THE MEASUREMENT OF CURRENT 49 



In many cases the damping is not an inherent property of the 

 galvanometer but depends both on the instrument and on the 

 circuit to which it is attached (see page 40). 



Resistance. The resistance should be appropriate for the 

 measurement in hand so that the maximum sensibility of method 

 may be obtained. 



Freedom from Effects of Mechanical Disturbances. Great 

 care should be exercised in making and mounting the movable 

 system so that symmetry about the axis of rotation is attained; 

 this contributes much to the stability of the system when it is 

 subjected to mechanical disturbances. Choice of location for 

 the instrument and the method of setting up are important. 



Freedom from Stray-field Effects. It is essential that the 

 indications be uninfluenced by the unavoidable variations of the 

 local field. 



Definiteness of Zero Reading. -The zero reading should bo 

 definite and the deflect imi- shmild come promptly to their final 

 values with no viscous action of the controlling spring. 



Law of Deflection. Throughout its useful range the deflection 

 as read from the scale should be proportional to the current. 



Visibility of Suspended Parts. When the instrument is set 

 up ami ready for use, it should be possible to see the movable 

 parts and to satisfy oneself that the clearances are properly 

 adjusted. 



Accessibility for Repairs. -It should be possible to take out 

 easily the entire movable system with its suspension. 



Temperature Effects. The effect of temperature on the sensi- 

 tivity should be small and inequalities of temperature should 

 not set up thermo-electric currents in the galvanometer circuit. 



Optical System. The definition obtained by the optical 

 em used in reading the deflections should be so perfect that 

 readings to the limit of accuracy of the instrument may be 

 obtained without undue fatigue. 



THE JULIUS DEVICE FOR ELIMINATING THE EFFECTS OF 

 MECHANICAL DISTURBANCES ON GALVANOMETERS 11 



The object to be attained is the suspension of the movable 

 in of the instrument from a point which is practically 

 stationary. 



