396 OPTICAL PROJECTION 



be in the field. There should be 1 inches clear between 

 the two poles, and after all the plates are firmly screwed to- 

 gether and the ends filed fiat for the usual pole-pieces, the 

 magnet is wound until the coils come nearly into contact. 

 With such a magnet, two to four bichromate or Grove 

 cells will give ample power for most experiments, using either 

 small discs, or bars of metal or other substances j inch to 1 

 inch in length, for projection. 



The same apparatus may be used to show that a cube or 

 disc of copper suspended between the pole-pieces, and rotated 

 by twisting the thread, is almost immediately stopped as 

 soon as the current is switched on. 



For experiments on flame and vapours, however, a 

 magnet must be constructed differently, with the limbs much 

 farther apart, in order that the flame or vapour may be 

 introduced beneath the pole-pieces. The vapour of iodine 

 dropped upon a piece of heated metal is easily projected, and 

 the repulsion will be plainly distinguished. It is unnecessary 

 in dia-magnetic projections, that more than the magnetic 

 pole terminals should be seen in the field. 



The various heaping of liquids, according to their mag- 

 netic character, is fairly visible in profile ; but by turning the 

 magnet so as to lie horizontally on the vertical attachment, 

 the heaping of liquids in a thin watch-glass will be con- 

 spicuously shown by the strong refraction produced at the 

 inclined surfaces. 



237. Static Charges. Most simple phenomena of this 

 character are demonstrated by projecting an electroscope, 

 especially of the gold-leaf kind, as shown in fig. 81. Static 

 induction is readily shown in the same way. All the usual 

 experiments with suspended pith-balls, chimes, feathers, hair, 

 &c., are too obvious to need any explanation. Any apparatus 

 which cannot be reduced to the size of the condensers, or the 

 lens in fig. 106, may generally be projected by the shadow 

 method ( 109). 



