404 



OPTICAL PROJECTION 



there is no room for misunderstanding. The arrangement 

 known as Eoget's Spiral gives another very elegant illus- 

 tration of the attraction of parallel currents. The wire carry- 

 ing the current is wound into a vertical helix, suspended 

 at the top from one terminal, and dipping at the bottom into 

 mercury, which forms the other terminal. The coils should 

 be not less than an inch in diameter, and wound closely 

 to be very easily movable : then on passing the 



current the coils draw 

 more closely together, 

 which is very conspicuous 

 upon the screen. If more 

 powerful action is needed 

 an iron core may be intro- 

 duced, as in the figure, but 

 this will rarely be neces- 

 sary. 



Any of the usual ap- 

 paratus for showing the 

 rotation of currents, mag- 

 nets, or galvanic cells, is 

 fc easily constructed small 

 % enough for projection, and 

 S need not be further dwelt 

 upon. 



For many electro-mag- 

 netic experiments it is 



convenient to arrange two parallel horizontal brass rods pro- 

 jecting from beneath the front of the condensers, communi- 

 cating with binding screws, as devised by Mr. G. M. Hopkins. 

 Then small pieces of apparatus, such as here shown, connect- 

 ing with metal spring hooks underneath, have only to be laid 

 on the rods and pushed home to one side, to be in contact. 

 Fig. 225 illustrates the simple magnetic effect of a current, 

 the wire being wound into a small coil at the top of each 



FIG. 224. Bogefs Spiral 



