Several references have been made in the preceding chapter to the specific use of electricity without a description, however, of its source or application. The author does not deem it necessary in this volume to go into the principles of electricity, and takes it for granted that the practitioner is sufficiently familiar with a knowledge of the rudiments of the subject and that he understands the meaning of an electric cell commonly known as a battery. =The Electric Battery.=—An electric cell or battery is made up of two poles which are named positive, designated by the + (plus) sign, and negative by the - (minus) sign. In the usual form of cell used the parts are made up of a carbon and zinc cylinder placed into a glass jar containing the electrolyte or actuating fluid. The latter is either an aqueous solution of potassium bichromate or salammoniac contained in a glass jar. For continuous use or open circuit work the Le Clanche type of cell is most practicable. [Illustration: FIG. 502.—ELECTRIC WET CELL.] In Fig. 502 a cell of this type is shown in which the positive pole or element is composed of a solid piece of carbon forming a cover to the glass jar as well, and the negative element is of zinc. The covering over of the jar prevents evaporation of the solution and adds much to its life. =The Voltage or Electromotive Force.=—The voltage or electromotive force from such a cell averages about 1.5 volts. Voltage represents the force or propelling power of current known scientifically as the electromotive force and designated EMF. Owing to the great resistance of the body to the electric current, a proportionate force is required to attain therapeutic results. The unit measure of the quantity of current is known as the ampère. As this is too great for therapeutic use, the thousandth part, or _milliampère_, is employed, and for the purpose of measuring the amount of current given the patient the milliampèremeter is included in the circuit or flow of current. The unit of resistance is termed the Ohm, and to simplify the method of electrotherapeutic administration the practitioner may refer to Ohm’s law as a guide. He must remember the average resistance to the current of the parts to be operated on by this process. The law is as follows: EMF or Voltage C or Current in Ampères = ------------------ R or Resistance, or commonly written C = R/EMF