102 PHYSIOLOGICAL PHYSICS. [Chap. x. 



the more intense current of the two. When used in 



this way the galvanometer is said to be DIFFERENTIAL. 



To obtain the acme of sensibility of the 



instrument, the following procedure should be adopted. 

 Place the galvanometer so that the coils face due east 

 and west, the galvanometer looking west towards the 

 lamp and scale, which are about three feet distant. 

 Having carefully levelled the instrument by the level- 

 ing screws, remove the glass shade, and see that the 

 needle swings freely in its small cellular space within 

 the coils. By gently moving the milled head of the 

 brass pin from which the needles hang, the needles 

 may be raised or lowered. Free suspension being 

 obtained, put on the glass shade, the controlling 

 magnet being removed. In the position of the 

 galvanometer, the system not being absolutely astatic, 

 the needles will take up a position of rest in the plane 

 of the magnetic meridian. Now put the controlling 

 magnet on its support quite at the top, with its 

 north pole pointing north, and slowly slide it 

 down its support. The aim is to obtain a position in 

 which the controlling magnet quite neutralises the in- 

 fluence of the earth's magnetism. As the magnet is 

 moved down, the needles will at first dance backwards 

 and forwards, but as the magnet approaches the 

 proper position the oscillations of the needles become 

 fewer and much more slow. It should be noted 

 that the more sensitive the needle is, the greater is the 

 time occupied by one oscillation, or, in other ivords, the 

 longer is the period of oscillation. This gives an 

 important indication in adjusting the magnet. Now, 

 as the magnet nears the position in which it renders 

 the needles astatic, the slightest movement of it to right 

 or left will cause the beam of light to travel from one 

 side to the other of the scale and even beyond the scale, 

 consequently the fine adjustment must now be used if 

 it is wished to turn the controlling magnet. Just 



