426 



ELECTRICAL MEASUREMENTS 



control the speed of the electric motor which drives the contact- 

 making device or the alternator which furnishes the current. It 

 is presupposed that the voltage of the supply is kept constant. 

 Hand regulation may be employed if some very sensitive form of 

 detector be used which will at once make evident a change of 

 speed, but in careful work this necessitates an additional observer 

 whose only duty is to note the speed indicator and keep the con- 

 trol rheostat adjusted. Therefore, some arrangement which 

 will be automatic in its action is required. 



The Giebe-Helmholtz Speed Regulator. 29 A shunt-wound 

 direct-current motor may be controlled by a form of centrifugal 

 governor suggested by Helmholtz and developed by Giebe. 



Supply 

 FIG. 250. Giebe speed regulator. 



Fig. 250 shows the diagram of the circuits and a general view 

 of the governor. The resistance X is for the rough adjustment 

 of the speed. 



When the speed rises too high, the governor short-circuits a 

 resistance in the field circuit and thus decreases the speed. 



Referring to Fig. 251, the frame D carries the mechanism and is 

 rigidly attached to the shaft R which is directly connected to the 

 motor. The electrical connections to the contacts KI and K z 

 are made through the slip rings V. The weight P slides on a 



