, whose middle point O is connected to the primary OA 

 of the remaining transformer. The secondaries Si and S 2 of the two 

 transformers are arranged to have the same number of turns, while 

 the primary BC of the first transformer has \/3 times the number of 

 turns in the primary OA of the second transformer. From this it 

 follows that the e.m.f.'s induced in the secondaries are equal, and 

 since we have already shown that they are in quadrature ( 6) with 

 each other, they will form a two-phase system. It is clear that along 

 with the phase transformation any desired voltage transformation 

 may be effected by suitably arranging the ratios of transformation. 



60. Choking Coils 



By a choking coil, impedance coil, or reactance coil, is meant a 

 coil having a low resistance and high self-inductance. The primary 

 of a transformer whose secondary is open forms a very powerful 

 choking coil. In many cases, it is desirable to have some simple 

 means of adjusting the reactance, and for this purpose various 

 arrangements may be used. Thus, the coil may be provided with a 

 sliding core, which may be clamped in any desired position relatively 

 to the coil. Or the coil may be wound on the middle limb of a 

 [-shaped core, and provided with a movable yoke, which may be 

 placed at varying distances from the projecting ends of the core, 

 thereby altering the length of the air-gaps in the magnetic circuit. 



A choking coil affords a ready means of reducing the voltage of 

 supply without any appreciable loss of power such as would occur 

 if a resistance were used for the same purpose. This is due to the 

 very low power factor of the coil, which enables it to take a large 

 current at a high p.d. without the absorption of any large amount of 

 power. 



