XXVI.] 



ELECTRICAL KEYS RHEOCHORD. 



161 



passes to zero. This method of using the key we may call that for 

 "making and breaking a current." 



3. (2.) When the key is dosed the 

 current is said to be " short-circuited." 

 Apparatus. Daniell's cell, detector, 

 four wires, and a Du Bois key. 



(a.) As in scheme (tig. 86) connect 

 the + pole of the battery to the outer 

 binding screw of one brass bar of the 

 key, and the - pole to the outer binding 

 screw of the other brass bar. Then 

 connect the inner binding screws of 

 both brass bars with the detector. 



(/>.) Observe when the key is de- 

 pressed or closed, there is no deflection 

 of the needle, i.e., when the current is 

 cut off from the circuit beyond the key 

 or bridge ; when the key is raised, the 

 needle is deflected. 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 through the wires beyond FlQ u ^ isjleymond , 8 Key 



the key ; so feeble is it that it does not 

 affect a nerve. On raising the key, the whole of the current passes 



FIG. 85. Scheme of Du Bois Key. 

 B. Battery ; K. Key ; N. Nerve ; 

 M. Muscle. 



FIG. 86. Scheme of Du Bois Key 

 for Short-Circuiting. N. Nerve ; 

 M. Muscle ; B. Battery ; K'. Key 



through the detector or nerve, as the case may be. This method 

 of using the key is called the method of " short-circuiting." 



