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 tlie ketj is dosed the 

 current is said to be " short-circuited." 

 Apparatus. DanielPs 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. 



(b.) 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 Flfl a Lois . Reyraond , s Key . 



the key ; so feeble is it that it does not 

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



Fro. Ss.Si-lieme of Du Bois Key. 

 B. Battery ; A'. 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." 



