Page:The American Cyclopædia (1879) Volume VI.djvu/524

516 ELECTRO-MAGNETISM needle, instead of across or around it, as the theory of Ampere would indicate. Mr. Stur- geon, in England, was the first to construct an electro-magnet, which consisted of a piece of iron wire bent hi the form of a horseshoe, in- sulated with a coating of sealing wax, over which was loosely coiled a few feet of copper wire. (See fig. 2.) When the current was sent through the lat- ter, the iron be- came magnetic, and exhibited in proportion to its size a very in- tense action. The first person, how- ever, who exhi- bited the great power of the gal- ^ vanic current in Prof. Henry of Washington. He found that by surrounding a large bar of iron bent into the form of a horseshoe with a number of coils of wire, so connected with the battery of a single element that the current in each wire would move in the same direction, a magnetic power of astonishing magnitude could be produced with a comparatively small galvanic apparatus. A magnet constructed on this principle, now in the cabinet of the college of New Jersey, at Princeton, will readily sup- port 3,500 Ibs. In order, however, to produce a maximum effect of this kind, it is necessary that great care be taken in the insulation of the wires, that there be no cutting across from one wire to another ; and for this purpose the ends of two wires intended to be soldered to the positive pole of the battery should project together, while the two ends intended to be united to the negative pole of the battery should also be associated. If the magnetic power of the iron is to be developed by means of a compound battery, then a single long wire may be employed instead of a number of short ones. The power of the electro-magnet depends on the following conditions: on the energy of the current, the dimensions and form of the iron, the nature of the iron the softer the better the perfect insulation of the wire, and the proper adjustment of the length of the wire to the intensity of the battery. By means of an electro-magnet of the kind we have mentioned, the instantaneous develop- ment of an immense magnetic power is pro- duced, by which discoveries have been made in regard to this mysterious agent, of the highest interest. Prof. Faraday showed by the application of this instrument that mag- netic property is possessed by all bodies, either in the direction of the greatest length of the body, the form in which it is ordinarily de- veloped, or at right angles to this length. He found, for example, that when different substan- ces are made into bars and suspended by means of a fibre of silk between the poles of a power- ful electro-magnet, they arrange themselves with the longer axis in the direction of the pole or with the shorter axis in the same direction. Bodies of the former class are called magnetic ; those of the latter class are called diamagnetic. This property is even possessed by gases. (See DIAMAGNETISM.) An electro-magnet even of immense power can be magnetized, unmagnet- ized, and remagnetized in an opposite direction, by instantaneous changes in the direction of the current of the galvanic battery. The large magnet we have mentioned as at Princeton can be loaded with several hundred pounds, and while in this condition may be so rapidly unmagnetized and remagnetized with the op- posite polarity that the weight has not time to commence its fall before it is arrested by the attraction of the reverse magnetism. This sudden change of polarity affords a means of producing mechanical movements of consider- able power through the agency of electro-mag- netism, which have by some been considered as a rival to steam power. The first machine moved by this power was invented by Prof. Henry immediately after his experiments in developing electro-magnetism, and an account of it was published in the " American Journal of Science " in 1831. It consisted of an oscil- lating iron beam surrounded by a conductor of insulated copper wire. A current of elec- tricity was sent through this in one direction, which caused one end to be repelled upward and the other attracted downward by two stationary magnets. The downward motion of the one end of the beam near its lowest point brought the conducting wires in contact with the opposite poles of the battery, which produced the reverse motion, and so on continually. In a subsequent arrangement, the velocity of motion was regu- lated by a fly-wheel, and electro-magnets substi- tuted for the permanent magnets at first used. Prof. Ritchie of the London university after- ward produced a rapid rotatory motion between the two legs of an inverted horseshoe magnet in a piece of iron around which a current of electricity was made to revolve, and the magnet- ism reversed at each semi-revolution. Modifi- cations of these two forms of the apparatus have since been made in almost every part of the civ- ilized world. A large electro-magnetic engine was constructed by Prof. Jacobi of St. Peters- burg, by which a small boat was propelled at the rate of several miles an hour. But the largest and most efficient engine of this kind was constructed by Prof. Page of Washington, at the expense of the government. It exhibited sufficient power to propel with considerable velocity a railway car, and afforded the best means which has yet been presented of esti- mating the comparative cost of the application of electricity as a motive power. From all the experiments which have been made, it appears that though the electro-magnetic power can be applied with less loss in the way of effective work than heat by means of the steam engine,