ELECTROMOTIVE FORCE AND CURRENT. 7 



the velocity of the conductor in a direction at right angles 

 to P, N will represent the vertical velocity, and N P will 

 represent its horizontal velocity in magnitude. This will be 

 true whatever position the conductor P may be in. 



If it is known that when the conductor is moving 

 vertically the electromotive force induced in it is V volts, 

 then the voltage induced when the conductor is moving in 

 any other direction, such as the direction P L (Fig. 2), is less 

 than V, since the vertical velocity of the conductor is 

 less, and its actual value will be given by 

 , _ y vertical velocity at time considered 



vertical velocity when conductor moves vertically 



= V x 0-p = V sin 0, 



if is the angle P Q through which the conductor has 

 moved from the vertical axis. 



Thus the variations of the induced electromotive force 

 are the same as those of the sine of an angle which passes 

 through all values from to 360. 



The values of the electromotive force may be readily 

 plotted graphically from Fig. 2, since the length of N may, 

 always be taken to represent the electromotive force generated 

 on the same scale of volts upon which the radius P repre- 

 sents the maximum voltage. This has been done in Fig. 4, 

 where the path of the conductor P is indicated by a circle, and 

 for each value of the angle Q P, measured horizontally in 

 degrees, the length N is set up vertically to represent the 

 value of the electromotive force for that position of the 

 conductor, a curve being drawn through the points thus 

 obtained. 



It will be seen that the curve obtained in this way is a 

 sine curve. The change of direction of the electromotive 

 force, which occurs when the conductor begins to cut the 

 lines in the reverse direction, is shown by the curve crossing 

 the zero line. Ordinates above the line indicate electro- 

 motive forces in one direction along the conductor. Ordinates 

 below the line represent those in an opposite direction. For 

 the sake of uniformity it is usual to call electromotive forces 

 measured below the line Negative. 



The curve shown in Fig. 4 may be taken to represent the 

 variations of voltage given by an alternator. For example, 

 the maximum voltage on the vertical scale of the diagram is 25. 

 It would be possible to obtain from the curve the exact 



