MUSCLE 



619 



logical observation s 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 neighbourhood 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. 207) . 

 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 gal- 

 vanometer coils. If both are so sur- 

 rounded, each must be within a separate 

 coil, and the current must pass in oppo- 

 site directions in the two coils, otherwise 

 they would neutralize each other. In 

 the d' Arson val 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 de- 

 flected, carrying with it a small mirror 

 attached to the suspending filament. A 

 great advantage of this galvanometer in 

 many situations is that it is unaffected by 

 neighbouring currents. 



The deflection of a magnet by a current 

 of given strength is proportional to the 

 number of turns of wire around it. 

 Where an increase in the number of turns 

 does not sensibly cut down the current, 

 as in experiments on tissues like nerves, 

 whose resistance is large in comparison 

 with that of the galvanometer, an instru- 

 ment with a great number of turns of wire that is, a high-resistance 

 galvanometer is suitable. The resistance of the galvanometers 

 generally used in electro-physiology varies from 3,000 or 4,000 ohms 

 up to five times as much. 



The string galvanometer of Einthoven has peculiar merits for 

 certain physiological purposes. It consists of a silvered quartz-fibre 

 stretched in a very strong magnetic field. When traversed by a cur- 

 rent the fibre is deflected, and by means of a beam of light the deflec- 

 tion is greatly magnified. By photographing the beam a record of 

 the swing of the fibre caused by a momentary current can be obtained. 

 In this way the instrument may be employed to record the electrical 

 changes occurring in the human heart with each beat (p. 732). 



A rheocord is an instrument by means of which a current may be 

 divided, and a definite portion of it sent through a tissue (Fig. 208) . 



FIG. 207. ASTATIC PAIR OF 

 MAGNETS. 



SN and NS are the mag- 

 nets, fixed to the vertical 

 piece P. M is a mirror. 

 The arrow-heads show the 

 direction of a current which 

 deflects both magnets in the 

 same direction. 



