4 ELECTROMOTIVE FORCE AND CURRENT. 



then grasp the conductor they will point in the direction in 

 which a north pole would be urged by the magnetic field 

 round the conductor. 



In machines, conductors are generally grouped together 

 in windings forming coils or solenoids. In such cases the 

 same relative direction of current and magnetic field must 

 exist, but usually in these cases it is the field formed within 

 the coil which is the most important, and the rule for easily 

 determining this direction may be modified as follows : 



Place the fingers of the right hand so as to follow the 

 direction of the current round the coil, and the extended 

 thumb will indicate the end of the solenoid which will be a 

 north pole. 



Action of a Current on a Magnetic Field. As already 

 stated, a conductor moved across lines of force has an electro- 

 motive force induced in it tending to produce a current. 

 Conversely, if a conductor situated in a magnetic field carries 

 a current it will tend to move across the lines of force. In 

 other words, the field will exert a force upon a conductor 

 carrying a current. The direction of the force is such that 

 if the conductor is free to move, it will do so in a direction 

 perpendicular to the lines of force, so as to cut them in the 

 way necessary to produce an electromotive force opposing 

 the current. Thus a " back electromotive force " is set up 

 in any conductor moving under the mutual action of 

 its current and a magnetic field, opposing the current 

 which causes the motion. In this case, which is that of 

 the conductors in the armature of a motor, the relation 

 between the direction of motion, lines of force, and current 

 is given by Fleming's rule, as previously stated (see Fig. 1), 

 except that the left hand must be used instead of the right, 

 the fingers indicating the direction of the same quantities as 

 before. 



The magnitude of the force between a conductor carrying 

 current and the field in which it is situated is given by the 

 following formula, absolute C.G.S. units being employed 

 Force acting on conductor = 



(in dynes) 



Current x Length of conductor x Strength of field. 



(in absolute units) (in cm.) (in lines per cm. 2 ) 



In this form the statement is only true when the conductor 

 is at right angles to the lines of force, and the direction of 



