x CLASSIFICATION AND NOTATION. 



II. SYNCHRONOUS MACHINES, which comprise a constant mag- 

 netic field, and an armature receiving or delivering alternating 

 currents in synchronism with the motion of the machine; i. c., 

 having a frequency equal to the product of the number of pairs 

 of poles and the speed of the machine in revolutions per 

 second. 



III. SYNCHRONOUS COMMUTATING MACHINES : These in- 

 clude : (i) Synchronous converters, commonly called "con- 

 verters"; i. e. t converters from alternating to direct, or from 

 direct to alternating current, and (2) Double-current generators ; 

 i. e. t generators producing both direct and alternating currents. 



A converter is a machine employing mechanical momentum in 

 changing electric energy from one form into another. 

 A converter may be either : 



a. A direct-current converter, converting from a direct current 

 to a direct current, or 



b. A synchronous converter, formerly called a rotary con- 

 verter, converting from an alternating to a direct current or vice 

 versa. 



Phase converters are converters from an alternating-current 

 system to an alternating-current system of the same frequency, 

 but in different phase. 



Frequency converters are converters from an alternating-cur- 

 rent system of one frequency to an alternating-current system of 

 another frequency, with or without change in the number of 

 phases. 



IV. RECTIFYING MACHINES, OR PULSATING- CURRENT GENERA- 

 TORS, which produce a unidirectional current of periodically vary- 

 ing strength. 



V. STATIONARY INDUCTION APPARATUS, i. e., stationary appa- 

 ratus changing electric energy to electric energy through the 

 medium of magnetic energy. These comprise : 



a. Transformers, or stationary induction apparatus in which 

 the primary and secondary windings are electrically insulated 

 from each other. 



