The tension of galvanic is far less than that of frictional electricity, but by greatly multiplying the number of pairs in a battery the tension of frictional electricity may be approached. Thus, a battery constructed by Mr. Gassiot of 3,520 zinc and copper pairs, having electrodes -1/50of an inch apart, gave a series of sparks across the interval which lasted for several months. It is usual to say that the quantity of frictional electricity is small and its tension great, while the quantity of a galvanic pair is great in proportion to its tension. An experiment of Faraday's showed that two wires, one of zinc and one of platinum, each 1/13 of an inch in diameter, immersed in acidulated water 3/20 of a second, had a greater effect on a magnetic needle than that of 23 turns of the large electrical machine of the royal institution. The physiological effects of galvanic electricity vary with intensity and quantity; they are treated in the article Medical Electricity. The astatic galvanometer cannot be used to measure currents of much strength, on account of its too great delicacy. For this purpose the tangent galvanometer and the sine galvanometer are employed.

The tangent galvanometer consists of a vertical circle made of a band of copper the two ends of which are connected with the poles of a battery. In the centre of this vertical circle a small magnetic needle is placed, in length about 1/12 of the diameter of the circle. When the needle is no longer than this, the tangent of the angle of deflection will be proportional to the strength of the current. In using the instrument the plane of the vertical circle is placed in the plane of the magnetic meridian. The sine galvanometer, invented by Pouillet, is represented in fig. 19. A longer magnetic needle may be employed in this instrument, because it is kept at right angles to the axis of the coil through which the current •passes. A horizontal, graduated circle, containing a declination needle, is fixed within a vertical circle, the two turning on a vertical axis which passes through the centre of a lower stationary, horizontal, graduated circle, an index being used to measure the arc of revolution. A stout copper wire, covered with silk, is passed one or more times around the rim of the vertical circle, according to the strength of the current which is to be measured. For weaker currents the coils are increased.

In using the instrument the plane of the vertical circle is placed in the plane of the magnetic meridian. The needle and index will then each stand at 0, respectively on the upper and lower horizontal circles. If a current is now sent through the wire, the needle will be deflected; and if the vertical circle is rotated till the needle lies in its plane, and therefore again points to 0, the deflection will be marked by the index on the lower circle. The deflecting force of a current acting at right angles to the axis of the needle exactly balances the magnetic force of the earth, which is proportional to the sine of the angle which the needle makes with the magnetic meridian. An instrument called a differential galvanometer is sometimes used to measure at the same time the difference in strength of two currents. For this purpose two separate coils of the same sized wire are passed an equal number of times around the same needle. When two currents are sent in contrary directions through the coils, the amount of deflection produced will indicate the difference in strength between them. Sir William Thomson's mirror galvanometer (fig. 20) measures a delicate galvanic current with more precision than any other instrument that has been invented.

A magnet is suspended within a coil of wire which varies in size and length according to the size and length of the conductor through which the current has already passed. If it has passed through long circuits containing bad conductors, the coil should be long and of fine wire, because the current will have been so much weakened that a fine long wire is now sufficient to conduct it, and therefore it may be used to induce a considerable magnetic force. The coil is placed within the cylinder mounted upon the rectangular box shown in the figure, and to one side of the magnet suspended within it there is attached a mirror which reflects a ray of light upon a horizontal graduated screen in front of it, and behind which there is placed a lamp which sends a ray of light through an orifice. A slight deflection of the magnet, which together with the mirror weighs only a few grains, gives the reflected ray a wide range over the graduated screen. A bar magnet, S, placed in the magnetic meridian, is used to counteract the earth's magnetism and thereby increase the delicacy of the instrument. Another bar magnet, T, perpendicular to the magnetic meridian, is used to adjust the instrument to zero when no current is passing.

An instrument called a rheostat, invented by Wheatstone for the purpose of comparing resistances, is represented in fig. 21. Two cylinders of equal diameters turning upon their axes are held in a frame. One of them, A, is of metal, and the other, B, of some non-conductor, as vulcanite or baked wood. There is a spiral groove in the non-conducting cylinder in which a wire, connected with the binding screw C, is wound for an indefinite distance, and then transferred to the other cylinder and wound upon it to its further end. By turning the crank connected with one of the cylinders the wire may be all transferred from one to the other. A binding screw connects with the metal cylinder, and when this and the other binding screw are connected with the electrodes of a battery a galvanic current will pass through the wire which is wound upon the non-conductor, and also through the metal cylinder, so that it will be easy to interpose in the circuit any desired length of wire having any desired area of cross section. Establishing a certain length of a certain sized wire as a unit of measure, a comparison may be made between the resistances of various media.