=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. [Illustration: FIG. 503.—SERIES CONNECTION.] These two poles when brought in contact with human tissue exhibit different action and effect. Without going into electro-chemistry it may be said the current of the positive pole is sedative and that of the negative irritant or destructive. That oxygen and acids are freed at the positive pole and hydrogen and alkalies at the negative pole. It is due to these properties of the current that it is employed therapeutically, but to properly employ it the current must be controlled so that the exact amount given or used can be estimated. This is accomplished first of all by the interposition of resistance within the circuit. This resistance should be such that the current can be increased or decreased at will. It has been referred to and is called a rheostat. Its position in the circuit is shown in Fig. 504. [Illustration: FIG. 504.—SHUNT RHEOSTAT CONNECTION.]