June 25, 1891] 



NAIURE 



hung an endless chord, and the two bights carry similar 

 pendant pulleys, c, D, from which again hang weights, 

 E, F. The weight of the cord being negligible, the sys- 

 tem is devoid of potential energy ; that is, it will balance, 

 whatever may be the vertical distance between c and D. 

 Since either pulley, A, B, may turn independently of the 

 other, the system is capable of two independent motions. 

 If A, B turn in the same direction and with the same 

 velocity one of the pendant pulleys, c, D, rises, and the 

 other falls. If, on the other hand, the motions of A, B 

 are equal and opposite, the axes of the pendant pulleys 

 and the attached weights remain at rest. In the electri- 

 cal analogue the rotatory velocity of a corresponds to a 

 current in a primary circuit, that of n to a current in a 

 secondary. If, when all is at rest, the rotation of a be 

 suddenly. started, by force applied at the handle or other- 

 wise, the inertia of the masses E, f opposes their sudden 

 movement, and the consequence is that the pulley B turns 

 backwards, i. e. in the opposite direction to the rotation 

 imposed upon A. This is the current induced in a second- 

 ary circuit when an electromotive force begins to act in 

 the primary. In like manner, if A, having been for some 

 time in uniform movement, suddenly stops, B enters into 

 motion in the direction of the former movement of A. 

 This is the secondary current on the break of the current 

 in the primary circuit. It might perhaps be supposed by 

 some that the model was a kind of trick. Nothing could 

 be further from the truth. The analogy of the two 

 things was absolutely essential. So far was this the case 

 that precisely the same argument and precisely the same 

 mathematical equations proved that the model and the 

 electric currents behaved in the way in which they had 

 seen them behave in the experiment. That might be con- 

 sidered to be a considerable triumph of the modern dyna- 

 mical method of including under the same head pheno- 

 mena the details of which might be so different as in this 

 case. If they had a current which alternately stopped 

 and started, and so on, for any length of time, they, as it 

 were, produced in a permanent manner some of the 

 phenomena of electrical induction. The particular ap- 

 paratus by which he proposed to illustrate those effects of 

 the alternating current was devised by a skilful American 

 electrician, Prof. Elihu Thomson, and he had no doubt it 

 would be new to many. The alternating current was led 

 into the electro-magnet by a suitable lead ; if another 

 electric circuit, to be called the secondary circuit, was 

 held in the neighbourhood of the first, currents would be 

 induced and might be made manifest by suitable means. 

 Such a secondary circuit he held in his hand, and it was 

 connected with a small electric glow lamp. If a current 

 of sufficient intensity were induced in that secondary 

 circuit it would pass through the lamp, which would be 

 rendered incandescent. [Illustrating.] It was perfectly 

 clear there was no conjuring there ; the incandescent 

 lamp brightened up. One of the first questions which 

 presented itself was, what would be the effect of putting 

 something between .'' Experimenting with a glass plate, 

 he showed there was no effect, but when they tried a 

 copper plate the lamp went completely out, showing that 

 the copper plate was an absolute screen to the effect, 

 whatever it might be. Experiments of that kind, of course 

 in a much less developed and striking form, were made 

 by Faraday himself, and must be reckoned amongst some 

 of his greatest discoveries. 



Before going further, he might remark on what strong 

 evidence they got in that way of the fact that the propa- 

 gation of the electric energy which, having its source in 

 the dynamo downstairs, eventually illuminated that Httle 

 lamp, was not merely along the wires, but was capable of 

 bridging over and passing across a space free from all 

 conducting material, and which might be air, glass, or, 

 equally well, vacuum. Another kindred effect of a striking 

 nature, devised by Prof. Elihu Thomson, consisted in 



NO. I I 30, VOL. 44] 



the repulsive action which occurred between the primary- 

 current circulating around a magnet and the current in- 

 duced in a single hoop of aluminium wire. Illustrating 

 this by experiment, he showed that the repulsion was so 

 strong as to throw the hoop up a considerable height. 

 ThoiC effects were commonly described as dependent 

 upon the mutual induction between two distinct cir- 

 cuits, one being that primarily excited by a battery or 

 other source of electricity, while the other occurred in a 

 detached circuit. Many surprising effects, however, de- 

 pended on the reactions which took place at different 

 parts of the same circuit. One of these he illustrated by 

 the decomposition of water under the influence of a single 

 Daniell cell with the aid of self-induction. 



About tfte time that the experiments of which he had. 

 been speaking were made, Faraday evidently felt wr- 

 easiness as to the soundness of the views about electricity- 

 held by his contemporaries, and to some extent shared by- 

 himself, and he made elaborate experiments to remove all! 

 doubt from his mind. He re-proved the complete identity 

 of the electricity of lightning and of the electricity of the 

 voltaic cell. He was evidently in terror of being misled 

 by words which might convey a meaning beyond that 

 which facts justified. Much use was made of the term 

 " poles " of the galvanic battery. Faraday was afraid of 

 the meaning which might be attached to the word 

 "pole," and he introduced a word since generally sub- 

 stituted, "electrode," which meant nothing more than 

 the way or path by which the electricity was led in. 

 " Electric fluid " was a term which Faraday considered 

 dangerous, as meaning more than they really knew about 

 the nature of electricity, and as was remarked by Max- 

 well, Faraday succeeded in banishing the term " electric 

 fluid " to the region of newspaper paragraphs 



Diamagnetism was a subject upon which Faraday 

 worked, but it would take him too long to go into that 

 subject, though he must say a word or two. Faraday 

 found that whereas a ball of iron or nickel or cobalt, 

 when placed near a magnet or combination of magnets, 

 would be attracted to the place where the magnetic force 

 was the greatest, the contrary occurred if for the iron was 

 substituted a corresponding mass of bismuth or of many 

 other substances. The experiments in diamagnetism 

 were of a microscopic character, but he would like to 

 illustrate one position of Faraday's, developed years after- 

 wards by Sir Wm. Thomson, and demonstrated by him 

 in many beautiful experiments, only one of which he now 

 proposed to bring before them. Supposing they had twa 

 magnetic poles, a north pole and a south pole, with an 

 iton ball between them, free to move along a horizontal 

 line perpendicular to that joining the poles, then, 

 according to the rule he had stated, the iron ball 

 would seek an intermediate position, the place at 

 which the magnetic force was the greatest. Conse- 

 quently, if the iron ball be given such a position, 

 they would find it tended with considerable force to a 

 central position of equilibrium ; but if, instead of using 

 opposite poles, they used, e.g., two north poles, they 

 would find that the iron ball did not tend to the central 

 position, because that was not the position in which the 

 magnetic force was the greatest. At that position there 

 was no magnetic force, for the one pole completely 

 neutralized the action of the other. The greatest force 

 would be a little way out, and that, according to Fara- 

 day's observations, systematized and expressed in the 

 form of mathematical law by Sir Wm. Thomson, was 

 where the ball would go. [This was illustrated by experi- 

 ment.] 



The next discovery of Faraday to which he proposed 

 to call attention was one of immense significance from a 

 scientific point of view, the consequences of which were 

 not even yet fully understood or developed. He referred 

 to the magnetization of a ray of light, or what was called 



