ON MAGNETISM. 541 



develope the means of rendering this agent one of the great assistants to 

 human power. An attractive force can be created and destroyed at plea- 

 sure, and thereby an alternation'of action, so necessary to dynamical effects, 

 can be produced. The electro-magnetic telegraph is based on the same 

 principles.* A wheel has twenty-four conductors placed on its circum- 

 ference at equal intervals, so that when it is turned through a complete 

 revolution, the voltaic circuit is completed and broken twenty-four times. 

 This wheel is placed at one station, and another wheel, together with 

 an electro-magnet, at the other; a pair of wires sufficing to effect the 

 communication between them. When the circuit is complete, the electro- 

 magnet is in action and causes its accompanying circle to move through 

 one division ; that is to say, each turn of the one wheel causes a similar 

 movement in the other. Now to every division is attached a letter of the 

 alphabet. If then the instrument be standing at C, and it be requi- 

 site to convey the letter F, the first wheel must be turned through three 

 divisions, by which D, E, and F are successively presented to the ob- 

 server at the other station ; the last of which only is suffered to rest. 

 The close analogy between the agents which produce the varied forms of 

 electricity and magnetism is rendered still closer by the beautiful dis- 

 coveries of Faraday and others. When a current is passing along a wire, 

 it induces a similar current along a wire placed near the first, at the times 

 of making and of breaking the contact. Now we have seen that a current 

 of electricity passing round a bar of iron renders it a magnet, and it was 

 easy to conjecture that, conversely, a magnet should produce a current in 

 a coil wound about it. Faraday f proved that this is the case at the moment 

 only of its becoming or ceasing to become a magnet. The coil was 

 wrapped round a piece of soft iron, the extremities of which could be 

 brought simultaneously in contact with the ends of a horseshoe magnet. 

 At the instant of forming this contact a current of electricity was produced 

 along the coil, the effect of which was sensible to the galvanometer. Soon 

 after this discovery, all the usual electrical effects were produced in this 

 way, and in 1832 was constructed, by M. Pixii, J a very powerful magneto- 

 electric machine. This machine, as improved by Saxton and Clarke, || 

 consists of a compound horseshoe magnet of a large size fixed in a given 

 position. A piece of soft iron, of much the same shape, has a quantity of 

 insulated copper wire wound round it, and is so placed as to be capable of 

 rapidly presenting its ends alternately to the poles of the fixed magnet. By 

 this means it becomes constantly magnetized, demagnetized, and oppositely 

 magnetized. Thus the conditions requisite for the development of an 



* Wheatstone, Mech. Mag. 1840. Walker's Electr. Mag. vol. ii. Sturgeon's 

 Annals, v. 337. Steinheil, Ueber Teleg. 4to, Munch, 1838. Morse, Ann. de Ch. 

 Ixxii. 219. Lenz, Ueber die Praktischen Anwendungen des Galv. Petersb. 1839. 

 De Heer, Theorie de la Teleg. Electr. Bullet, des Sci. Phys. en Neerland, 1839. 

 Finlayson, The Application of the Electric Fluid to the Useful Arts. For the 

 application of galvanism to gilding, &c. see Jacobi, Galvanoplastik, St. Petersb. 

 1840. Smee's Metallurgy. 



t F?*a*day, Ph. Tr. 1832. Experimental Researches in Electricity, 1839. 



J Ann. de Ch. 1. 322. Ph. Mag. ix. 262. 



|| Sturgeon's Ann. i. 145. 



