PHASE METERS 549 



ends just behind the white indexes. In the instrument of this 

 class made by Siemens and Halske, all the reeds are fixed to a 

 single metallic bar which is so mounted on a spring support that 

 it may be gently vibrated by the action of an electromagnet 

 excited from the circuit. The particular reed which is in tune 

 with the circuit responds with the maximum amplitude. 



FIG. 333. Hartmann and Braun frequency meter. 



These frequency meters may be used as speed indicators. 

 In this case, a toothed wheel or some other form of rotary key 

 is attached to the shaft of the machine and serves to interrupt 

 a direct current which is sent through the meter. Knowing the 

 number of contacts per revolution of the shaft and the number of 

 impulses as read from the scale, the revolutions may be calcu- 

 lated. 



The Siemens and Halske arrangement in a modified form is used 

 as a revolution indicator for steam engines and other machinery. 



