Uniform- Electric - Current Accumulator. 63 



longer than the radius ; the continu- 

 ation, AB, at right angles to this in 

 the plane of the wheel, of a length 

 equal to the radius; and the completion 

 of the circuit by a fork, B C, extend- 

 ing to an axle bearing the wheel. If 

 all of the wheel were cut away except 

 a portion, C T, from the axle to the 

 point of contact at the circumference, 

 the circuit would form a simple rect- 

 angle, CTAB, except the bifurcation of the side B C. Let a fixed 

 magnet be placed so as to give lines of force perpendicular to the 

 wheel, in the parts of it between C the centre and T the point 

 of the circumference touched by the fixed conductor ; and let 

 power be applied to cause the wheel to rotate in the direction 

 towards A. According to Faraday's well-known discovery, a 

 current is induced in the circuit in such a direction that the 

 mutual electromagnetic action between it and the fixed magnet 

 resists the motion of the wheel. Now the mutual electromag- 

 netic force between the portions A B and C T of the circuit is 

 repulsive, according to the well-known elementary law of Am- 

 pere, and therefore resists the actual motion of the wheel; hence, 

 if the magnet be removed, there will still be electromagnetic in- 

 duction tending to maintain the current. Let us suppose the 

 velocity of the wheel to have been at first no greater than that 

 practically attained in ordinary experiments with Barlow's elec- 

 tromagnetic disk. As the magnet is gradually withdrawn let 

 the velocity be gradually increased so as to keep the strength of 

 the current constant, and, when the magnet is quite away, to 

 maintain the current solely by electromagnetic induction between 

 the fixed and moveable portions of the circuit. If, when the 

 magnet is away, the wheel be forced to rotate faster than the 

 limiting velocity of our previous supposition, the current will be 

 augmented according to the law of compound interest, and would 

 go on thus increasing without limit were it not that the resist- 

 ance of the circuit would become greater in virtue of the eleva- 

 tion of temperature produced by the current. The velocity of 

 rotation which gives by induction an electromotive force exactly 

 equal to that required to maintain the current, is clearly indepen- 

 dent of the strength of the current. The mathematical deter- 

 mination of it becomes complicated by the necessity of taking 

 into account the diffusion of the current through portions of the 

 disk not in the straight line between C and T ; but it is very 

 simple and easy if we prevent this diffusion by cutting the wheel 

 into an infinite number of infinitely thin spokes, a great number 

 of which are to be simultaneously in contact with the fixed con- 



