134 



EXPERIMENTAL PHYSIOLOGY. 



current is made, and when it is opened the current is broken 

 (Fig. 38). Apparatus. Use a charged Daniell's cell and detector 

 as before, three wires, and a du Bois key screwed to a table. 



(a.) As in the scheme (Fig. 38) connect one wire from 

 the battery to one brass bar of the key. Connect the other 

 brass bar with one binding screw of the detector. Connect 

 by means of the third wire the other binding screw of the 

 detector with the - pole of the cell. 



(b.) Observe on depressing the key (i.e., making the 

 circuit) the needle is deflected, on raising it (i.e., breaking 

 the circuit) the needle passes to zero. This method of using 

 the key we may call that for " making and breaking a 

 current." 



This method is never used for an interrupted current applied 

 to a nerve or muscle. 



3. (2) When the key is closed the current is said to be " short- 

 circuited." Apparatus. Daniell's cell, detector, four wires, and 

 a du Bois key screwed to the table. 



(a.) As in the scheme (Fig. 39) connect the wire from 

 the positive pole of the battery 

 to the outer binding screw of one 

 brass bar of the key, and the 

 other battery wire to the outer 

 binding screw of the other brass 

 bar of the key. Then connect 

 the inner binding screws of both 



brass bars with the 

 screws of the detector. 



binding 



(b.) Observe when the key is 

 depressed or closed, there is no 

 deflection of the needle i.e., 

 when the current is cut off from 

 the circuit beyond the key or 

 Fi 39.-Scheme of du Bois .bridge; when the key is raised, 



K^^slTBT the Seedle is defl ^ d 



battery ; K', key. 



When the key is depressed, 



the current is said to be " short-circuited," for the key acts like a 

 bridge, and so a large part of the current passes through it back 

 to the battery, while only an excessively feeble current passes 



