398 OPTICAL PROJECTION 



apparatus is in fact too sensitive for most lecture purposes; 

 but diminishing this by using shorter fibres, and adjusting a 

 hori.-.onlal strip of glass behind the ball. di\ ided as a scale, 

 many useful quantitative experiments may be made ill a very 

 simple ai nanner. 



The mere fact that, the density of a charge is greater at 

 the ends of a conductor than in the middle may bo shown by 

 Prof. Weinhold's very simple apparatus (tig. k J17). Four 



pieces Of paste-board with rounded sides and edges are couivd 

 with tinfoil, and fastened into a box like shape b\ strips of 

 ribbon also covered with foil, or by thick foil itself. To tho 

 middle of one angle are suspended by conducting threads a 

 pan of small pith balls ; and close to the middle of the oppo- 

 site angle are fastened two slicks of sealing' wax as insulating' 

 handles. The whole apparatus is shown at. A. If both 

 handles are so held thai the plates take the form of i:. and t lie 

 apparatus is gi\en a small charge, a little di\ergeitce of the 

 pith-balls will be observed; but if now tho handles be so 



turned that the plates take the form of c, the divergence will 

 be increased ver\ considerably. The apparatus max be made 

 quite small, but is more effective if the pieces are about six 

 inches square, when it can bo projected by the shadow method 

 ( 109). 



L 



Fiu. 218 



288. Static Electrical Stress. Dr. Kerr's experiments 

 showing by optical methods the state of stress produced by 

 electrical charge in di electrics, are easih projected, tig. '218 

 b, in-: a .-vneral sketch of the arrangements. Tho rays from 

 the lantern pass through a polarising apparatus, M, and then 

 throu.-rh a glass cell, P. In this cell are arramr> d t\\o copper 

 plates a very small distance apart, say from ^ to J inch, which 



