774 TEXT-BOOK OF PHYSIOLOGY 



tube is put in connection with a platinum wire (a), and the acid in the vessel 

 with a second wire (b). When a constant current passes into the apparatus 

 in the direction from b to a the mercury is pushed up the tube, and, upon 

 the breaking of the current, it may or may not return to the zero-point. For 

 the purpose of measuring in millimeters of mercury the pressure necessary 

 to compensate this change in the capillary constant produced by the electro- 

 motive force of polarization, the apparatus is provided with a pressure- vessel, 

 H, and a manometer, B. This electrometer can be applied to any microscope 

 having a reversible stage. The oscillations of the mercury can then be 

 observed with the microscope provided with an ocular micrometer (Fig. 350). 

 The special advantage of the electrometer is, that it will respond instantly 

 to any variation in the electro-motive force and indicated a difference of 



FIG. 349. VON FREY'S CAPILLARY ELECTROMETER. 



FIG. 350. CAPILLARY 

 ELECTROMETER. R. 

 Mercury in tube; capil- 

 lary tube. 5, Sulphuric 

 acid. q. Hg. B. Ob- 

 server. M. Microscope. 



potential, according to Lippmann's observation, as slight as the r0 Q fl6 of a 

 Daniell. These rapid oscillations can be recorded by photographic methods. 



In using either the galvanometer or the electrometer for detecting the 

 existence of electric currents or differences of potential in living tissues, it is 

 absolutely essential that non-polarizable electrodes be employed in con- 

 nection with it. 



The String Galvanometer. The string galvanometer (Fig. 351) as de- 

 vised by Einthovens is a far more sensitive apparatus than either the usual 

 galvanometer or the capillary electrometer, and responds to feeble electric 

 currents more quickly than either. It is based on the principle that a con- 

 ductor placed near a stationary magnet will tend to change its position when 

 a current is passed through it, just as a magnet, e.g., a delicate magnetic 

 needle, placed near a conductor will change its position when a current is 

 passing through the conductor. For the purpose of developing a high degree 



