. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . her types of alternatingcurrent motor simpler in their operation are known as the induction motorand the repulsion motor. In these the alternating current is sent throughthe field coils, while the armature is short circuited. The rotation isproduced by the action between the alternating magnetic field and thecurrents induced by it in the armature. One disadvantage of the alternating current motor is that the speedis largely determined by the frequency of the alternations, and thereforecannot be con
. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . her types of alternatingcurrent motor simpler in their operation are known as the induction motorand the repulsion motor. In these the alternating current is sent throughthe field coils, while the armature is short circuited. The rotation isproduced by the action between the alternating magnetic field and thecurrents induced by it in the armature. One disadvantage of the alternating current motor is that the speedis largely determined by the frequency of the alternations, and thereforecannot be controlled so simply as that of motors operated with adirect current. Where it is necessary to obtain greatly differing speeds,some special mechanical device such as friction gearing may be used. THE TRANSFORMER, ETC. The Transformer.—The transformer is a device for changing the formof electrical energy from high to low potential, or xice versa. It consistsessentially of two coils of wire completely insulated from each other,wound on the same iron core (Fig. 60). Then on account of the mutual. Diagram (theoretical) of a transformer. induction between the coils (p. 75) any variation of the current in onewill induce an E. M. F. in the other. The iron core is designed toform a closed magnetic circuit, so that all the lines of force producedby a current in either coil will penetrate the other, which gives amaximum effect (see p. 66). In practice an alternating current is sentthrough the primary coil, and therefore an alternating E. M. F. isinduced in the secondary.^ One coil usually consists of a large number of turns of rather finewire, while the other has a much smaller number of turns of coarser extreme cases only one turn is used. Either coil may be used as theprimary or inducing coil. As the induced E. M. F. is proportional tothe number of turns in the secondary coil, the fine wire coil is used as asecondary when the E. M. F. is to be raised, and the other when theoppos
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