10 



ELECTRICAL MEASUREMENTS 



A set of five shields cut from ordinary soft iron water pipes, 

 and annealed, gave the results quoted below. 



DATA FOR FIVE SHIELDS OF SOFT IRON PIPE 

 Length of shields 30 cm. 



SHIELDING RATIOS OBTAINED BY USING THE ABOVE 



Shields used 



Shielding ratios 



1 



1+2 



1+2+3 



1+2+3+4 



1+2+3+4+5 



19.3 

 104.1 

 252.0 

 723.1 



2,724.8 



These figures show that a Thomson galvanometer can be ren- 

 dered practically immune from the effects of local field variation 

 by the use of multiple shields with air spaces. 



A commercial form of galvanometer in which spherical shields 

 are employed is shown in Fig. 7. 



A simple and effective instrument, the one from which the 

 data concerning the shields were taken, is shown in Fig 10 on 

 page 22. 



Under present-day conditions it is useless to attempt to employ 

 an unshielded Thomson galvanometer. 



Suspensions. In Thomson's original instrument, the needle 

 system was suspended by a single silk fiber. This does well 

 enough for instruments of moderate sensitivity. When the high- 

 sensitivity is desired, the controlling field is reduced to a 

 unimum. The torsional properties of the suspension fiber then 

 ome important. Silk fiber suspensions are exceedingly trouble- 

 for they are greatly affected by changes of hygrometric con- 



