ELEMENTARY EXPERIMENTAL PHYSIOLOGY. 5 



metal blocks can be connected by a thick brass bar attached to an 

 insulated movable handle. This key, like the mercury and spring 

 key, may be used as a simple make and break key (Fig. 5) ; but its 



Fia. 5. Plan of the use of a Du Bois key, as a simple make and break key. 



Flo. C. Arranged as a short-circuiting key: key shut. 



Fio. 7. Arranged as a stort circuiting key : key open. 



proper use is as a short-circuiting key (Figs. 6 and 7) ; and when 

 a Du Bois key is directed to be used, it must be inserted into the 

 circuit as a short-circuiting and not as a simple key. 



Fio. 8A. The Pohl's reverser. A and B the two side 

 cups ; C, D, B and F the four corner cups ; S the handle 

 made of glass or vulcanite. 



FIG. 8s. Universal key (Gotch). The key is used by 

 rotating the arm containing the screws connected with 

 the wires A and B, which come from the battery. In 

 the position shown the current flows from the wire of 

 C to that of D ; if rotated through 45* there is a complete 

 double break of the battery -circuit ; if rotated through 

 90 then the current is remade and the current flows 

 from the wire of D to that of C. 



