. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . circuit is formed of two metals there will be a contact potentialdifference at each junction; but when the temperature is the same, thesewill be exactly equal and will neutralize each other. There will there-fore in this case be no current. If, however, the junctions are at dif-ferent temperatures, then, since the difference of potential depends uponthe temperature, they will no longer balance and there will be aresultant E. M. F. producing a current. This device is called a thermo-electric couple.
. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . circuit is formed of two metals there will be a contact potentialdifference at each junction; but when the temperature is the same, thesewill be exactly equal and will neutralize each other. There will there-fore in this case be no current. If, however, the junctions are at dif-ferent temperatures, then, since the difference of potential depends uponthe temperature, they will no longer balance and there will be aresultant E. M. F. producing a current. This device is called a thermo-electric couple. The energy of the current is drawn from the heatsupplied to maintain the difference of temperature. The E. M. F. produced by a single thermo-electric couple is very small,amounting to only a small fraction of a volt. The effect may be in-creased by having a number of junctions of two metals, the alternateones being heated (Fig. 22). Such an arrangement is called a thermo-yile. The currents generated, however, are seldom sufficient to be ofpractical importance. Fig. 22 Fig. 23 ffor I COID Ll. Diagram of thermopile. Voltaic cell. The Voltaic Cell.—As stated above, when a circuit of uniform temper-ature is entirely metallic, the various potential differences balance eachotiier. If one of the metals is replaced by an electrolyte, the balance ELECTRICAL GENERATORS 55 is destroyed, and when the circuit is closed a current is produced. Thisis the principle of the voltaic or galvanic cell. The energy is derivedfrom the chemical action which takes place between the electrolyte andthe metals during the passage of the current. A simple form of voltaic cell consists of two plates, one of copper andone of zinc, dipping in a solution of sulphuric acid (Fig. 23). The zincplate is found to be at a potential about half a volt lower than the acid,while the copper is nearly an equal amount above, so that the copperis about one volt higher than the zinc. The conditions when the circuitis open a
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