_The Faradaic Battery._--In Fig. 10 (see p. 29), a Faradaic battery, worked by two Leclanché cells, is shown; but I find it better to employ either one or two ordinary bichromate cells instead of the Leclanché, as the former can be kept in order by the operator himself without much trouble. The primary coil is fixed upon a pedestal, the secondary is movable, and can be lifted over or thrust away from the primary. The degree of action in the secondary coil being proportionate to the extent to which it is brought under the influence of the primary, this arrangement admits of the most perfect graduation of the current; and it has been for some time in use in all well-constructed instruments. The innovation I have made consists in limiting the primary coil to its legitimate purpose of induction, and rendering the secondary alone available for application to a patient. I have been long satisfied that therapeutically the distinction between the primary and secondary coil entirely consists in the greater tension of the current of the secondary coil enabling it to penetrate easily several thicknesses of muscle, but there is no therapeutic indication that cannot be fulfilled by this secondary coil; and at its lowest power I have frequently applied it to the conjunctiva. The rapidity of vibration of the interrupting hammer is varied by increasing or decreasing the distance between the point of the needle and the electro-magnet by the protrusion or retraction of the screw, of which the needle forms the end--that is, by increasing or decreasing the space through which the hammer passes in its vibration, and also by altering the pressure of its spring, but there is seldom therapeutic need for change of vibration; and unless this exists it is better _not to alter the adjustment so long as the instrument acts well_.[9] After considerable use the point of the needle, and the exact spot of the platinum disk of the hammer against which this needle impinges, become oxidized, causing weakening or stoppage of the current. This platinum disk has been constructed to rotate, and a hole has been drilled in its circumference (Fig. 11). By inserting a little lever furnished with the instrument into this hole, the slightest twist given to the disk is sufficient to bring a new surface of platinum into contact with the needle point. This will usually be all that is required, but, if not, the needle can be unscrewed, and its point cleaned with emery paper. When in course of time the disk becomes dotted over with spots of oxidation, the screw fixing the hammer in position must be unscrewed, the hammer lifted out, and its surface similarly cleaned. [Illustration: FIG. 11. The Platinum Disk and Lever.]