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 closed the 

 current is said to he " short-circuited." 

 Apparatus. — Darnell's cell, detector, 

 four wires, and a Du Bois key. 



(a.) As in sclieme (fig. 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 

 the key ; so feeble is it that it does not 



Fig. 84.— Du LoisReymoncrs Key. 



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 

 forShort-Circuitiiig. N. Nerve ; 

 M. Muscle ; B. Battery ; b". Key 



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

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



L 



