There are many such instruments on the market of both dry and moist cell type. The moist cells usually require a bichromate of soda or potash solution and are so constructed that the carbon and zinc poles are taken out of the electrolyte or solution and placed into water-tight compartments provided for them. Such an apparatus is shown in Fig. 510_a_. [Illustration: FIG. 510_a_.—PORTABLE WET CELL DIRECT CURRENT APPARATUS.] The best cell for this purpose is the silver chloride battery. It is compact, light in weight, and gives a steady current. The only objection is the high cost. Portable batteries should be furnished with a milliampèremeter. A type of a compact dry cell direct current apparatus is shown in Fig. 510_b_. In the end the best apparatus proves the most economical. [Illustration: FIG. 510_b_.—DIRECT CURRENT DRY CELL APPARATUS WITH RHEOSTAT AND INTERRUPTED CURRENT ATTACHMENT.] =Electrodes.=—Having the circuit or current under control, it now becomes necessary to attach electrodes to the free poles to be able to properly apply it to the patient. These electrodes vary considerably according to their use. The author will refer to only those that are of service in electrolysis. _Sponge Electrode._—The one electrode held by the patient is usually made of a metal disk covered with felt or sponge attached to a wooden handle and is shown in Fig. 511. [Illustration: FIG. 511.—SPONGE ELECTRODE.] This electrode represents the positive; the negative pole is held by the operator. When used, the felt or sponge is moistened with warm water to which a little salt has been added and is placed into the palm of the hand, sponge inward. The author prefers to use a plain metal disk with the sponge and places a piece of absorbent cotton or gauze over it when in use for hygienic reason. When the operator prefers he may resort to arm or wrist electrodes which can be clamped upon the limb and be held in position and shown in Fig. 512. [Illustration: FIG. 512.—ARM ELECTRODE.] The hand electrode is of greater service since the patient can regulate or make and break the current at will, a matter of no small consequence when fairly large currents are being used to destroy a growth upon the skin of the face. _Needles and Needle Holders._—For the negative electrode the operator uses a needle holder with a needle of proper form and material. Two needle holders are shown in Figs. 513 and 514. [Illustration: FIG. 513. FIG. 514. ELECTROLYTIC NEEDLE HOLDERS.] When the operator desires he may employ an interrupting needle holder with which he can make and break the current at will during the operation. It is shown in Fig. 515. [Illustration: FIG. 515.—INTERRUPTING CURRENT NEEDLE HOLDER.]