53 6 A MANUAL OF PHYSIOLOGY 



the Thomson galvanometers the magnet is very light. A strip or 

 two of magnetized watch-spring does very well. The magnet is 

 * damped,' that is, its tendency, when once displaced, to go on 

 oscillating about its new position of equilibrium is overcome by 

 enclosing it in a narrow air space. In the Wiedemann instrument the 

 magnet is heavier (Fig. 149 ). It swings in a chamber with copper 

 walls. Every movement of the magnet ' induces ' currents in the 

 copper ; these tend to oppose the movement, and so * damping ' is 

 obtained. It is usual to read the deflections of the Wiedemann 

 galvanometer by means of a telescope. An inverted scale is placed 

 over the telescope at a distance of, say, a metre from the mirror; an 

 upright image of the scale is formed in the telescope after reflection 

 from the mirror, and with every movement of the latter the scale 

 divisions appear to move correspondingly. The method of reading 

 by a telescope can be applied to any mirror galvanometer, and is 

 often extremely convenient in physiological work. Sometimes a 

 small scale is fastened on the mirror itself, and observed directly 

 through a low-power microscope. 



A suspended magnet, if no other magnets are near, takes up a 

 definite position under the influence of the earth's magnetism ; its 

 long axis, in the position of rest, lies in a vertical plane, called the 

 plane of the magnetic meridian at the given place. The * marked ' or 

 north pole points north, the south pole south. If the magnet is dis- 

 turbed from this position, it tends to return to it as soon as the dis- 

 turbing force ceases to act. If, for instance, the north pole is displaced 

 in an eastward direction, the earth's magnetism will produce a couple 

 (a pair of parallel forces acting in opposite directions), one member 

 of which may be considered to pull the north pole towards the west, 

 and the other to pull the south pole towards the east. Displacement 

 of the magnet, then, is opposed by this couple ; and where the dis- 

 placing force is small, that is, the current passing through the galva- 

 nometer weak, as is usually the case in physiological observations, it 

 becomes important to reduce the effect of the magnetism of the 

 earth, in other words, the strength of the magnetic field, as much as 

 possible. This can be done by bringing a magnet into the neigh- 

 bourhood of the galvanometer with its north pole pointing north. 

 This pole, which is the one attracted by the earth's north pole, is 

 magnetized in the opposite sense ; and by properly adjusting its 

 distance from the galvanometer magnet, the influence of the earth 

 on the latter can be almost neutralized, and the system made nearly 

 'astatic.' In many galvanometers the magnets attached to the mirror 

 form an ' astatic ' pair (Fig. 150). Two small magnets of nearly equal 

 strength are connected to a light slip of horn or an aluminium wire, 

 with their poles in opposite directions. The earth's magnetism affects 

 them oppositely, so that the resultant action is nearly zero. It is not 

 possible to make the magnets exactly equal in strength, nor is it 

 desirable, for then the system would not tend to come to rest in 

 any definite position, and the zero point would be constantly shifting. 

 Either one or both magnets may be surrounded by the galvanometer 

 coils. If both are so surrounded, each must be within a separate 



