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
| R | ||
| C | = | ——— |
| EMF |
The Rheostat.—When we consider that the resistance between electrodes placed on the palm of the left hand and the back of the neck is about 4,000 Ohms, it may be readily understood that considerable voltage is required to overcome this resistance before the proper amount of current can be employed. Since each cell, for quick reference, may be said to represent one volt, at least twelve and not more than sixteen cells would be required for electrolysis. Not all of the current given off by a battery of such number of cells should be used on a patient for electrolytic purpose. Some method must be employed to reduce this voltage and to control it at will. This is necessary since the life of a cell varies and its current capacity is limited according to the use the cell is put to. An instrument of this nature is called a rheostat and is usually made of graphite or metal wire. Water resistances are also used, but they do not permit of a constant current because of the consequent heating and decomposition of the water into its elements at the two metal poles exposed to the water. The proper instrument will be referred to later.
The electric cell represents a certain voltage; to add to this more cells are needed and connected with each other so that each adds its voltage to other or the circuit. The method of connecting cells in this manner is called series connection, in which the carbon element of one cell is connected with the zinc of the next, and so forth, until the last cell, leaving two free poles, one carbon and a zinc to which the wires to hold the electrodes for the patient are connected. As has been said, the carbon is the positive pole and the zinc the negative. The method of connection is shown in [Fig. 503].
