162 



PRACTICAL PHYSIOLOGY. 



fxxvi. 



(c.) Test the effect of a galvanic current by applying the 

 electrodes to the tip of the tongue. 



N.B. In using the key to apply an induction current to excite 

 a nerve or muscle, always use this key by the second method, i.e., 

 always place a short-circuiting key in the secondary circuit. 



4. Mercurial Key. Where a fluid contact is required the wires 

 dip into mercury. Study the use of this key. It is used in the 

 same way as a Du Bois key. 



5. Morse Key (fig. 87). If it is desired to make or break a 

 current rapidly, this key is very convenient. If this key be used 

 to make and break the primary circuit, connect the wires to B 



and C ; when the style of 

 the lever, I, is in contact 

 with c, the current does 

 not pass in the primary 

 circuit. On depressing the 

 handle, K, the primary 

 circuit is made. If, how- 

 ever, the wires be con- 

 nected to A and B, the 

 current passes and is 

 broken on depressing K. 

 To use this key as a short- 

 circuiting key, connect the wires from the battery to A and B, and 

 those of the electrodes to A and C. The current is short-circuited 

 until K is depressed, when the current passes from C to A through 

 the electrode wires. 



6. The Contact- or Spring-Key (fig. 88) is also very useful for 



FIG. 87. Morse Key. The connections are con- 

 cealed below, but are B to I, A to c, C to C'. 



FIG. 88. Spring-Key. 



FIG. 89. Plug-Key. 



rapidly making and breaking a circuit, or for giving a single shock, 

 as in estimating the work done during the contraction of a muscle. 

 The current can only pass between the binding screws when the 

 metallic spring is pressed down. The left end of the spring is in 

 metallic contact with the upper binding screw, while the second 



