Z, zinc element; C, carbon element; P, binding-posts for the primary coil; S, binding-posts of the secondary coil; a, the interrupter when the circuit is passing to the helix; b, the interrupter when the circuit is broken. The screw (shown in contact with a) allows of the adjustment of the interrupter to the bundle

of soft-iron wires within the primary helix, thus making the interruptions fast or slow at the will of the operator. The patient is connected with the battery in action by means of cords attached to the binding-posts at P or S. These cords are not

"Practical Suggestions respecting the Varieties of Electric Currents and the Uses of Electricity in Medicine," by Ambrose L. Ranney, M.D.: D. Appleton & Co., 1885.

shown in the diagram, but are shown in Fig. 1. Ishow the direction of the currents.

The arrows

The zinc is marked as the

negative element (—), and the carbon as the positive (+) element of the battery. Note that the wire of the primary coil is represented as coarser than that of the secondary; that the secondary coil has no connection with the elements of the cell; that the current going to the primary binding-posts is generated by the iron core, and is not that which originates in the galvanic cell; and that the interrupter has a small piece of platinum soldered upon it where it comes in contact with the screw, so as to prevent oxidation at that point. Patients feel the current made by the "break" more than that from the "make" of the circuit; hence one electrode apparently gives a stronger current.

The primary current is that which results from the inductive action of the coils of the inner helix upon each other, strengthened by the inducing influence of the magnet.

The secondary current is that which passes to and fro in the outer helix; each electrode is alternately anode and kathode, but the direct current is so much the stronger that it may be alone considered, and this current being understood, we may properly speak of anode and kathode in reference to faradism.

The strength of the induced current is in direct proportion to the number of coils in the outer helix,

and to the strength of the exciting galvanic current. It may be increased by withdrawing the metallic cylinder which surrounds the helix, and thus lessens the inductive influence.

The faradic current has no marked chemical nor heating effects; it has not the penetrating power possessed by galvanism.

CHAPTER II.

FORMS OF ELECTRICAL APPARATUS FOR MEDICAL AND

SURGICAL USE.

Constant Elements.-In the simple "one-fluid cell" already described, it is found that, practically, the current is not constant. The zinc is rapidly dissolved, the fluid becomes saturated with salts, and the hydrogen from the electrolytic decomposition of the water accumulates on the carbon plate, polarizing it, and so preventing the passage of electricity to and through it. All these influences aid in rapidly reducing the current to a minimum.

The destruction of the zinc and the weakening of the current by the formation of numerous small couplets between particles of zinc and the impurities they contain is prevented by amalgamation. Polarization is prevented by having two fluids so arranged as to chemically utilize the liberated hydrogen. Cells so constructed are said to be constant.

Of single fluid cells, the Grenet may be taken as the type; the elements are zinc and carbon im

mersed in a dilute solution of sulphuric acid, bichromate of potassium being added as a partial depolarizer. This form is often used in faradic batteries. The Daniell is a good example of the "two-fluid element." A glass vessel contains solution of sulphate of copper and a perforated copper cylinder with a rim above; on the rim rest crystals of copper sulphate, which keep the solution saturated. Inside of the copper cylinder is a porous earthenware vessel containing a zinc plate immersed in solution of dilute sulphuric acid. The hydrogen set free by electrolysis decomposes the copper sulphate, forming copper and sulphuric acid, the former being deposited on the copper cylinder, the latter going to reinforce the sulphuric acid already present. Good cells of this type are those of Siemens and Halski, Leclanche, the chloride of silver, etc.

For electrolysis, Stöhrer's elements are specially commended by Bartholow.

For luminous and heating effects, we use either large cells or small elements connected in surface. Among the best are Piffard's and Byrnes's. Polarization or storage cells are also used for these purposes. "When two plates of platinum are dipped