20 ELECTRICAL MEASUREMENTS 



and winding of the coils, on the construction of the suspended 

 magnetic system, on the strength and position of the neutralizing 

 magnets which produce the controlling field, on the torsional 

 constant of the suspension fiber, and on its initial torsion. This 

 last must be removed when the galvanometer is set up, as its 

 presence necessitates a strong controlling field to bring the 

 needle to its zero position. 



Ampere Sensitivity. Microampere Sensitivity. Quantita- 

 tively, the current sensitivity of a galvanometer is the deflection, 

 as read from the scale, per unit current. This simple statement is 

 not sufficiently definite, so in order to obtain results that admit of 

 comparisons being made between different instruments, some con- 

 vention must be adopted as to the conditions under which the 

 sensitivity is to be measured. 



It will be assumed that one of the mirror and scale methods of 

 reading is used. The current, 7 G , will be stated in amperes, the 

 scale deflection D, in millimeters, and the scale distance L, in 



meters (1,000 scale divisions). Then, S f = y-y- is the deflection 



in millimeters which would be produced by unit current if the 

 scale had been at a meter's distance. S f will be a very large 

 number; consequently, for convenience in writing, the micro- 

 ampere (10~ 6 ampere) is frequently used as the unit of current. 

 This gives the microampere sensitivity. 



Relation between Time of Vibration and Current Sensitivity. 

 By common consent the sensitivity is measured when the sus- 

 pended system has a stated time of vibration. 



The connection of the sensitivity with the time of vibration of 

 the needle system will be seen from the following. If the damp- 

 ing due to air friction be neglected, the time of vibration, T of 

 a suspended magnetic system, having a moment of intertia P, 

 and a magnetic moment M } when placed in a field of strength H, 

 will be 



The current through the instrument is given nearly enough by 



/ - 1QH D 

 G G 2000L 



