486 DYNAMO FOR ARC LAMPS [Cn. XIII 



676. Cycle. With alternating current where the current 

 flows first in one direction and then in another with a change in 

 polarity for each reversal, a cycle includes a change in polarity to 

 the opposite, and back to the starting point. That is, a cycle 

 includes flow in two directions and consequently includes two 

 polarities ; and this is repeated over and over again. 



677. Frequency. The number of cycles per second with an 

 alternating current is called its frequency. The frequencies in 

 most common use are: 25 cycles, 60 cycles and 135 cycles per 

 second. The 60 cycle frequency is most generally used for lighting 

 circuits and the 25 cycle frequency is mostly employed for long 

 distance transmission, and frequently for motors. The 130 or 135 

 cycle frequency is now uncommon. 



SPECIAL DYNAMO FOR ARC LAMPS 



678. The characteristics of the arc are that the potential 

 difference between the electrodes is dependent upon the arc length 

 but not upon the current (see 743). It is required to supply this 

 arc with a constant current regardless of the differences in arc 

 length. This may be done with a constant potential supply by 

 using a rheostat in series with the arc, or it may be done by using a 

 constant current generator. Since the early days of arc lighting, 

 street arcs have been connected in series and are supplied by a 

 direct current dynamo of this type, no resistance being used. These 

 dynamos have an automatic controlling device which increases the 

 voltage when the current falls slightly below the rated value (6.6 

 amperes) and which decreases the voltage should the current rise 

 slightly above this value. For street lighting this regulation must 

 be very close, but for projection purposes the regulation need be 

 only approximate. There are some types of dynamos which have 

 the proper characteristics to be connected directly to an arc lamp 

 without intervening resistance. The characteristics of such a 

 dynamo must be that a slight momentary increase in current 

 caused by a lowering in the potential difference at the arc will be 

 met by a decrease in the voltage generated, and conversely a 



