SCIENCE AND PRACTICE. 337 



current induced in a wire of known length, moving in a 

 magnetic field of known intensity. 



If a current of the intensity I circulate in a straight 

 conductor of the length L, placed at right angles to the 

 lines of force of a magnetic field whose intensity is ft, the 

 force F in operation will be the same as if the conducting 

 wire were circular, of unit radius, and the magnetic lines 

 radiated from a pole of the intensity s, in its centre. 



By equation (4, this force is equal to s L I. 



Going back to the straight conductor, let us suppose it to 

 move with a velocity = V, perpendicularly to its own 

 length, and to the lines of magnetic force ; a magneto- 

 electric current would be induced in it, which would have 

 the effect of offering a resistance to its motion, and this 

 resistance would be that expressed by the force s L I. 



To overcome the resistance due to this retarding influence 

 of the current, work has to be performed ; its amount being 

 proportional to the force, and to the velocity with which 

 the conductor passes across ; or, if W represents the work, 



W=Y(*LI) . . . . (5, 



which is equivalent to the work done by the current. 

 According to (3, however, 



W = P R t, 

 whence the resistance, 



s and I can be obtained in absolute measure ; the second 

 member, therefore, contains no unknown magnitude, and we 

 have R in absolute electro -magnetic units. 



With R, and I in absolute units, with aid of Ohm's law, 

 we find the electro -motive force E, 



E = Y s L, 



that is to say, the electro- motive force produced in a straight 

 conductor, moving perpendicularly to its length and to the 



