32 ANIMAL ELECTRICITY. LECTURE II. 



through a wash-bottle half full of water. Through 

 the electrodes T and L the current of injury of 

 the otherwise undisturbed nerve passes to the key- 

 board. The other pair of electrodes {Exc.) are 

 connected with an induction coil ; a circular com- 

 mutator revolving once a minute sets up excitation 

 at minute intervals, and each time this happens there 

 is a negative variation of the current of injury. A 

 reverser enables the direction of the exciting currents 

 to be changed at will. 



The keyboard is made up of four keys, K, con- 

 nected with the nerve, K. with a galvanometer, K^ 

 with a compensator, K^ with a second galvanometer. 



When all four keys are closed, the board is simply 

 a square of metal. When K, is opened, a mouth is 

 made by which the nerve currents are let into the 

 square. When in addition K3 is opened, a mouth is 

 made by which the nerve currents are let out of the 

 square to the galvanometer. On the same principle 

 when K^ is opened, a standardising current is let into 

 the square, and thence through the nerve and the 

 galvanometer, and if need be the fourth key, K^, is 

 connected with a second galvanometer ; if there is no 

 such galvanometer in use, K^ is left closed. 



The galvanometer, by which your nerve currents 

 are measured, consists essentially of a suspended 

 system of litde magnets fixed to the back of a light 

 mirror, which reflects a beam of light on to a scale. 

 The nerve currents traverse a coil of fine wire sur- 



