Galvanism, or the dynamic form of electricity, is excited by the contact of two metals, or other conducting bodies, with the presence of a fluid capable of chemical action on one only of the two, or on one more than on the other. A change of electrical condition takes place in the one most easily affected chemically; the other assumes an opposite condition; and at the moment of communication between them, whether by the absolute contact of the two, or by means of another metal, an attempt to restore the equilibrium takes place, with the effect of developing electrical phenomena. The apparent current of force is from the metal chemically affected, through the liquid, to the one not affected, and then through the connecting material to the point of origin. Hut, as the cause is constantly operating, the electrical condition is constantly disturbed, and no equilibrium is in fact established, but a constant circle of action maintained until the exciting agency is exhausted. Anything susceptible of change by electrical influence, if placed in the circuit, will feel the effects in a degree proportionate to its susceptibility, and the force developed. From a simple circle, as above described, but slight effect is obtained;, yet enough to be very sensible.

To increase the effect, these simple circles or elements must be multiplied, so as to form what are called galvanic batteries, or voltaic piles; and the power developed is in proportion to the number of the elements employed. Numerous methods of attaining this end have been devised, which it is not my object to describe. One thing is common to all of them; namely, that the associated metals, or other conductors, in each pair, and the several pairs themselves always bear the same relation to one another; and the successive pairs must be connected by a conducting medium. The extremes of the arrangement are consequently of opposite character; and, when they are made to communicate, the accumulated force of the whole battery is exerted in the line of communication. These extremes are usually denominated poles, the one at which the metal most acted on is placed being the negative, and the other, towards which the current of force is directed, the positive.

Zinc and either copper or silver are the metals usually chosen, and a diluted acid, especially the sulphuric or nitric, the exciting and conducting liquid; but other metals and other liquids, and even non-metallic substances may be substituted, and various other arrangements have been shown to have a similar agency; chemical action, however, being common to all.

As before stated, the tension of galvanic electricity is very slight, and consequently but slight sparks are afforded by approaching the poles, and relatively slight sensation produced; but the influences dependent on quantity are strong, such as chemical decomposition, the development of caloric, and, in the animal system, the production of organic change.

Application. The simplest galvanic arrangement may be applied with great facility. A small circular or oval plate of zinc, and a silver coin an inch in diameter, placed in the mouth, one above and the other below the tongue, and then allowed to touch, afford evidence of their action to the sense of taste; and a similar pair, soldered together, may be used for very gentle stimulation to these parts; the saliva acting as the exciting liquid.

Another simple arrangement is to place upon two separate parts of the body, between which it is desired to establish a galvanic current, two thin oval or circular plates of zinc and silver, an inch or two in diameter, one on one of the parts, and the other on the other, and to connect the two by means of a delicate wire attached to an eye, upon the outer surface of each plate. The skin beneath should be moist, so as to allow the galvanic influence to penetrate through the cuticle, which is a bad conductor; and for this purpose a layer of any wet conducting substance may intervene between the plates and the skin. Even distant parts of the body may be connected in this way.

Galvanic Chains

A series of small hexagonal plates, composed each of a zinc and a silver plate soldored together by one of their surfaces, and connected by wires so as to move freely, forms a sort of chain, which may be worn next the body, and becomes active through the perspiration. The chain of Pulvermacher, made of couples of minute coils of wire around cylinders of wood, and connected together by wire, acts with considerable energy. The number of elements may extend to several hundreds. Excited by being steeped in an acid liquor, it continues to act for several hours, and may be usefully employed in the treatment of superficial affections. The advantage of these and other arrangements with small elements is, that the peculiar galvanic stimulation may be obtained with less of the heating effect, which is proportionate, in sumo degree, to the size of the metallic plates, while the former depends more upon their number.

Batteries or piles are made of various forms, and of variable numbers of pairs, according to the amount of effect desired. They are applied by means of wires connected at one end with their opposite poles, and terminating at the other in various modes for convenience of use, by which the galvanic influence is conveyed to the person at any distance from the battery. These conductors must be insulated, in some portion of their course, by glass tubes or otherwise, so that the operator himself may not interrupt the current. Of the different methods of termination of these conductors more will be said, in connection with other electric arrangements to be considered immediately. I may mention here that the object is sometimes effected by applying the agency through the medium of water. Thus, both feet may be placed in vessels of water connected with the opposite poles, or both hands, or one foot and one hand; and thus the currents varied in their direction through or over the body. Indeed, the whole body may be immersed; so that, when the bath is connected with one pole, and the body with the other, the electric current may diffuse itself through the system, in its attempts to escape at the surface. Disadvantages of the ordinary batteries used for chemical purposes, as Cruikshank's, Wollaston's, etc., are the disagreeable odour given out in consequence of the decompositions which take place, their unwieldy size, and the difficulty of suitably regulating their action. They are not, therefore, much employed; though circumstances occasionally arise which render a resort to them advisable.

The ordinary galvanic batteries, though they give a continuous current of a certain duration, are unable to furnish that continuity and duration of the galvanic action which is necessary to fulfil all the demands of the most recent galvanic therapeutics. To obviate this defect, several batteries have been contrived, known from their authors as Grove's, Bunsen's, and Daniell's batteries, which have stood the test of trial, and one of which is now generally used, where the greatest simple galvanic effects are required.