=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. [Illustration: 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. [Illustration: FIG. 506.—CELL SELECTOR AND BATTERY ARRANGEMENT.] =Milliampèremeter.=—The fact that a proper resistance has been forced in circuit is not alone sufficient to permit of the proper use of current for electrolysis. A measuring device should be included, as has been referred to and called the Milliampèremeter or Milliammeter. It is shown in Fig. 507. [Illustration: FIG. 507.—MILLIAMPÈREMETER.] The method of connecting this instrument in series with the current from the rheostat has been shown in Fig. 509. =The Electric Current.=—Where the operating room of the physician is provided with street current it will be found more economical and cleaner to use that current for this purpose. Usually the direct current is furnished of a voltage varying from 100 to 125 volts. To utilize such a current a wall plate is employed and connected to the circuit, as shown in Fig. 508. The resistance of an electric lamp is added to guard against injuring the patient if by any accident or negligence the circuit has been improperly closed. [Illustration: FIG. 508.—DIRECT CURRENT SWITCH BOARD OR WALL PLATE.] Whether the street or battery current is used with such a plate makes no difference except that with a battery circuit the lamp is not used. The connections are given in Fig. 509. [Illustration: FIG. 509.—WALL-PLATE CONNECTIONS.] It will be observed that a current changing switch has been added to the wall plate. This is included in the circuit to permit of changing the poles to the patient without interfering or disconnecting the electrodes if desired at any time during treatment. =Portable Batteries.=—The above instruments and circuits refer to those to be used in the operating room and are stationary. The physician may be called upon to treat a patient at a distance and for this purpose must have a portable battery.