. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . ^ one or two amperes, while a direct E. M. F. of the sameamount would give 100 amperes/ When the effect of the resistance isnegligible, the current is one-quarter of a period behind the E. M, F.,the case shown in Fig. 52. Maximum and Effective Values.—In speaking of an alternating currentor E. M. F., it is necessary to distinguish between its maximum valueand what is called its effective value, i. e., the value of the direct currentor E. M. F. which would produce the same heating effect in a givenc
. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . ^ one or two amperes, while a direct E. M. F. of the sameamount would give 100 amperes/ When the effect of the resistance isnegligible, the current is one-quarter of a period behind the E. M, F.,the case shown in Fig. 52. Maximum and Effective Values.—In speaking of an alternating currentor E. M. F., it is necessary to distinguish between its maximum valueand what is called its effective value, i. e., the value of the direct currentor E. M. F. which would produce the same heating effect in a givenconductor. The relation between the two depends upon the form of thewave; in the sinusoidal current the effective value is of the maximum,while with a current such as is represented by Fig. 53 the ratio is muchsmaller. When the value of an alternating current or E. M. F. isstated without qualification, the effective value is alwaj^s meant. Fig. 53. Non-sinusoidal alternating current. Measuring Instruments.—As the types of galvanometer or ammeterpreviously described are not affected by an alternating current, instru-ments depending upon other principles must be used. The most impor-tant of these may be classified under four heads: 1. The Mutual Action of Currents.—li the currents in two coils aresimultaneously reversed, the force between the coils will always be inthe same direction. An electrodynamometer (p. 72) will thereforemeasure an alternating as well as a direct current. 2. The Attraction of Soft Iron by a Coil.—A piece of soft iron isattracted by a solenoid in whichever direction the current flows. Theforce is therefore unchanged by the reversal of the current. 3. The Heating Effect.—In the hot wire instruments the currenttraverses a fine platinum wire, heating it and thus causing it to expansion is transferred to a dial, which may be graduated in termsof the current. The heating effect is the same (see
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