Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.

If two portions of straight, movable wires have constant gal- vanic currents coursing through them, they are then mutually attractive and attracted, while the current is still moving in the same direction ; but if one of them is reversed, so that they run in contrary directions, then they are repelling and repelled; and this is not instantaneoiis, as with static electricity, but is continuous as long as the current continues to traverse the conductors. Thus the ascertained action of an electric current upon a magnetized needle had furnished the means in every respect for determining the existence, and for appreciating the force, of any sort of electric current. Immediately M. Schweig- ger, a German philosopher, applied these principles to the con- struction of the first galvanometer multiplier, which was em- ployed by M. Nobili in his wonderful electro-physiological researches. M. Dubois-Reymond then constructed on this same principle a galvanometer of the utmost sensitiveness, and as remarkable for its accuracy, by employing more than 24,000 elliptical convolutions of insulated fine wire, by which he was enabled to detect the presence of electric currents even in almost all the tissues of the living animal body; and by this superior aid he arrived at his fundamental and special laws in electro-physiology. Very soon after Oersted's discovery, Arago demonstrated that if a copper wire that is well covered with silk thread and varnish be rolled into the form of a helix around a bar of soft iron, and an electric current is then caused to pass through the wire so coiled and situated, the soft iron becomes a power- ful magnet, and remains so as long as the current runs. He showed that it is with the greatest rapidity that soft iron is mag- netized and demagnetized by the electric current. Such tem- porary magnets are termed e/ec^ro-magnets, in order to dis- tinguish them from permanent magnets of steel. Thus on, he showed that the electric current imparted a magnetic property to pieces of soft iron temporarily, to steel permanently, also to other bodies that did not possess it previously. The term " electricity of induction" strictly means the de- velopment of electricity by the influence of other electricity in 12*