NATURAL PHILOSOPHY. 149 



the most economical way of producing 1 electricity, it afforded a 

 valuable addition to our knowledge of electric induction. 



The apparent paradox was this: that a small magnet, capable 

 of supporting only 40 pounds, was capable of exciting a current 

 of electricity that would cause a large electro-magnet to support 

 1,800 pounds. The power required to drive the machine was more 

 than a hundred times that excited in the large electro-masrnet : 



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this, though material as far as economy of production is concerned, 

 did not account for the fact that a small amount of magnetic force, 

 in co-operation with mechanical force, was capable of exciting an 

 indefinitely large amount of magnetic force. The true cause of 

 this indefinite power of increased excitation of magnetism is the 

 comparative slowness of magnetic excitation in iron as compared 

 with the quick excitation of electricity in a copper conductor when 

 passing through a sphere of magnetic force. In other words, the 

 magnetic polarization of iron requires an appreciable time to 

 arrive at a maximum; but the maximum of an induced current 

 of electricity is acquired almost instantaneously. This being so, 

 an indefinitely small induced current of electricity may excite an 

 indefinitely large amount of magnetism in iron by repetition, the 

 only limit being the number of currents that can be excited within 

 the time of magnetic saturation of any given piece of iron. Now, 

 iron requires not only a definite time for saturation, but also a 

 definite time for demagnetization. Moreover, there always re- 

 mains in most specimens of iron, for a considerable time after 

 magnetization, a small amount of magnetism, which has been 

 termed residual magnetism. In this new apparatus this residual 

 magnetism is made use of and multiplied to almost an indefinite 

 degree by the current of electricity excited by itself in co-opera- 

 tion with mechanical motion. 



Prof. Wheatstone, for the purpose of showing that the least 

 appreciable amount of magnetism might be multiplied by electric 

 induction to the full saturation of any electro-magnet, took a 

 piece of boiler plate one-half inch wide, and 15 inches long, bent 

 it in the horseshoe form, and coiled it with 640 feet of No. 14 cov- 

 ered copper wire. Between the pales of this electro-magnet he, 

 caused a Siemens keeper to revolve, having first excited the 

 magnet by means of a small electric current or by bringing it in 

 contact with a permanent magnet. The current being stopped or 

 the permanent magnet withdrawn, the keeper was then set in 

 motion by the force of two men. The small current of electricity 

 excited in the coil of the keeper by means of the residual magnet- 

 ism in the electro-magnet being passed round the electro-magnet, 

 its magnetism was thereby augmented, and consequently its in- 

 ductive power; so that the magnetism of the magnet and the 

 electric current of the keeper reacted on each other till they both 

 acquired a maximum of power by the conversion of mechanical 

 force into electricity and magnetism. When a length of platinum 

 wire was inserted in the circuit, when the excitation was at its 

 maximum, 4 inches of the wire were made red hot. 



Prof. Wheatstono considers it easy to prove that the residual 

 roagnetisnj of the electro-magnet is the determining cause of this 



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