14 ELEMENTS OF ELECTRICAL ENGINEERING. 



brushes will be passing from one segment to the next at the in- 

 stants when the alternating current is passing through the zero 

 value. The sparking, however, is serious at best, and in order 

 to reduce this sparking, it is necessary to connect resistances 

 s and s r as shown. Sometimes one only of these resistances, 

 generally s f , is used. Thus, in the compound alternator which 

 is shown in Fig. 5, the brushes of the rectifier are connected to 

 the series field winding and also to a resistance which is mounted 

 inside of the bearing pedestal of the machine. 



The Tirrell field regulator. The complications involved in 

 the compounding of an alternator, as above explained, add to the 

 cost of the machine, the compounding cannot be adjusted to com- 

 pensate completely for variations of the terminal voltage, espe- 

 cially when the alternator supplies current to inductive circuits, and 

 the rectifying commutator becomes very troublesome when it is 

 attempted to use it to rectify the large current output of a large 

 alternator. It is therefore not usual to compound large alternators. 

 Voltage control of large alternators is usually accomplished by 

 a switchboard attendant, although an automatic regulator, known 

 as the Tirrell regulator,* is now coming into use, especially for 

 alternators of medium size. This regulator is essentially an auto- 

 matic field rheostat which is adjusted, not by varying the amount 

 of the resistance in circuit, but by repeatedly short-circuiting a 

 fixed resistance, the duration of each short-circuit being varied 

 automatically. Thus if a resistance of I o ohms is in circuit with 

 a field coil for one-third of the time, the effect is the same as if 

 31/3 ohms were in circuit continuously, provided that the 10 ohms 

 is short-circuited repeatedly during very short intervals of time. 

 3. Advantages and disadvantages of alternating current. The 

 electrical transmission of a given amount of power may be ac- 

 complished by a large current at low voltage qr by a small cur- 

 rent at high voltage. In the first case very large and expensive 

 transmission wires must be used or the loss of power in the trans- 



* See General Electric Company's Bulletin, No. 4,351, July, 1906, for a detailed 

 description of the Tirrell regulator. 



