THE AMERICAN NATURALIST. [Vol. XXXVII. 



roughly adjusted bv moving the copper wire through the cork 

 and wedging it in place by a small wooden wedge occupying 

 with the wire the hole in the cork. The contact of the mercury 

 with the platinum wire serves to close the circuit of the battery, 

 the other wire from the battery being connected to the mercury 

 by means of the screw and piston in the side tube used to adjust 

 the height of the mercury column. Into this single-cell circuit 

 is put a standard 150-ohm telegraphic relay mac hine of the pat- 

 tern used by the Western Union Telegraph Co. 



The closure of this single-cell circuit magnetizes the core of 

 the electro-magnet (J/.), which, pulling against the delicate spring 

 (S.), overcomes it and moves the armature (A.) away from the 

 post (P.), thereby breaking the heating current. 



When, owing to the interruption of the heating current, the 

 bath cools, the mercury in the regulator recedes from the plati- 

 num point, thus breaking the relay circuit, the electro-magnet 

 becomes demagnetized, and the delicate spring pulls the armature 

 into contact with the post and thus closes the heating current. 

 The finer adjustment of the distance between platinum point and 



