THE MEASUREMENT OF CURRENT 



9 



open at the ends and the diameter is made small compared with 

 the length; for instance, if the internal diameter is 2 or 3 cm., the 

 length may very well be about 30 cm. See Figs. 7 and 10. 



The shielding ratio, that is, the ratio of the strength of the ex- 

 ternal local field to the corresponding field within the inner shield 

 is greatly affected by the arrangement of the iron. If the iron is 

 all concentrated in a single cylindrical shield, having an outside 

 radius five times the inner radius, the shielding ratio will be about 

 98 per cent, of that for a shield of infinite thickness. If a single 

 shield of permeability 202 be used, the maximum possible shield- 

 ing ratio is about 50. This shows the futility of trying to thor- 

 oughly protect a galvanometer by a single shield of great weight. 



If a given weight of iron is used in several concentric shields, 

 with air spaces between them, its effectiveness is vastly increased. 



Given the innermost and outermost radii of a system of three 

 shields, the shielding ratio is a maximum when the radii of the 

 shells are in geometrical progression. 



Cyclic annealing of the shields at high temperatures materially 

 increases their effectiveness through an increase of the perme- 

 ability. The following experimentally determined values give an 

 idea of the protection afforded by a multiple shield. No joints 

 or lateral openings are allowable. 



DATA FOR THREE SHIELDS OF CAST SILICON STEEL* 

 Length 29.3 cm. 



SHIELDING RATIOS OBTAINED BY USING THE ABOVE 



Shielding ratios 



