618 A MANUAL OF PHYSIOLOGY . 



ray of light is allowed to fall on the mirror, from which it is reflected 

 on to a scale ; and the rotation of the mirror is magnified and 

 measured by the excursion of the spot of light on the scale. In 

 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. 206). 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 



FIG. 206. SCHEME OF WIEDEMANN'S GALVANOMETER (WITH TELESCOPE READING). 



T, telescope ; S, scale ; M, mirror ; m, ring magnet suspended between the two 

 galvanometer coils G, the distance of which from m can be varied ; F, fibre 

 suspending mirror and magnet. 



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 

 disturbed from this position, it tends to return to it as soon as the 

 disturbing 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 oi parallel forces acting in opposite direc- 

 tions) , 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 displacing force is small that is, the current passing 

 through the galvanometer weak, as is usually the case in vphysio- 



