. Dental electro-therapeutics. them in series in the formof a battery. Leclanche Dry Cell.—This cell is made on the same prin-ciple as the wet cell described. Instead of the glass jar, thecase of the cell is made of zinc which is used as the positiveelement; in the centre is the carbon nega-tive element surrounded with a layer ofmanganese dioxide as the electrolyte is a pasty substance com-posed of some preparation of ammoniumchloride. The cell is sealed at the topand is of small size, the smallest measur-ing If x If x 3f inches. The currentproduced by one of these small cells
. Dental electro-therapeutics. them in series in the formof a battery. Leclanche Dry Cell.—This cell is made on the same prin-ciple as the wet cell described. Instead of the glass jar, thecase of the cell is made of zinc which is used as the positiveelement; in the centre is the carbon nega-tive element surrounded with a layer ofmanganese dioxide as the electrolyte is a pasty substance com-posed of some preparation of ammoniumchloride. The cell is sealed at the topand is of small size, the smallest measur-ing If x If x 3f inches. The currentproduced by one of these small cells isabout equal to a wet cell of ordinarysize and it will last a fairly long resistance of the Leclanche cellis a factor in its construction, for whichmakers have devised several methods toovercome. Messrs. Schall & Son makeone form which they supply in theirbatteries in which zinc is placed inside a cylinder of carbon,separated from it by the electrolyte. The advantages of the Leclanche dry cell over the wet. Fig 8.—Leclanchedry cell. 56 CELLS are that they are encased in metal and are unbreakable;they contain no liquid which would be liable to spill, theyare sealed, and there is no corroding of the terminals, theinternal resistance is slightly less and E. M. F. slightlygreater than the ordinary wet cell. The principal disadvantage they possess is that theycannot be recharged, when used up they become worthless,but they last long enough to make this disadvantage hardlyworthy of mention. Two years in constant use in a dentalpractice might well be the life of a dry cell. The Smee Cell.—This is a simple cell made of plates ofamalgamated zinc to form the positive element. Theseare placed parallel to each other and separated by a plateof silver coated with a thin layer of platinum to form thenegative element. These two metals, zinc and platinum, arefar apart in the contact series and the electro-motive forcegenerated is greater than it would be if the negative pla
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