Dirt is a non-conductor of electricity, and the studs of the dial must be kept clean with emery paper or plate-powder, as also the under surface of the needle, key, and binding screws, which unscrew to admit of removal. In the daily use of a battery the chief work is usually thrown upon the first half (say in a battery of forty cells, upon the first twenty-five), and various arrangements have been added to batteries by ingenious instrument-makers to enable the operator to vary his selection of the cells to be brought into use, and thus to relieve the first half of his battery, or, in other words, to equalize its work. But this unequal work question is more a theoretical than a practical evil; for if the initial cells grow weaker a greater number can be placed in use. I have carefully studied all the proposed modifications, and have found in all of them the remedy worse than the disease, unless the graduating dial be doubled (an original suggestion of my own), so that the initial cells of one week may be made the terminal cells of the next. [Illustration: FIG. 7. Graduating dial with needle at “0”.] When desired batteries can be constructed with this double dial (Fig. 8), but it adds to the complexity of the instrument, and I do not myself use it. [Illustration: FIG. 8. New form of Graduating Dial.] In Fig. 9 a similar battery is shown to that just described; but the elements consist of carbon and zinc, and are lifted into and out of a bichromate solution. As it can be recharged by the owner without the necessity of sending it to the maker, it is especially suited for country and colonial practitioners. [Illustration: FIG. 9. 40-Cell Voltaic Battery with zinc and carbon elements, and lifting apparatus.] [SN: Essentials of a Medical Voltaic Battery.] To recapitulate. The essentials of a medical Voltaic battery are-- _a._ A constant supply of electricity of sufficient quantity and quality. _b._ A means by which this electricity may be administered in measured doses. _c._ A means by which the direction of its current may be changed. _d._ A means by which it may be instantly discontinued. FARADISM. [SN: The Faradaic Current.]