PHYSIOLOGIC APPARATUS. 



721 



and the mercury ascends the tube toward the negative pole. From facts 

 such as these Lippmann constructed the capillary electrometer, a con- 

 venient modification of which devised by M. v. Frey, is shown in Fig. 371. 

 This consists of a glass tube, A , forty millimeters in length, three millimeters 

 in diameter, the lower end of which is drawn out to a fine capillary point. 

 The tube is filled with mercury and its capillary point immersed in a 10 per 

 cent, solution of sulphuric acid. The vessel containing the acid is filled to 

 the extent of several millimeters with mercury also. The mercury in the 

 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 



FIG. 371. VON FREY'S CAPILLARY ELECTROMETER. 



FIG. 372. CAPILLARY 

 ELECTROMETER. R. 

 Mercury in tube; capil- 

 lary tube. s. Sulphuric 

 ac'id. q. Hg. B. Ob- 

 server. M . Microscope. 



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. 372). 

 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 

 potential, according to Lippmann's observation, as slight as the rTr g"g"o of a 

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

 In using either the galvanometer or the electrometer for detecting the 

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

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