548 ELECTRICAL MEASUREMENTS 



these modern instruments there is a bank of steel reeds so tuned 

 that the natural periods of successive reeds differ by one alterna- 

 tion. This bank of reeds is acted upon by an electromagnet 

 traversed by current taken from the line. Only the reeds very 

 nearly in tune with the frequency of the circuit respond visibly, 

 the reed most nearly in tune showing the maximum amplitude. 

 If it is exactly in tune, the amplitude is very large. This is well 

 illustrated by Fig. 332, which shows the amplitude of vibration 

 of a reed tuned to a frequency of 90 alternations when currents 



Frequency 



90 89.5 89 88.5 88 87.5 87 



FIG. 332. Showing effect of frequency on the amplitude of vibration of a 

 reed in a frequency meter. 



of various frequencies are sent through the magnet. A varia- 

 tion from 90 to 89.5 alternations reduces the amplitude over 50 

 per cent, (compare with the vibration galvanometer, page 434). 



In order to insure reliability and long life the butts of the reeds 

 must be firmly held. The arrangement adopted by the firm of 

 Hartmann and Braun and one form of the complete instrument 

 are shown in Fig. 333. 



If the reeds are unpolarized they will be drawn toward the 

 magnet at each alternation, so that the reed having twice the 

 frequency of the current responds. If they are polarized, by 

 either permanent or electro-magnets, the reed having the same 

 frequency as the circuit will have the maximum amplitude of 

 vibration. 



The reeds are approximately tuned by making them of dif- 

 ferent lengths, the final tuning being effected by altering their 

 weights by filing away drops of solder which are at their outer 



