THE MEASUREMENT OF CURRENT 21 



The deflection is 



200L/ G G 

 ~H~ 



D 200G _KMG 2 _ K'MVfcTJ 

 ' ' S ' ~ IJ. = H ~P -p- 



Therefore, with any given magnetic system, the sensitivity is 

 proportional to the square of the time of vibration. If this be 

 doubled by changing H, the sensitivity will be increased fourfold, 

 for to double the time of vibration the directive force must be 

 reduced to one-fourth of its previous value, so the deflection due 

 to the same value of the current is quadrupled. 



If the needle system is very light the damping due to air fric- 

 tion will be so great that there will be a considerable departure 

 from the above relation. The sensitivity of modern research 

 galvanometers with very delicate suspended systems is more 

 nearly proportional to the first than to the second power of the 

 time of swing. Instruments with exceedingly light systems are 

 sometimes operated in vacuo. 



In comparing instruments, the sensitivity must be reduced 

 to the value it would have if the movable system had some definite 

 time of vibration: 10 sec. for a complete swing is that commonly 

 taken. 



Normal Sensitivity. It is unfair to compare instruments which 

 have very different resistances and different times of vibration. 

 It is desirable to reduce the sensitivities to the values they would 

 have with the galvanometers wound to a standard resistance, 1 

 ohm, with a wire having an insulation of zero thickness, and 

 with a needle system whose time of vibration is 10 sec. From the 

 previous discussion the normal current sensitivity is 



D 1Q y 



~ - * - 



Volt Sensitivity. Microvolt Sensitivity. The volt sensitivity 

 under any given conditions is the deflection per unit voltage and 

 is consequently the current sensitivity divided by the resistance. 

 When dealing with moving coil galvanometers (page 38) the 

 condition is frequently imposed that the resistance of the circuit 

 shall be such that the galvanometer is critically damped. 



