150 



EXPERIMENTAL PHYSIOLOGY 



fig. 119). The resistance of such a galvanometer is very high, from 

 10,000 to 20,000 ohms, and therefore tends to weaken the current. 

 This does not matter, however, when we are studying the current 

 yielded by a piece of tissue whose resistance is very high, and are 

 employing unpolarisable electrodes whose resistance is also very 

 great. Attached to the upper magnet is a light mirror by means of 

 which a beam of light is reflected, and thus any rotation of the 

 magnet and attached mirror is detected by the movement of the 

 reflected beam. 



The galvanometer is set up so that the mirror faces to the west, 

 and the magnets and coils therefore lie in the magnetic meridian. 



The coil is provided with a shunt by means 

 of which we can vary the amount of the 

 current allowed to pass through the gal- 

 vanometer when we are dealing with rela- 

 tively large currents. This consists of three 

 resistances of ^th, ^th, and o^th of the 

 resistance of the galvanometer, and is con- 

 nected up in parallel with the galvanometer, 

 i.e. as a deriving circuit. By means of a plug 

 we can utilise either of the three resistances, 

 or by leaving out the plug send the whole 

 current through the galvanometer. If the 

 plug be inserted so that the resistance of 

 ^th is in parallel with the galvanometer 

 then the current is divided, 1 part passing 

 through the galvanometer and 9 parts 

 through the shunt, i.e. ^Vth of the total cur- 

 rent is sent through the galvanometer. 

 Similarly by means of the other resistances 



Fig. 119. 



we can send 



Tiro^h 



or -nrnTrth of the cur- 



rent through the galvanometer. The shunt 

 is further provided with a short-circuiting 

 key, so that all the current can be sent 

 through the shunt and none allowed to 

 pass through the galvanometer. 



To observe the movements of the needle 

 a source of light is condensed on to a 

 narrow vertical slit, and the light from this is collected by a pro- 

 jecting lens and then thrown on to the mirror of the galvano- 

 meter, and so from this on to the scale (see fig. 118), where it is 

 focussed sharply by adjusting the position of the projecting lens. 

 First determine the nature of the deflection by sending a small 



Astatic couple of magnets n s, 

 s n, suspended by a silk fibre and 

 carrying a mirror (indicated by 

 the dotted circle) : the surrounding 

 line and arrows indicate the dis- 

 position of the coils ; N s is the 

 neutralising or controlling magnet. 

 All these parts are represented as if 

 viewed by an observer standing- 

 west, i.e. in the position of the 

 lamp hi fig. 118. (Waller.) 



