552 Prof. Thomson on the Applications of Mechanical Effect 



cally equal to the clcctro-motivc force, wliich will be denoted by 

 F, thus inductively produced, since the unit of electro-motive 

 force adopted by those who have introduced or used absolute 

 units in electro-dynamics is that which would be produced in the 

 same circumstances if the velocity of the motion were unity. If 

 the ends of the moveable conductor be pressed on two fixed con- 

 ductors, connected with one another either simply by a wire, or 

 through any circuit excited by electro-motive forces, so that a 

 cuiTcnt of strength 7 is sustained through it, it will experience 

 an electro-magnetic force in a direction perpendicular to its own 

 length and to the lines of magnetic force in the field across 

 which it is moving, of which the amount will be the product of 

 7 into the intensity of the magnetic force, or, since this is unity, 

 simply to 7*. The motion of the conductor being in that line, 

 the force will be directly opposed to it when the current is in the 

 direction in which it would be if it were produced solely by the 

 electro-motive force we are considering; and therefore, if we 

 regard 7 as positive when this is the case, the work done in 

 moving the conductor during any time will be equal to the pro- 

 duct of 7 into the space through which it is moved, and will 

 therefore in the unit of time be F7, since F is numerically equal 

 to the velocity of the motion. But this work produces no other 

 efiect than making the electro-motive force act, and therefore 

 the electro-motive force must produce some kind of efiect me- 

 chanically equivalent to it. Now if an equal electro-motive force 

 were produced in any other way (whether chemically, thermally, 

 or by a common frictional electrical machine) between the same 

 two conductors, connected in the same way, it would produce 

 the same efi'ects. Hence, universally, the mechanical value of 

 the work done in a unit of time by an electro-motive force F, on 

 a circuit through which a current of strength 7 is passing, is F7. 



4. If the algebraic signs of F and 7 be different, that is if the 

 electro-motive force act against the direction of the current, the 

 amount of work done by it is negative, or effect is gained by 

 allowing it to act. This is the case with the inductive re- 

 action, by which an electro-magnetic engine at work resists the 

 current by which it is excited, or with the electrolytic resistance 

 experienced in the decomposition of water. 



5. The application of the proposition which has just been 

 proved, to chemical and thermal electro-motive forces is of much 

 imoortance. I hope to make a communication to the Royal 

 Society of Edinburgh before the end of this year, in which, by 

 the application to thermal electro-motive forces, the principles 



* This statement virtually expresses the definition of the " strength " of 

 a current, according to the electro-magnetic unit now generally adopted. 



