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].
Fig. 504.—Shunt Rheostat Connection.
Cell Selector.—The physician may do without such a rheostat and use a cell selector with the object of adding one or more cells to the circuit at will. Such instrument is composed of a marble or wooden base with a number of disks upon it, each disk representing a cell of the battery. A metal arm is made to slide over these disks, and as it advances over each disk the current from that cell is added to the circuit. It may have a second arm which is used to cut out the current from the cell or cells at the beginning of the circuit—in fact, will permit of the selection of any cell in the circuit by proper manipulation. Such a selector is shown in [Fig. 505].
Fig. 505.—Cell Selector.
The connection of the cells of the battery when a selector is used varies from that just mentioned. The proper wiring with the disks of the Selector is shown in [Fig. 506].