22 ELECTRICAL MEASUREMENTS 



Megohm Sensitivity. The megohm sensitivity is the number 

 of megohms which must be inserted in a circuit containing an 

 e m f of 1 volt in order to obtain a galvanometer deflection of 

 1 mm., at a meter's scale distance. This is the same, numerically, 

 as the microampere sensitivity, referred to on page 20. 



Design for a Reflecting Galvanometer. 5 A modern design 

 for a reflecting galvanometer with magnetic shielding is shown 

 in Fig. 10. 





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FIG. 10. Shielded Thomson galvanometer. 



The support for the coils and the suspended system is shown 

 at the left hand of the figure. It consists of a brass rod, 3.8 cm. 

 in diameter. This is divided longitudinally along the axis and 

 the two parts are hinged together at de. A groove ran is milled 

 lengthwise in both halves and serves to accommodate the sus- 

 pended system. When the instrument is ready for use, the 

 space between the coils is about 2J^ mm., just sufficient to allow 

 the needle system to turn around. There are small holes along 

 the axis of the coils, denoted by I and transverse saw cuts in 

 the uprights, denoted by s; by this means the needle system can 

 be observed when the coils are in their proper position. 



