1891.] Sir W. Thomson on Electric and Magnetic Screening. 345 



WEEKLY EVENING MEETING, 



Friday, April 10, 1891. 



William Crookes, Esq. F.E.S. Vice-President, in the Chair. 



Sib William Thomson, D.C.L. LL.D. Pres. R.S. M.B.I, 



Electric and Magnetic Screening. 



There are five kinds of screening against electric and magnetic 

 influences, which are quite distinct in our primary knowledge of 

 them, but which must all be seen in connected relation with one 

 another when we know more of electricity than we know at present : 

 — I. Electrostatic screening ; II. Magnetostatic screening ; III. Varia- 

 tional screening against electromotive force ; IV. Variational screening 

 against magnetomotive force ; V. Fire-screens and window-blinds or 

 shutters. 



I. 



Electrostatic screening is of fundamental significance throughout 

 electric theory. It has also an important place in the history of 

 Natural Philosophy, inasmuch as consideration of it led Faraday 

 from Snow Harris's crudely approximate but most interestingly sug- 

 gestive doctrine of non-influence of unopposed parts and action in 

 parallel straight lines betv\'een the mutually visible parts of mutually 

 attracting conductors, to his own splendid theory of inductive attrac- 

 tion transmitted along curved lines of force by specific action in and 

 of the medium intervening between the conductors. 



A continuous metallic surface completely separating enclosed air 

 from the air surrounding it acts as a perfect screen against all electro- 

 static influence between electrified bodies in the portions of air so 

 sej)arated. This proposition, which had been established as a theorem 

 of the mathematical theory of electricity by Green, in the ninth 

 article of his now celebrated essaj^,* was admirably illustrated by 

 Faraday, by the observations which ^ he made inside the wooden 

 cube covered all around with wire netting and bands of tinfoil, which 

 he insulated within this lecture-room ij *' I went into the cube and 

 lived in it ; and, using lighted candles, electrometers, and all other 

 tests of electrical states, I could not find the least influence upon 

 them, or indication of anything particular given by them, though all 

 the time the outside of the cube was powerfully charged, and large 

 sparks and brushes were darting off from every point of its outer 

 surlace." 



* See pp. 14 and 48 of the reprint edited by Ferrers, 

 t 'Experimental Kescarclics,' 1173-1174. 



