CHAPTER VI 

 ELEMENTARY PRINCIPLES OF ALTERNATING CURRENT 



44. Alternating Currents Wave Shape Frequency. 

 An alternating current is one which periodically changes its 

 direction of flow, assuming alternately positive and nega- 

 tive values. The current generally completes a period 

 (complete cycle of values) in a very short time, perhaps 

 one-fifteenth of a second or less. Also the sequence of 

 values assumed during the negative half of the cycle is 

 exactly similar to that of the positive half. In other words 

 if the instantaneous values of current are plotted with 

 time for the other co-ordinate, a wave is obtained, the two 

 halves of which are similar in a peculiar way. In Fig. 

 116 is shown one complete cycle of an alternating current; 

 if the loop B is moved back on the time axis to B', then 

 A and B' are symmetrical with respect to the time axis. 

 Practically all alternating current and voltage waves have 

 tiiis characteristic symmetry. 



Alternation-Frequency. One-half a cycle is called an 

 alternation; in Fig. 116 the time CE is called the period 

 of the alternating current and the number of periods passed 

 through by the current in one second is called the periodicity 

 or frequency. A current of a frequency of 60 cycles for 

 example, passes through 60 cycles per second; the frequency 

 is expressed sometimes, however, in alternations per minute 

 and we might say that this 60-cycle current had a frequency 

 of 7200 alternations per minute. 



Frequencies in Common Use. The frequencies in common 

 use for lighting and power purposes are 60 cycles and 25 



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