ELECTROMOTIVE FORCE AND CURRENT. 5 



the force is then perpendicular to the plane containing the 

 conductor and the lines forming the field. 



If the conductor is inclined at an angle 6 to the lines of 

 force, the pull = length x current x field strength x sin Q. 



When converted into more convenient units, the force is 

 given by the following equations : 



r 1 H C* = current in amps. 



/(kilogrammes) = >in nnn 1 = length of conductor in cm. 



H = lines per sq. cm. 



, ,,, . G. 1." H. \" in inches. 



72,904,350 H in lines per sq. cm. 



,,,, . (7. r. H". \" in inches. 



11,303,000 H* in lines per sq. inch. 



Production of an Alternating Electromotive Force. An 



electrical generator is a machine in which 'conductors are 

 made to cut lines of force in order that they may have an 

 electromotive force generated in them. The most convenient 

 way of causing conductors to cut lines of force is to 

 make either the conductors or the magnetic field rotate. 

 The conductors usually- pass alternately across poles of north 

 and south polarity, and in doing so the direction of the 

 electromotive force induced is reversed as the conductor 

 passes from pole to pole. Consequently, in both direct and 

 alternating generators the electromotive force produced in 

 the armature conductors (and also the current) is an alterna- 

 ting one. The difference between a direct and alternating 

 current generator lies in the method of collecting the current 

 from the armature conductors, rather than in the nature of 

 the currents induced. In the direct-current machine the 

 current is rectified at the commutator, while in the alternator 

 the current in the external circuit has the same direction and 

 variations as it has in the armature conductors. 



Nature of Alternating Electromotive Force. In order to 

 study the manner in which the alternating electromotive force 

 in a generator varies, we may take the case of a single con- 

 ductor travelling in a circular path at a uniform speed in a 

 uniform field between a pair of opposite poles. (See Fig. 2.) 



The electromotive force generated in the conductor is 

 numerically equal to the rate at which it cuts the lines of 

 force divided by 10 8 . Evidently this rate varies with the 

 position of the conductor. Referring to Fig. 2, the electro- 



