134 TRANSFORMERS. [Exp. 



If any combination of coils gives a voltage which is beyond 

 the range of the voltmeter, these tests can be made by using a 

 lower supply voltage; it may be found convenient to connect 

 the high potential side of the transformer to the line, thus step- 

 ping the voltage down to a lower voltage in the secondary. 



10. Prove that the voltage of the secondary is either in phase, 

 or 180 out of phase,f with the primary voltage. To do this, 

 join together one terminal of the primary and one terminal of 

 the secondary, so that the two windings are in series; the supply 

 voltage is connected to the terminals of the primary. Measure 

 the voltage across the primary, the voltage across the secondary, 

 and the voltage across the two, measured between the terminal 

 of the primary and the terminal of the secondary which are not 

 joined together. Either the sum or the difference of the first 

 two readings will equal the third reading; whether it is the sum 

 or the difference will depend on which terminals of the two 

 windings are connected together. If the two voltages were of 

 different phase, the total would be found to be not the algebraic 

 sum but the vector sum. 



11. Use as an Auto-transformer. As ordinarily used, a 

 transformer has two independent circuits, a primary and a sec- 

 ondary, and any particular winding is used as part of one of 



f ( lOa). The secondary eletromotive force is in the same phase as the 

 primary counter electromotive force, being induced by (substantially) the 

 same flux ; hence it is opposite in phase to the primary impressed or line 

 electromotive force. It follows that the secondary current, when the 

 transformer is loaded, is nearly opposite in phase to the primary current, 

 this being discussed more fully in Exp. 5-C. This opposition of currents 

 is verified by the auto-transformer test, u. 



That primary and secondary currents are opposite to each other in 

 phase may be further illustrated by the following experiment. Take a 

 straight upright core surrounded by a primary circuit. Place around it 

 (loosely) a closed ring forming a secondary circuit. Connect the primary 

 to an alternating current supply. When the primary circuit is closed, the 

 secondary will be thrown off violently, showing that the currents in the 

 two circuits are in opposite directions. The secondary ring may be held 

 down by threads, so as to float as a halo. 



