700 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



overcome by enclosing it in a narrow air space. In the Wiedemann 

 instrument the magnet is heavier (Fig. 222). It swings in a chamber 

 with copper walls. Every movement of the magnet ' induces ' cur- 

 rents 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. * VfC 



Sometimes a small scale 

 is fastened on the mirror 

 itself, and observed di- 

 rectly through a low- 

 power microscope. 



Fig. 223. Astatic Pair of 

 Magnets. SN and NS are 

 the magnets, fixed to the 

 vertical piece P. M is a mir- 

 ror. The arrow-heads show 

 the direction of a current 

 which deflects both mag- 

 nets in the same direction. 



F-ig. 224. Diagram of String Galvanometer. The 

 string or fibre CC is stretched between the poles of 

 a powerful electromagnet. When a current passes 

 down the string it is deflected in the direction of the 

 large arrow a i.e., at right angles to the magnetic 

 field NS. When the current is reversed, the string 

 moves in the opposite direction. The movements 

 of the string can be observed by a microscope, A 

 (objective E), passing through a hole bored through 

 the centre of the magnet poles. For obtaining 

 records a source of light is placed at B and concen- 

 trated on the fibre by a condenser, F, and the move- 

 ments of the shadow are recorded by photography 



In many galvanometers the magnets attached to the mirror form an 

 ' astatic ' pair (Fig. 223). 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 op- 

 positely, so that the resultant action is nearly zero. Either one or both 

 magnets may be surrounded by the galvanometer coils. If both are so 

 surrounded, each must be within a separate coil, and the current must pass 

 in opposite directions in the two coils, otherwise they would neutralize 

 each other. In the d 7 Arsonval galvanometer the current passes through 

 a small coil of fine wire suspended in the field of a strong magnet. When 

 the current passes the coil is deflected, carrying with it a small mirror 

 a ' tached to the suspending filament. A great advantage of this galvano- 

 meter in many situations is that it is unaffected by neighbouring currents. 



