. Dental electro-therapeutics. is no chemical depolarizer in this cell, the depolariza-tion is mechanical, that is, the gas bubbles are unable tocling to the surface of the platinum plate and the platekeeps comparatively clear of this polarizing agent, but itis an imperfect means of depolarizing. The zinc of the cell must be withdrawn when the currentis not being used, for the constant chemical action of theacid on the zinc destroys it. The electro-motive force of the cell is about 2 volts andwhen in use it is rapidly reduced on account of the collec-tion of hydrogen gas which collec
. Dental electro-therapeutics. is no chemical depolarizer in this cell, the depolariza-tion is mechanical, that is, the gas bubbles are unable tocling to the surface of the platinum plate and the platekeeps comparatively clear of this polarizing agent, but itis an imperfect means of depolarizing. The zinc of the cell must be withdrawn when the currentis not being used, for the constant chemical action of theacid on the zinc destroys it. The electro-motive force of the cell is about 2 volts andwhen in use it is rapidly reduced on account of the collec-tion of hydrogen gas which collects on the negative type of cell has been much used in the past for electro-therapeutic purposes, but for dental work it has no advantageover the Leclanche cell. Bichromate of Potash Cell.—This cell is constructed of aplate of zinc which forms the positive element, and twoplates of carbon set one on either side of the zinc whichform the negative element. The electrolyte is dilute sul- PERSULPHATE OF MERCURY CELL 57. phuric acid. A mixture of strong sulphuric acid, powderedbichromate of potash and water is the depolarizer. Theelectrolyte is sometimes varied by the use of chromic acidinstead of potash, because its chemical action lessens theinternal resistance. The chemical action of the cell is con-stant and the metal element is arranged so that it can beremoved from the electrolyte whenthe cell is not in use. It is oftenmade in the form of a bottle witha cork through which passes a rodattached to the zinc, which providesfor the lifting of the zinc clear of theliquid when the cell is not beingused. By the chemical action of the elec-trolyte on the plates when the circuitis closed, a strong electric current isexcited, positively charged ions ofhydrogen and metallic radicle passfrom the zinc to the carbon element, at the same time negatively charged ions of oxygen andacid radicle pass from the carbon to the zinc. The E. M. F. of the cell is about 2 volts. It is used wh
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