\GNETISM AXD F.LF.CTRTCITY 399 



can bo charged -f and by connecting with tho coats of 

 aLeydon jar. or the terminals of a Wiinshurst machine. The 

 Space between tlie platert is focussod on I lie sereen by tlie lens 

 D, the rays passing thro. analysing Niool, N. If 



tho plates arc vertical or horizontal, the Nicola are aet at an 

 i >ut crossed to gi\e (lie dark field. The coll is 

 best filled with carbon disulphido. On charging the plates, 

 the Held at once becomes bright, and if the charge id increased 

 splendid colour-effects are produced, as with glass in a state 

 of mechanical stress described in '209. 



With charges HuflicienHy powerful, similar stresa can be 

 shown in glass itself, pierced so that Iho terminals may oomo 

 within a small distance. 



At o may bo introduced a compensator of quartz to 

 measure the optical elTcct of charges, thomsolvea measured by 

 any sullicicnlly accurate electromotor. 



ii,T.. Static Discharge. The sparks from ajar or battery, 

 or Winishurst machine, are beat projected by a little apparatus 

 arranged for the 

 fit-Id of the lan- 

 tern, and which 

 needs no descrip- 

 tion beyond tlie 

 diagram in fig. 

 '21!), from Mr. G. 

 M. Hopkins. 1 The 



halls being adjusted for striking distance, arc focussod on tbe 

 screen, and the light then reduced to as low a point as will 

 just barely make tJiem visibk. Tho place of tho image of 

 the discharge spark upon the screen is thus localised, which 

 in this case makes just nil the dillcrem-e. in a scroon demoli- 

 sh ;it mu, and renders it very satisfactory; the form of tho 



i Tin <lu i m. ami fig*. 225, 220, 227, 281, 2B2, are from artirloi bj 



Mr. llopkiiiN in tli.- .sv/Vntylc American. 



