Electromagnetism 175or, self-induced emf, ddvolteLit(5.9)Similarly, when the switch is subsequently opened, the ﬂ ux produced by coil 1 will collapse to zero. The galvo will again indicate that a momentary emf is induced in coil 2, but of the opposite polarity tothe ﬁ rst case. Thus, an emf has been induced into coil 2, by achanging current (and ﬂ ux) in coil 1. This is known as a mutually induced emf. If the changing ﬂ ux can link with coil 2, then it must also link withthe turns of coil 1. Thus, there must also be a momentary emfinduced in this coil. This is known as a self-induced emf. Anyinduced emf obeys Lenz ’ s law. This self-induced emf must therefore be of the opposite polarity to the battery emf. For this reason, it is also referred to as a back emf. Unfortunately, it is extremely difﬁ cult to demonstrate the existence of this back emf. If a voltmeter was connected across coil 1, it would merely indicate the terminal voltage of the battery. 5.17 Self-Inductance Self-inductance is that property of a circuit or component which causes a self-induced emf to be produced, when the current through it changes. The unit of self-inductance is the henry, which is deﬁ ned as follows: A circuit has a self-inductance of one henry (1 H) if an emf of one volt is induced in it, when the circuit current changes at the rate of one ampere per second (1 A/s). The quantity symbol for self-inductance is L . From the above deﬁ nition, we can state the following equation Leitd/dhenryNotes:1 The minus sign again indicates that Lenz ’ s law applies. 2 The emf symbol is e, because it is only a momentary emf. 3 The current symbol is i, because it is the change of current that is important.4 The term di/dt is the rate of change of current.