44 



THE PRODUCTION AND USE OF LIGHT 



right till the compartments B and B are quite small and the 

 compartments A and A are quite large, the valves are shifted to 

 the left so that the gas then enters the compartments B and B 

 and escapes from A and A. The two are so attached that one 

 shifts one-fourth of a complete vibration before the other does. 



FIG. 24. A dry gas meter, the front 

 and top casings removed. 



FIG. 25. Ideal diagram showing the 

 operation of the gas meter. 



The gas pressure is then exerted upon the right side of the 

 disks and they move to the left whenever gas is being used. 

 Thus, so long as gas is being consumed the disks keep mov- 

 ing back and forth. This to-and-fro movement of the disks 

 operates the valves and the dial pointers. 



Exercise 19. To Study 'the Construction and Operation of a Gas Meter 



Stand at one end of a meter from which the casing has been removed 

 as shown in Fig. 24. l Grasp the two flags, one in each hand. By 

 pressing the disks to right and left alternately you will soon learn so 

 to operate them as to cause the crank always to revolve in the right 

 direction. Watch carefully the movement of the valves and the 

 pointers. 



1 A worn-out meter can usually be obtained from the local gas company. 



