ELECTRICAL KEYS, RHEOCHORD. 



135 



through the wires beyond the key, so feeble is it that it does not 

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

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

 method of using the key we may call the method of " short- 

 circuiting." 



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 short-circuit an induction current. 



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. Plug Key (Fig. 40). Two brass 

 bars are fixed to a piece of vulcanite. 

 The circuit is made or broken by insert- 

 ing a brass plug between the bars. Each 

 brass bar is provided with two bind- 

 ing screws, to which one or two wires 

 may be attached, so that it can be used 

 like a du Bois key, either by the first 

 or second method. 



Fig. 40. Plug Key. 



6. Morse Key (Fig. 41). 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 G ; when 

 the style of the lever, 

 I, is in contact with c, 

 the current does not 

 pass in the primary cir- 

 cuit. On depressing the 

 handle, K, the primary 

 circuit is made. If, 

 however, the wires be 

 connected 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 B and 0. The current is short-circuited until K 

 is depressed, when the current passes from to B through the 

 electrode wires. 



7. The " Trigger or Turn- over Key " is referred to in Lesson 

 XXXII. 



Fig. 41. The Morse Key. The connections 

 are concealed below, but are B to I, A to c, 

 C to c'. 



