PRINCIPLES OF ALTERNATING CURRENT 199 



figure represents the exciting current of a transformer; 

 its shape indicates the presence of the third and fifth har- 

 monics. Fig. 119 shows the e.m.f. wave of an alternating 

 current generator which the manufacturers guaranteed 

 would give a sine wave; this shows how poorly a machine 

 may sometimes be designed. Certain types of alternating 

 current apparatus would not operate at all if supplied with 

 power from such an alternator; if this machine were used 

 in making tests upon a circuit in which condensers were 



E.M.F. wave with 

 eleventh harmonic 



FIG. 119. Reproduction of an e.m.f. Wave from an a-c. Generator, 

 Supposed to Give a Sine Curve. 



used, the results obtained would be very difficult to 

 interpret. 



45. Virtual * Values Form Factor. As the magnitude 

 of an alternating current varies from instant to instant, 

 assuming all values between its maximum positive and 

 maximum negative values, it is apparent that we must 

 establish some method of determining what is the virtual 

 numerical value of such a current in amperes. The equiv- 

 alence between the ampere of continuous current and the 

 ampere of alternating current is determined by the effect 



* The word virtual has recently^ been adopted by the A.I.E.E. to 

 signify the maximum value -4- -y/2. Previously the term ej 

 Value had been used in this sense. 



