80 



PRACTICAL PHYSIOLOGY 



.have to be used (see Part II. ), but these do not concern the 

 junior student. 



The current is thrown into or cut out of the structure to be 



FIG. 82. Plan of the use of a Du Bois key, as a simple make and break key. 



acted upon (made or broken) by means of some form of KEY such 

 as those shown in Figs. 79, 80, 81 (arranged in the circuit as 

 shown in Figs. 82, 83, 84). 



FIG. 83. Arranged as a short-circuiting key : key shut. 



They may be used either to make the current when closed in 

 the circuit (Fig. 82), or to short-circuit the current back to the 

 battery when closed (Fig. 83). 



FIG. 84. Arranged as a short-circuiting key : key open. 



I. The Use of the Galvanic Current. 



To Test the Action of the Current. Connect thick covered wires 

 with the terminals marked G on the table, + for positive (anode) 

 and for negative (cathode) from the constant current supplied 

 from a generator, or connect them to the terminals of three Daniell 

 cells in series. Insert into the circuit a mercury key, as shown in 

 the diagram, so that when it is closed the current will flow through 

 the terminals (Fig. 73). Hold the ends of the wires, one on each 

 side of the tongue, and note the sensations produced when the 

 current is allowed to pass (made) by closing the key, when it is 

 cut out (broken) by opening the key, and when the current is 

 flowing. Note whether the sensations are different or similar at 

 the two poles on closing and on opening and during the flow. 



