ELECTRICITY METERS 



519 



that the movable system may be enabled to make three complete 

 revolutions without complications arising from the spring being 

 twisted too tightly. 



The damping disc D 2 is of copper and rotates between two 

 sets of magnets M and M f . Each magnet is adjustable vertic- 

 ally, so that the strengths of the magnetic fields through which the 

 disc moves may be varied. In this manner the strengths of the 

 currents induced in the disc when it turns, and consequently the 

 retardation experienced by it, may be altered and the rapidity 

 with which the instrument responds to changes of load adjusted. 

 It is intended that the magnets be so set that with a constant 

 load, 90 per cent, of the registration is produced in 5 consecutive 

 minutes. 



The hand HI is driven from the spindle by a system of gear- 

 ing, and moves over a dial graduated in kilowatts. As it moves 



10 15 



Time in Minutes 



FIG. 300. Characteristic of General Electric type W demand indicator. 



it pushes before it the hand H 2 , which is loose on the shaft and 

 provided with a ratchet R and a light spring ', which tends to 

 turn the hand back against the ratchet. The result is that H 2 

 is pushed up to the maximum by HI and left there, when, owing 

 to the decrease of the load, Hi returns toward zero. The instru- 

 ment may be set by opening the case and raising the ratchet 

 which allows S' to return the pointer H 2 to zero, H 1 being pre- 

 viously turned back to that point. It will be seen that this de- 

 vice gives the power which is being used at any time, as well as 



