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LXXIX. Applications of the Principle of Mechanical Effect to the 

 Measurement of Electro-motive Forces, and of Galvanic Resist- 

 ances, in absolute Units. By Prof. W. Thomson*. 



1. TN a short paper "On the Theoiy of Electro -magnetic In- 

 A duction," communicated to the British Association in 

 1848t^ I demonstrated that "the amount of mechanical effect 

 continually lost or spent in some physical agency (according to 

 Joule, the generation of heat) during the existence of a galvanic 

 current in a given closed wire_, is, for a given time, proportional 

 to the square of the strength of the current f^ and I showed that 

 Neumann's beautiful analytical expression for the electro-motive 

 force experienced by a linear conductor moving relatively to a 

 magnet of any kind, is, in virtue of this proposition, an imme- 

 diate consequence of the general principle of mechanical effect. 

 At that time I did not see clearly how the reasoning could be 

 extended to inductive effects produced by a magnet (either of 

 magnetized matter or an electro-magnet) of varying power upon 

 a fixed conductor in its neighbourhood, or to " the induction of 

 a varying current on itself ;'' but I have recently succeeded in 

 making this extension, and found that the same general principle 

 of mechanical effect is sufficient to enable us to found on a few 

 elementaiy facts, a complete theory of electro-magnetic or electro- 

 dynamic induction. The present communication, which is ne- 

 cessarily very brief, contains some propositions belonging to that 

 part of the theory which was communicated to the British Asso- 

 ciation ; but it is principally devoted to practical applications 

 with reference to the measurement of electro-motive forces arising 

 from chemical action, and to the system of measurement of 

 '^ galvanic resistance in absolute units,'' recently introduced by 

 Weber {. 



2. Prop. I. — If a current of uniform strength be sustained in a 

 linear conductor, and if an electro-motive force act in this con- 

 ductor in the same direction as the current, it will produce work 

 at a rate equal to the number measuring the force multiplied by the 

 number measuring the strength of the current. 



3. Let the electro-motive force considered be produced by the 

 motion of a straight conductor of unit-length, held at right 

 angles to the lines of force of a magnetic field of unit-intensity, 

 and carried in a direction perpendicular to its own length and to 

 those lines of force. The velocity of the motion will be numeri- 



* Communicated by the Author. 



t Report, 1848; Transactions of Sections, p. 9. 



X " Messungen galvanischer Leitungswiderstande nach einem absoluten 

 Maasse ; " von Wilhelni Weber. — Poggendorff's Annalen, March 1851, 

 No. 3. 



