538 



A MANUAL OF PHYSIOLOGY 



coil, and the current must pass in opposite directions in the two coils, 

 otherwise they would neutralize each other. 



The deflection of a magnet by a current of given strength is pro- 

 portional 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 instrument 

 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. 



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. 151). 



The electrometer con- 

 sists (i) of a small table 

 carrying a parallel -sided 

 glass vessel containing mer- 

 cury and sulphuric acid. 

 (2* The capillary tube, 

 which can be moved in 

 two directions at right 

 angles to each other, and 

 so adjusted in the field of 

 the microscope. (3) A pres- 

 sure-vessel, and a mano- 

 meter connected with it 

 for measuring the pres- 

 sure. (4) Two binding- 

 screws connected by wires 

 to the mrrcury in the 

 capillary tube and in the 

 parallel-sided vessel. The 

 binding -screws can be 

 short-circuited by closing 

 the friction-key shown at 

 the right side of the figure, 

 thus preventing any dif- 

 ference of electromotive 

 force between two points 

 connected with the screws 

 from affecting the electro- 

 meter. 



FIG. 153. CAPILLARY ELECTROMETER (AFTER FREV), AS ARRANGED FOR 

 MOUNTING ON THE MICROSCOPE STAGE. 



A compensator is simply a rheocord from which a branch of a 

 current is led off, to balance or ' compensate ' any electrical difference 

 in a tissue, like that which gives rise to the current of rest of a 

 muscle, for example (Fig. 152). 



An electrometer is an instrument for measuring electromotive 

 force, that is, differences of electric potential. Lippmann's capillary 

 electrometer is being more and more employed in physiology. A 

 convenient form of it is shown in Fig. 153. A simpler form, suitable 

 for students working in a class where a considerable number of 

 copies of the instrument is needed, can be conveniently made as 

 follows. A glass tube is drawn out to a capillary at one end and 



