Peltier in 1834 discovered an effect opposite to the foregoing, namely, that when a current is passed through a couple of dissimilar metals the temperature is lowered or raised at the junction of the metals, depending on the direction of the current. This is termed the Peltier The variations of temperature are found to be proportional to the strength of the current and not to the square of the strength of the current as in the case of heat due to the ordinary re sistance of a conductor. This latter is the C'R law, discovered experimentally in 1841 by the English physicist, joule. In other words, this important law is that the heat generated in any part of an electric circuit is directly proportional to the product of the resistance of this part of the circuit and to the square of the strength of current flowing in the circuit.
In 1822 Sweigger devised the first galvanom eter (q.v.). This instrument was subsequently much improved by Wilhelm Weber (1833). In 1825 William Sturgeon of Woolwich, Eng land, invented the horseshoe and straight bar electromagnet, receiving therefor the silver medal of the Society of Arts ((Trans. Society of Arts,' 1825). In 1837 Gauss and Weber (both noted workers of this period) jointly in vented a reflecting galvanometer for telegraph purposes. This was the forerunner of the Thomson reflecting and other exceedingly sensi tive galvanometers once used in submarine sig naling and still widely employed in electrical measurements. Arago in 1824 made the im portant discovery that when a copper disc is rotated in its own plane, and if a magnetic needle be freely suspended on a pivot over the disc, the needle will rotate with the disc. If on the other hand the needle is fixed it will tend to retard the motion of the disc. This effect was termed Arago's rotations. Futile attempts were made by Babbage, Barlow, Herschel and others to explain this phenomenon. The true explanation was reserved for Faraday, namely, that electric currents are induced in the copper disc by the cutting of the magnetic lines of force of the needle, which currents in turn react on the needle. In 1827 George Simon Ohm (q.v.) announced the now famous law that bears his name, that is: Electromotive force Current= Resistance.
In 1831 began the epoch-making researches of Michael Faraday (q.v.), the famous pupil and successur of Humphrey Davy (q.v.) at the head of the Royal Institution, London, relating to electric and electromagnetic induction.
Faraday's studies and researches extended from 1831 to 1855 and a detailed description of his experiments, deductions and speculations are to be found in his compiled papers, entitled (Experimental Researches in Electricity.' Fara day was by profession a chemist. He was not in the remotest degree a mathematician in the ordi nary sense—indeed it is a quest on if in all his writings there is a single mathematical formula.
The experiment which led Faraday to the discovery of electric induction was made as fol lows: He constructed what is now and was then termed an induction coil, the primary and secondary wires of which were wound on a woodcn bobbin, side by side, 'and insulated from one another. In the circuit of the primary wire he placed a battery of approximately 100 cells. In the secondary wire he inserted a galvanometer. On making his first test he ob served no results, the galvanometer remaining quiescent, but on increasing the length of the wires he noticed a deflection of the galvanome ter in the secondary wire when the circuit of the primary wire was made and broken. This was the first observed instance of the development of electromotive force by electromagnetic induc tion. He also discovered that induced currents are established in a second closed circuit when the current strength is varied in the first•wire, and that the direction of the current in the sec ondary circuit is opposite to that in the first circuit. Also that a current is induced in a secondary circuit when another circuit carrying a current is moved to and from the first circuit, and that the approach or withdrawal of a mag net to or from a closed circuit induces mo mentary currents in the latter. In short, within the space of a few months Faraday discovered by experiment virtually all the laws and facts now known concerning electro-magnetic induc tion and magneto-electric induction. Upon these discoveries, with scarcely an exception, depends the operation of the telephone, the dynamo ma chine, and incidental to the dynamo electric machine practically all the gigantic electrical industries of the world, Including electric lighting (q.v.), electric traction, the opera tion of electric motors for power purposes, and electro-plating (q.v.), electrotyping (q.v.), etc.
In .his investigations of the peculiar manner in which iron filings arrange themselves on a cardboard or glass in proximity to the poles of a magnet, Faraday conceived the idea of mag netic "lines of force') extending from pole to pole of the magnet and along which the filings tend to place themselves. On the discovery being made that magnetic effects accompany the passage of an electric current in a wire, it was also. assumed that similar magnetic lines of force whirled around the wire. For conven ience and to account for induced electric_ty it was then assumed that when these lines of force are ((cut') by a wire in passing across them or when the lines of force in rising and falling cut the wire, a current of electricity is devel oped, or to be move exact, an electromotive force is developed in the wire that sets up a current in a closed circuit.