There are several modes of ascertaining the current strength. The voltameter (Fig. 8) is an instrument for collecting the oxygen and hydrogen produced by the decomposition of water. The quantity of gas per minute evolved by the galvanic action is an absolute measure of the strength of the current during that time. This instrument, although not so much used as the galvanometer for determining current strength, is not without practical utility, and it is employed by Ciniselli and others to ascertain the power of any given combination before proceeding in the operation of electrolysis in the treatment of aneurism. Galvanometers are more frequently used. A galvanic current made to traverse a wire, in the vicinity of a magnetic needle, deflection takes place according to the strength and direction of the current. This constitutes the simplest form of the galvanometer. It is really a galvanoscope, indicating the existence of a current and its direction, but not affording an exact measure of its strength. The astatic combination is much more sensitive: it consists of two needles of about the same strength, but placed in opposite positions as to polarity, so that the directive force of the earth's magnetism is neutralized (Fig. 9). The arrangement of an astatic galvanometer is shown in Fig. 10. To render such a galvanometer of use as a means of exactly representing the strength of the current, the scale must be graduated in mille-am-péres of absolute measurement, or millewebers. Such instruments, as now made, enable the operator to express in exact terms the current strength; instead of the number of cups, which affords so inexact a measure, the absolute galvanometer interpolated in the circuit, indicates in mille-ampéres, or millewebers, the actual strength. In the present condition of this subject, it has become necessary to express in an exact way the current strength, just as medicines are carefully weighed.

Fig. 8. Voltameter.

Fig. 9. Galvanometer.

Fig. 10. Galvanometer.

The mode of combining the cells of a galvanic battery is determined by the purpose to which it is to be applied. The circuit may be "simple" or "compound." In the former, the elements are united zinc to zinc and copper to copper, so that in a battery of twenty cells so united there is one large zinc and one large copper element, and the resistance is at the minimum, the chemical action yielding a very considerable quantity of electricity, the most of which is available. In the compound circuit the battery elements are arranged in series, the zinc of one connected with the copper or carbon of the next, and so on throughout the whole number. As every conductor opposes some resistance to the passage of the current—in other words, there is no perfect conductor—there must be considerable loss of electricity. It is sometimes said that the quantity of electricity is determined by the size of the zinc plates acted on, and that intensity is the result of combining numbers of elements. It must be understood that intensity is the power to overcome resistance, and as the resistance offered by the human body is enormous, it follows that a battery for the medical application of galvanism must have numbers rather than size. Those battery-cells furnish the smoothest current in which the internal resistance is about equal to the resistance of the human body. The zinc-carbon elements of Stöhrer cause considerable irritation and burning, while those of Siemens and Halske are smooth and bland. The most experienced electricians use the latter for this reason. It follows, therefore, that all those devices intended to diminish internal resistance are not to be commended in galvanic batteries for medical use, how desirable soever they may be in batteries for economic purposes. In galvanic batteries intended for transportation, every consideration, except efficiency, must be sacrificed to portability, and various mechanical arrangements have been made to combine these qualities. In the Stöhrer, Gaiffe, and chloride-of-silver batteries, the safe transportation of the cells is insured, but the current of such combinations is wanting in the uniformity and smoothness which are such important qualities in the permanent batteries.

The performance of any galvanic combination may be readily ascertained by the law of Ohm. The intensity is directly proportional to the electro-motive force, and inversely proportional to the resistance encountered within the cell and on the circuit. This law is represented in the following formula:

I is the symbol for intensity, Ε for electro-motive force, R for internal resistance (in the cup), and r for external resistance (on the circuit).

Or it may be stated that R = E/C ; that is to say, the resistance equals the electro-motive force divided by the current. The term electro-motive force merely means the difference in potential between the two poles.

A battery, whether portable or permanent, requires a "pole-board" for its working. An ordinary battery of a few elements may be manipulated by a simple sliding selector and polarity changer, such as that now attached to the Stöhrer portable arrangement ; but the permanent batteries of large size, such as are now employed for the office and consultation rooms of medical electricians, require more efficient arrangements. The battery of 60 to 100 elements placed in the cellar or some distant closet, is connected, in groups of cells, with the pole-board. By means of the selector, any desired number of cells can be put in the circuit. The pole-board should also be supplied with a current-changer, a rheostat, an interrupter, and resistance-coils. A water rheostat suffices for ordinary purposes. As water is an indifferent conductor, it follows that the galvanic current has a degree of difficulty in passing through it ; hence, any amount of resistance can be interpolated in the circuit. The most accurate mode of introducing a measurable resistance is the resistance-coil. This is made of German-silver wire of a certain length and cross-section. The resistance offered by such a wire to the passage of the current is directly as its length, and inversely as its sectional area ; that is, the longer the wire, and the less its size, the more the resistance to the transmission of the current. The unit of resistance, the ohm, is the resistance made to one volt by a wire two hundred metres in length. It is obvious that any number of ohms in resistance can be interpolated in the circuit. Resistance-coils of definite measures of resistance are now added to each properly equipped pole-board. They are especially necessary in applications of galvanism by the polar method.