96 Prof. A. W. Riicker on the Magnetic 



he lias protected the absolute magnetic instruments at Green- 

 wich from the disturbances due to a dynamo. 



In that case external space was protected against magnetic 

 forces produced inside the enclosure. 



The converse arrangement is sometimes adopted, and, as 

 in Lord Kelvin's Marine Galvanometer, instruments are 

 enclosed in iron and thus protected against external mag- 

 netic influence. 



In the particular case when the iron shields are spherical 

 shells, the method of attacking the mathematical problem 

 involved is well known. Maxwell gives the working in the 

 case of an internal space protected by a single shell (Ed. 3, 

 vol. ii. p. 59), and also discusses the precisely analogous 

 problem of the flow of currents interrupted by shells of dif- 

 ferent conductivity from the rest. 



I believe, however, that a number of facts, which are of some 

 practical importance, are not generally known, and I propose 

 therefore to deal with them in the following paper, illustrating 

 them by numerical examples. 



In the earlier part of the paper the general theory is 

 developed, chiefly for the sake of defining the notation used, 

 though this course involves some recapitulation from Maxwell. 

 The discussion of the best conditions for shielding, with 

 which I mainly deal, is, I believe, new. 



The principal point to which attention is directed is the 

 calculation of the advantage gained by lamination, i. e. by 

 using shields separated by air-gaps, like those employed by 

 Mr. Christie, instead of a continuous mass of iron. 



That lamination would be useful might be foreseen by the 

 aid of the hydrodynamical analogue to the magnetic field. 



If we suppose sources and sinks of equal power to be 

 placed within an enclosure, we may weaken the flow outside, 

 either by making the enclosure or part of it very permeable, 

 or by making the bounding surface very impermeable. 



In like manner in the magnetic problem there are two ways 

 of reducing the internal field, viz. by enclosing the central 

 region by shells of greater, or by shells of less, permeability 

 than the surrounding space. Although we cannot in practice 

 use any material of less permeability than the air, we can 

 introduce into an iron shell layers of material less permeable 

 than itself. These may tend to reduce the external field by 

 preventing as it were the force from penetrating to the 

 external shell. 



This analogy has long suggested that laminated shells, in 

 which both the permeability of the iron and the relative 

 impermeability of other materials are utilized, may be the 



