Advanced Physics Study Course, W. Bolton, P.131(John Murray, 1986) 有以下的一個問與答：

 Q If the secondary circuit of a transformer is open the current in the primary is very small. Why is this, .............. A With the open circuit for the secondary, the back e.m.f. in the primary is almost the same as the applied potential difference, thus the net e.m.f. causing the current through the primary resistance is very low. .....

Advanced Physics for Hong Kong, Tom Duncan, 3rd Ed., V1, P.284 有以下一段文字：

 Consider initially a mains transformer whose primary winding has a resistance of 10 ohm; on a 240 V supply the primary current should therefore be 240/10 = 24 A. An a.c. ammeter connected in the primary records 0.10 A. The difference is very large and is due to the fact that the alternating flux (arising from the a.c. in the primary), which induces an e.m.f. in the secondary, also induces an e.m.f. called a back e.m.f. in the primary. This e.m.f. opposes the applied p.d. and is nearly but not quite equal to it at every instant. The net e.m.f. in the primary is therefore quite small.

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