MAGNETISM AND ELECTRICITY 409 



others, whether or not the linage should be erected by a 

 prism will depend much upon both the lecturer and the 

 audience. 



The heating effects of the current may be shown by carry- 

 ing a coil of platinum wire through water and boiling it ; 

 or by making a piece 

 of platinum wire in- 

 candescent as usual. 

 The last method, 

 however, and the 

 effects of heat in in- 

 creasing resistance, 



are not as a rule pro- 



. . . . FIG. 231 



jection experiment?, 



and therefore need not be further referred to. 



Incandescence as shown in electric lamps is best projected 

 by Mr. Hopkins' method, of showing in the field of the 

 lantern the apparatus itself by as dim a light as will make it 

 j ust visible on the 

 screen, and then 

 sending the cur- 

 rent through, 

 when the light 

 itself will be also 

 in focus. Fig. 

 231 is a simple 

 little arrangement 

 for an incandes- 

 cent lamp, and 



fig. 232 gives an FIG. 232 



equally simple at- 

 tachment for projecting the carbon points and arc. No regu- 

 lator at all is needed for the latter, as anywhere within an 

 inch or two will project equally well by this method ; and there 

 being no mechanism to be worked, a smaller current will give 



