8 ALTERNATING CURRENTS 



4. Commercial Frequencies. In this country, frequencies are 

 standardized at 60 cycles and at 25 cycles per second, although 

 other frequencies are used. In California and in Mexico, for 

 example, 50 cycles is used on some of the large transmission 

 systems. In the early days of alternating-current development, 

 133 cycles was common, but few, if any, plants now generate at 

 this frequency. The principal advantage of higher frequencies 

 is that transformers require less iron and copper, and so are lighter 

 and cheaper. The flicker of lamps is not perceptible at 60 cycles, 

 but at 25 cycles it is very evident. On the other hand, the 

 voltage drop in transmission lines and in apparatus varies almost 

 directly as the frequency, so that better voltage regulation 

 throughout the system is obtained with low frequency. Power 

 apparatus, such as induction motors, synchronous converters, 

 alternating-current commutator motors, etc., operates better at 

 low than at high frequencies. However, with one or two ex- 

 ceptions, the operation is satisfactory at 60 cycles per second. 

 A power and lighting company would ordinarily operate at 60 

 cycles per second, because the flicker of lamps at 25 cycles per 

 second is objectionable and the transformers at this lower frequency 

 are heavier and more costly than they are at the higher frequency. 

 On the other hand, an electric company generating strictly for 

 power purposes would ordinarily use 25 cycles. This frequency 

 is used by the New York, New Haven and Hartford R. R. for 

 its electric locomotives; on the Norfolk and Western Ry. for 

 operating electric locomotives; and by the Boston Elevated Ry. 

 Co. for transmitting high-voltage power to its direct-current 

 sub-stations. In Europe, frequencies as low as 15 and even 12.5 

 cycles per second are common. 



6. The Alternating-current Ampere. Figure 7 (a) shows an 

 alternating-current sine wave, having a maximum value of 1.414 

 amp. At first thought it might seem that the value in amperes 

 of such a wave should be based on the average value. If the 

 wave over one complete cycle is considered, the average value is 

 zero, as there is just as much negative as positive current. A 

 direct-current ammeter, if connected to measure this current, 

 would indicate zero, as such an instrument reads average values. 



The value of an alternating current is not based on its aver- 

 age value but on its heating effect and may be defined as follows: 



