A manual of practical medical electricity : the Röntgen rays and Finsen light . he electro-motive force of thecombination. Now, it will be found that those metals which are placedat the electro-positive end of the contact series are alsothose which are most oxidizable, and, vice versa, thosewhich stand at the electro-negative end are least oxidiz-able. We are doubly justified, then, in choosing zinc as oneconductor, and copper, silver, platinum, or carbon(graphite) as the other. The excitant liquid should possess two properties—itshould be capable of chemically acting on the oxidizableconducto
A manual of practical medical electricity : the Röntgen rays and Finsen light . he electro-motive force of thecombination. Now, it will be found that those metals which are placedat the electro-positive end of the contact series are alsothose which are most oxidizable, and, vice versa, thosewhich stand at the electro-negative end are least oxidiz-able. We are doubly justified, then, in choosing zinc as oneconductor, and copper, silver, platinum, or carbon(graphite) as the other. The excitant liquid should possess two properties—itshould be capable of chemically acting on the oxidizableconductor ; it should be a conductor of electricity, orelectrolyte. Water acidulated with sulphuric acid, or a solution ofammonium chloride or of common salt, will answer thepurpose. Place a piece of pure zinc and a piece of pure copper Galvanic Electricity 23 in a glass vessel three-parts filled with a ten per cent,solution of sulphuric acid. So long as the two metals arenot brought into contact, no change will be observed. Ifwe let them touch, or if we connect them together by a. Fig. 12.—Simple Cell. wire, the zinc will be slowly dissolved by the acid, andbubbles of hydrogen will appear at the surface of thecopper plate, while a current of electricity will flow acrossthe connecting-wire. By bringing the two metals into contact we establish a 24 A Manual of Practical Medical Electricity potential difference between tliem—that is, we electrify oneplate positively and the other negatively—and as thereis in all unequal distribution of forces a universal tendencytowards equilibrium, an electrical current passes from theplate at the higher potential to that at the lower. Thiscurrent flows from the zinc to the copper through theliquid, and from the copper to the zinc through the wire. Owing, however, to the chemical changes which accom-pany its passage through the liquid, a fresh potentialdifference is set up between the plates, which is againfollowed by an equilibr
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