148 SCIENTIFIC APPARATUS. 



induction by Faraday, in 1831, gave them a new method of 

 investigation. 



One of the earliest results of the discovery of electro-magnetism 

 was the attempt to use the great forces capable of being developed 

 thereby for practical purposes, as a source of motive power. 

 When it was found that an electric current, circulating round a 

 piece of soft iron, could maintain an intense degree of magnetisa- 

 tion in the iron without its own strength being diminished, very 

 exaggerated expectations were formed in many quarters of the 

 practical results likely to follow from the employment of electro- 

 magnetic engines; and, although Jacobi, in 1840, and Joule and 

 Scoresby, in 1845, published investigations which ought to have 

 put an end to such misconceptions, we still occasionally meet with 

 evidence that they are not altogether extinct. 



In 1845, by making use of powerful electro-magnets, Faraday 

 made two of his most memorable discoveries, namely, first, that, 

 when a ray of polarized light passes through various transparent 

 substances in the direction of magnetic force, the plane of polar- 

 ization is altered ; and, secondly, that susceptibility to magnetic 

 force is a property of all substances, and not, as was previously 

 supposed, confined to iron and a few chemically analogous metals. 

 He showed that, with reference to the effect of magnetic force upon 

 them, all substances may be divided into two classes, one of them 

 comprising those which tend to move in the direction of increasing 

 magnetic force, and the other comprising those which tend to 

 move in the direction of decreasing magnetic force. Bodies of the 

 former class, of which iron may be taken as the type, he called 

 paramagnetic, and he distinguished bodies of the latter class, of 

 which bismuth is the chief example, as diamagnetic. Faraday's 

 investigations into the optical relations of magnetism and the 

 phenomena of diamagnetism have been followed up and extended 

 by several experimenters, among whom Verdet, Pliicker, and W. 

 Weber may be specially mentioned. 



The distribution of magnetism in long thin steel magnets was 



