44 ELECTRICAL MEASUREMENTS 



iC 2 \ 2 

 As the relation -- = 4Pr, which is necessary for critical 



damping, is to be preserved, two changes will be required, the 

 second to compensate for the effect of the first on the damp- 

 ing. 



For example, suppose that it is desirable to increase the total 

 resistance, R, of the circuit N times. If this is done, the instru- 

 ment will become under-damped and the voltage sensitivity 

 will be reduced N times. To restore the damping a damping loop 

 may be added. This will increase T Q somewhat, due to the 

 increased moment of inertia. As indicated by the equations, 

 other compensating changes are possible. 



The Einthoven String Galvanometer 9 . In this instrument 

 the movable element is either a very fine silvered quartz fiber 



FIG. 17. -Einthoven string galvanometer. 



about 8 or 10 cm. long, or a very fine wire. This is placed in the 

 strong field due to an electromagnet and so mounted that the 

 tension upon it may be varied. 



When a current is sent through the fiber, it moves across the 

 magnetic field. The motion is observed with a microscope having 

 a micrometer eyepiece, a magnification of about 100 diameters 

 being used. The deflections are proportional to the current. 



Fig. 17 shows the essential features and one design of the 

 complete instrument. 



