376 ELECTRIC CURRENTS OF MUSCLES. 



CHAPTER XXY. 

 ELECTRIC CURRENTS OF MUSCLES. 



I. The Natural Currents. Obs. I. Place the gah^anome- 

 ter A and scale B east and west (with lamp lighted) about three 

 feet apart, level the galvanometer with the levelling screws c, 

 carefully set the mirror free if needful by gently raising the 

 milled head seen on the top of the galvanometer when the 

 glass cover is removed, 1 and adjust the height of the lamp 

 by pulling in and out its brass neck, or moving it from side to 

 side until the light falls well on the mirror. 



The most convenient galvanometer for the purpose is Sir 

 William Thomson's. The one represented in the figure (fig. 

 286) is a differential one, but should be used as a single one 

 in the following observations by connecting the two central 

 binding screws a a with a piece of wire. 



Having put on the glass cover, screw the adjusting magnet 

 d with its upright e on to the top of the galvanometer. Let 

 the magnet, with its north pole directed towards the magnetic 

 north, be at first at the top of the upright ; gradually bring it 

 down, moving it from side to side, and carefully watching the 

 spot of light as it travels to and fro on the scale. Before the 

 magnet has descended very far, the student will have so far 

 gained command over the mirror, as, by moving the magnet 

 to a certain position right or left, to be able to bring the spot 

 of light nearly to zero. 



This done, shift the scale away from or towards the galva- 

 nometer, until the image of the slit/ through which the lamp 

 shines is well focussed on the scale. (If not provided on the 

 scale, affix an upright wire in the middle of the slit ; have the 

 slit wide, and use the shadow of the wire seen in the broad 

 spot of light, to determine the position of the mirror.) 



Now bring the magnet very gradually still lower down, 

 keeping the spot of light as near as possible to zero, and 

 watching attentively the rapidity with which the spot oscil- 

 lates on either side of that point. It will be found that as the 

 magnet descends the oscillations become slower and slower. 

 This indicates that the influence of the earth's magnetism is 

 becoming more and more neutralized by the magnetism of the 



1 If possible, the galvanometer should be carefully levelled and set 

 once for all, aiid kept so in some place where it need not be disturbed. 



