568 CURTIS: a vibration electrometer 



alternating currents or voltages. One of the first instruments 

 used for this purpose was the telephone. This is very sensitive 

 between the frequencies of 500 and 3000 cycles per second, but 

 at frequencies below 500 cycles the sensitiveness decreases 

 rapidly with the frequency, so that it is very insensitive at fre- 

 quencies below 100 cycles. It also responds to the harmonics 

 of the current as readily as to the fundamental. 



As a null instrument, a vibration galvanometer is often much 

 more satisfactory than a telephone. The moving system of a 

 vibrating galvanometer is adjusted to have the same period as 

 that of the current to be detected, so that any harmonics in the 

 current produce very little effect upon the deflection of the 

 instrument. Also most vibration galvanometers have their 

 maximum sensitiveness at low frequencies (50 to 200 cycles), 

 though at least one form may be had which will go to frequencies 

 as high as 3000 cycles. Since the impedance of these instru- 

 ments is relatively low, they require an appreciable current to 

 produce a deflection which can be observed. Hence in bridges 

 where the impedance of the arms is very high, they are not very 

 sensitive. 



The vibration electrometer described in this paper was designed 

 as a vibrating instrument having an impedance much higher 

 than a telephone or vibration galvanometer. The need arose 

 in connection with the measurement of some very low capacities 

 at low frequencies. Its usefulness is limited to those cases where 

 it is desired to detect very small currents at low frequencies. 

 Its principal use is as a detecting instrument in a bridge having 

 very high impedances in the arms. 



The instrument is a modification of a quadrant electrometer. 

 Instead of the quadrants there are four vertical plates, while a 

 thin vertical vane of twice the area of a single plate corresponds 

 to the needle of the electrometer. Two plates, separated by a 

 narrow vertical slit, are in one plane, while opposite them in a 

 parallel plane are the other plates. Midway between the planes 

 is the aluminum vane, which is suspended by a bifilar suspension. 

 This vane is maintained at constant potential by a battery, 

 while an alternating voltage having the same period as the natural 

 period of the vibrating system is applied to the plates. This 



