Electromagnet and the Law of the Dynamo. 3 



electric technology deals with the electromagnet and its 

 applications, it is of utmost importance to know what the true 

 law of the electromagnet is. If the technologist is left to 

 the mercy of the physicist, he has to choose between formulae 

 that are based on physical conceptions and which are utterly 

 unmanageable for his purpose, and formulas that are purely 

 empirical expressions and have no rational basis. 



But the study of the modern dynamo-electric machine has 

 brought us to a point that makes some decision a necessity. 

 The entire action of the dynamo is dependent on the magne- 

 tizing action of the electric current, and the law of the dynamo 

 is, in the very nature of things, not to be discovered, save by 

 the discovery of the law of the electromagnet. It is indeed 

 extraordinary that such able physicists as Mascart and Angot, 

 Mayer and Auerbach, Schwendler, and Herwig sought in vain 

 for the true law to connect the electromotive force of the 

 dynamo with its speed, the resistance of its circuit, and the 

 constants of its construction. Hopkinson came nearest to 

 the mark when in 1879 he described the properties of the 

 dynamo in terms of those of a certain curve, which we now 

 call the characteristic curve of the machine; but he did not 

 state the law of the dynamo algebraically, and apparently he 

 accepted as true Weber's formula for the electromagnet. 



But the law of the dynamo is now known, and expressions 

 have been deduced which are found to agree with the utmost 

 accuracy with the observed facts, The discovery is almost 

 entirely due to Dr. Frolich*, whose papers, published at intervals 

 from 1880 to 1885, are in their way already classical. The 

 results first obtained by Frolich related exclusively to that 

 form of dynamo in which the armature- coils and the field- 

 magnet coils are both included in the main circuit — the 

 series-dynamo — though they have been further extended by 

 the present writer, and by others, including Frolich himself, 

 to other forms of machine. A very brief resume of Frolich's 

 research will explain his method of arriving at the law. 



His theory is based upon (1) Faraday's law of induction, 

 (2) Ohm's law, (3) a curve, called by him the current-curve, 

 expressing certain results of experiments made on the series- 

 wound dynamo. 



Following Faraday's principle, the induced electromotive 

 force E will be proportional to the speed of the machine n and 

 to a quantity M which Frolich calls the u effective magnetism," 

 and which is itself proportional to the effective area of the 



* Frolich's chief papers are as follows :^Berl. Berichte, 1880, p. 978 ; 

 Elektrotechnische Zeitschrift, vol. ii. p. 134 (1881) ; ib. vol. iii. pp. 69 and 

 113 (1882) ; ib. vol. iv. pp. 60, 67, and 71 (1833) ; ib. vol. vi. pp. 128, 

 139, 227 (1885). 



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