216 



HELMHOLTZ 



wire, traverses the spirals b b. The magnet c c is rigidly 

 fixed in the lever dd, at the other end of which is a style; 



FIG. 102 



this makes a mark on a paper band, drawn by a clock-work, 

 as often and as long as c c is attracted by the magnetic action 

 of the electrical current. Conversely, by reversing the mag- 

 netism in the iron core of the spirals b b, we should obtain 



