34 ANIMAL ELKCTRICITV. LECTURE II. 



rounding the magnets, deflect them, and with them 

 the mirror and beam of Ho^ht which moves rieht 

 and left on the scale, serving as a pointer to the 

 amount of deflection, which is an indicator of the 

 amount of current. The light moves right and 

 left on the scale ; replace the scale by a screen 

 with a horizontal slit, arrange a photographic plate 

 to descend vertically behind the screen, darken the 

 galvanometer room, and your pointer of light makes 

 its mark, and all its movements come out on the 

 developed plate as black lines on a clear background ; 

 you have a recording galvanometer. 



It is a orreat convenience to have such an in- 

 strument by itself in a dark room well away from 

 coils and keys and moving people, and it is well 

 to leave the recorder severely to itself once adjusted 

 and started. But it is also very convenient to know 

 what is going on. This is effected by a second 

 galvanometer attached to K^, upon which all that 

 is happening in the dark room can be watched on 

 a scale in the usual way. , The resistance in circuit 

 added by the second galvanometer is of no moment^ 

 that of the bit of nerve between T and L is already 

 100,000 ohms, that of the second galvanometer is 

 perhaps 10,000. 



In the figure the galvanometer (and mirror) are 

 as if viewed from the back, the sensitive plate faces 

 you. Imagine K, and K3 open, so that the nerve 

 current passes through the galvanometer, and trace 



