THE EFFECTS OF THE GALVANIC CURRENT may be classified under physiological, mechanical, magnetic, heating, luminous, and chemical. The mechanical effects relate to the mutual attraction or repulsion of one current to another, or to a part of itself. These, along with the magnetic effects, will be found treated of under MAGNETO ELECTRICITY. The heating and luminous effects have been partly discussed under ELECTRIC LIGHT. We shall here only further refer to the heating of wires, and to the galvanic spark. The luminous effects of galvanic electricity of very high tension will be given under INDUCTION Corn. The chemical effects have been already referred to, but a fuller consideration of these will now be given under the head Electrolysis in this article.
The physiological effects, as shown by the convulsions of Galvani's frog preparation, were the first observed manifestation of the current. Frog-limbs, as prepared by Galvani, when included in a circuit, form a galvanoscope of excessive sensibility, which rivals the finest galvanometer in delicacy of indication. There is one peculiarity in their action which deserves to be noted. The limbs contract only when the circuit is completed and broken, and remain undisturbed so long as the current passes steadily through them. The more frequently, therefore, the current is stopped and renewed, the greater is the physiological effect. The same is experienced when a current is passed through the human body. When the terminal wires of a battery are lifted one by each hand, except it consist of a very large number of cells, almost the only sensa tion felt is a slight shock on completing and breaking the circuit. Du Bois Reymond, the great authority on animal electricity, states that the nerves of motion are affected only by changes in. the electric tension of the current, whereas the nerves of sensation are affected not only by these, but also by the steady continuance of the current, and that the excitation of the nerves dependent on the changes of tension increases with their frequency and suddenness. Frictional electricity in this way owes its superior physiological power to the instantaneous nature of its discharge. It is only currents of
great tension which affect the ordinary human nerves. The poles of a battery of 50 Bunsen cells, capable of giving a brilliant electric light, for instance, may be handled without much inconvenience. This may be attributed partly to the non-conducting nature of the skin. If the current enter the body by a cut or wound, the sensation is affected even when the current is weak. The physiological effect is also much heiolit ened by moistening the hands with salt and water, or by holding metal handles instead of wires, so as to improve the conducting connection. Another cause of this insensi bility may be attributed to the fact that the current is not restricted, as it is in part of the frog preparation, to the nerve, but passes through all the conductors of the system. The nerves of the palate can be affected by a very feeble current; that Of sight by one proceeding from a battery of one or two cells, and that of hearing by a battery of some SO cells. See ELECTRICITY, MEDICAL.
Heating a strong current passes throngh thin wires;an intense beat is produced, sufficient to bring them to a white heat, and to fuse them. This is turned to practical use in exploding gunpowder, in and mining operations. Two wires of a battery placed at a safe distance are insulated from each other, and their ends, which are connected by a fine iron wire. are sealed up in a tin cartridge filled with gunpowder, and laid in the exploding charge. When all is adjusted, the battery con nection is completed, and the current making the iron wire red hot, ignites the gun powder in the cartridge, and that again the charge. In this way, all clanger is avoided. Experiments on the heating effects of the current through wires hare proved that the heat developed is proportional to the resistance of the wires, and to the squares of the strength of the currents; and that the strength of the current being the same, any length of wire may be heated to the same redness.