An elementary book on electricity and magnetism and their applications . mical depolariza-tion. Daniell cell. The thirdmethod of depolarizing introducesmore complicated chemical reac-tions, but concerning these wreneed not give much detail. Bythis method cells are constructedwhich give excellent results incontinuous service, and which are,therefore, called closed-circuit of these is probably the mostcommonly used of any form ofwet battery. This is the or-dinary gravity battery, or coppersulphate battery, which is so much used in telegraphy. The original Daniell cell(Fig. 10), from wh
An elementary book on electricity and magnetism and their applications . mical depolariza-tion. Daniell cell. The thirdmethod of depolarizing introducesmore complicated chemical reac-tions, but concerning these wreneed not give much detail. Bythis method cells are constructedwhich give excellent results incontinuous service, and which are,therefore, called closed-circuit of these is probably the mostcommonly used of any form ofwet battery. This is the or-dinary gravity battery, or coppersulphate battery, which is so much used in telegraphy. The original Daniell cell(Fig. 10), from which thegravity cell came, is still oftenused in experimental workwhen a constant voltage isneeded in electrical measure-ments. The active liquid isdilute sulphuric acid in whichis immersed the zinc or nega-tive plate. The copper plateis immersed in a depolarizingsolution of ordinary coppersulphate or blue vitriol (some-Danieii cell. times called bluestone). The L*« ^Carbon Fig. 9. — Sal ammoniac cell withdepolarizer placed inside carbonand with zinc outside. Copper. Porov*Cup 20 ELECTRICITY AND MAGNETISM two solutions are separated by a porous cup made of unglazedporcelain. In general terms, the chemical action which occurswhen the battery is working is as follows: the sulphuric acidattacks the zinc, and zinc sulphate is formed. At the sametime hydrogen is liberated from the sulphuric acid and goeswith the electric current toward the copper plate, where itwould be deposited if it were not for the copper sulphate whichsurrounds the copper plate. When the hydrogen gets into thecopper sulphate solution, it goes into chemical combination,and copper is separated from the solution and deposited uponthe copper plate, which is accordingly kept bright and in goodworking condition. During the operation of the cell the chemical action, whichhas been briefly explained, causes a change in the characterof the solutions. The sulphuric acid in its action on thezinc changes to a solution of
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