Electricity and Heat as Moving Powers. 67 



rent of electricity derived from the consumption of 1*56 lb. of 

 zinc per hour, in a Daniell's battery. But the best electro- 

 magnetic engine that we can hope to see constructed, cannot 

 be expected to give more than half or one-fourth of this 

 power ; in any case, we see here the limit of power which 

 no perfection of apparatus can make it exceed. The pecu- 

 liar mode in which the electric current produces dynamic 

 effects, has led to much miscalculation respecting the power 

 obtainable from it. In every sort of electric engine, the 

 material to which the neighbouring current gives motion, 

 whether it be another moveable current, or, what is more 

 usual, a magnetic body, is impelled in one direction with a 

 constant force ; and this force, whether it be attraction, 

 repulsion, or deflection, is, like the power of gravity, sensibly 

 constant at all velocities, however fast the body recedes be- 

 fore the action of the force, provided only the same quan- 

 tity (per minute) of electric current be maintained. This is 

 quite different from the action of steam-power, in which the 

 faster the piston moves the greater is the volume of steam 

 per minute that must be supplied to move it, or else the less 

 will be the power with which it moves. 



This fact, then, that the force with which an electric cur- 

 rent of a given " quantity" moves the machine, is the same 

 at any velocity of motion, bears no analogy to the case of 

 steam, but would indicate that the dynamic result obtainable 

 from a given electric current might be indefinitely great ; 

 and so it would be, were it not that the part moved always 

 tends to induce a current in the wire in the reversed direc^ 

 tion ; and this inducing influence, which increases with the 

 velocity of motion, conflicts with the original current, and 

 reduces its quantity, and consequently reduces the power of 

 the motion, as well as the consumption of materials in the 

 battery. 



Some have imagined that possible alterations in the parts 

 of the machine, or in its mode of action, would avoid the 

 evil, or might even make the induced current to flow with 

 the primary current, instead of against it. The impossibility 

 of this, though not readily proved in detail, can be at once 

 proved by reference to general principles ; it would, if true, 



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