68 



ALTERNATING CURRENTS 



The induction watthour meter has certain advantages over the 

 direct-current meter. As there is no coil-wound armature in 

 addition to the disc, the rotating element of the induction meter 

 is much lighter than that of the direct-current meter. Moreover, 

 it has no commutator or delicate brushes, which are frequent 

 sources of trouble with the direct-current meter. 



The induction meter is also made in the polyphase type. Two 

 single-phase elements act on a common spindle. There are two 

 sets of damping magnets. (For a more detailed analysis, see 

 "Electrical Measurements" by F. A. Laws.) 



38. Frequency Indicators. Frequency indicators are based 

 on two principles, that of electrical resonance and that of 



45 



50 55 



1,1,1,1,1,1 



ilifilllHIH 



FIG. 65. Frahm vibrating-reed frequency meter. 



mechanical resonance. The latter type is the more common and 

 is simpler in operation. A number of steel reeds, each having a 

 white index on its end, are clamped between two metal strips. 

 Each reed has its own mechanical frequency of vibration. Be- 

 hind this bank of reeds there is an electromagnet, the coil of 

 which is excited by the circuit whose frequency it is desired to 

 measure. The reed whose frequency is that of the circuit will 

 vibrate with the greatest amplitude, Fig. 65. With the excep- 

 tion of one or two reeds near this one, none of the others will be 

 affected. Therefore the frequency is determined by noting the 

 scale reading opposite this reed. Were the reeds unpolarized, 

 they would be attracted equally well by either a north or a 



