1891.] 



on Electric and Magnetic Screening. 



347 



The fixing of an electric image is shown in two experiments : 

 (1) the image of a stick of sealing-wax in a thin plane sheet of 

 vulcanite, moistened, warmed, and dried under the electric influence 

 by the application and removal of a spirit-lamp flame ; (2) the glass 

 jar of a quadrant electrometer with a rubbed stick of sealing-wax held 

 projecting into it, while the outer surface is moistened, warmed, and 

 dried by the application and removal of a ring of flame produced by 

 cotton wick wrapped on an iron ring and moistened in alcohol. 



Fig. 5 is copied from a diagram of Clerk Maxwell's to illustrate 

 screening by a plane grating of parallel bars of approximately circular 



Fig. 5. 



cross section, with distance from centre to centre twelve times the 

 diameter of each bar.* It represents the lines of force due to equal 

 quantities of opposite electricities on the grating itself, and a parallel 

 plane of continuous metal (not shown in the diagram) at a distance 

 from the grating of not less than one and a half times the distance 

 from bar to bar. The shading shows the lines of force for the same 

 circumstances, but with oval bars instead of the small circular bars of 

 Maxwell's grating. It is interesting to see how every line of force 

 ends in a bar of the grating, none straying to an iu finite distance 

 beyond it, which is necessarily the case when the quantities of elec- 

 tricity on the gratiug and on the continuous plane are equal and 

 opposite. If an insulated electrified body, with electricity of the same 

 name as that of the grating, for example, is brought up from below, 

 it experiences no electric force differing sensibly from that which 

 would be produced by its own inductive effect on the grating, till it is 

 within a less distance from the grating than the distance from bar to 

 bar, when it experiences rejjulsion or attraction, according as it is 

 under a bar of the grating or under the middle of a space between 

 two bars. If there be a parallel metal plane below the grating, kept 



* 'Electricity and Magnetism,' vol. 1. art. 203, fig. xiii. 



