APPENDIX. 



Deschiption op Instkuments used in Lesson ix. 



du Bois Reymond's Key. This is shewn at C, 



Fig. 1. (See end of Appendix.) Note the arrange- 

 ment of the wires; when the key is closed the current 

 passes across the key and an infinitesimal part only 

 passes by the electrodes through the alternative 

 circuit ; in the text this is called ' arrangement of 

 key for short-circuiting the current.' If two wires 

 only are connected with the key, one on either side, 

 the current cannot pass when the key is open, but 

 can when the key is closed; in the text this is 

 called ' arrangement of key to break the current.' 



Morse Key. This is shewn at F, Fig. 1. The con- 

 necting wires of a Morse key are hidden by the 

 framework. The following diagram (Fig. 2) will 

 shew the way in which they are usually arranged. 



It is obvious that with wires arranged as at F, Fig. 1, 

 the current can pass from a to 6, except when the 

 end X of the lever is pressed down (cp. Fig. 2), then 

 the circuit is broken ; if the wires are connected with 

 i and c, the current cannot pass until the end x of 



