436 ELECTRICAL MEASUREMENTS 



effective length of which may be adjusted by turning the milled 

 head, A, at the upper end of the vertical screw, thus securing a 

 coarse adjustment. The fine adjustment is made by turning 

 the milled head, B, which, by means of the spring, S, controls 

 the tension on the suspension. 



On account of the large restoring moment which must be em- 

 ployed to obtain a high rate of vibration, the sensitivity of a 

 vibration galvanometer for direct currents is very small. It is 

 only when the period of the galvanometer and that of the cur- 

 rent coincide that the current sensitivity rises to a high value. 

 This is well illustrated by Fig. 255A. From the figure it is clear 

 that if the sensitivity is to .be maintained, the frequency of the 

 current must be constant; in the case shown a change of 0.2 per 

 cent, in the frequency reduces the current sensitivity by about 

 70 per cent. 



The characteristic of responding freely to only one frequency 

 permits many measurements to be made with non-sinusoidal 

 currents, provided the harmonics are not so pronounced that the)' 

 " force" the vibration of the movable system. In one very good 

 commercial form of vibration galvanometer the sensitivity for 

 the third harmonic is only 34>ooo> an< 3 for the fifth harmonic only 

 K 2>ooo of that for the fundamental. This selective sensitivity 

 is one reason why, within the range where they are both effective, 

 the vibration galvanometer is superior to the telephone as a 

 detector, unless the telephone is tuned to the frequency of the 

 current. 



As current of constant frequency is essential, it is not always 

 possible to use commercial electric circuits as sources of power 

 in those alternating-current measurements where the vibration 

 galvanometer is employed. 



Current Sensitivity. 31 The relation between current and 

 deflection for the vibration galvanometer is obtained by solving 

 equation 9, page 25, in which i = I N sin (Nu)t. JVco is N2nr times 

 the fundamental frequency of the current; for the fundamental 

 N = 1, for the third harmonic N = 3, etc. 



The equation according to which the vibration takes place is 



P + * + rB = ClN Sin Ww) * 



