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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



because they may lead to uncontrollable movements of the magnet, and a consequent shift- 

 ing of the point. There arc other forms of instruments, such as the Deprez-d' Arson val 

 mirror galvanometer, 1 which arc not affected by Buch influences. 



The galvanometer i- very sensitive to the presence of electric currents. Another appa- 

 ratus which is even more responsive to changes in electric potential of short duration is 

 tli*' capillary electrometer. 



The capillary electrometer (Fig. 63) consists of a glass tube (a) drawn out to form 

 a very fine capillary, the end of which dips into a glass cup with parallel sides (/) contain- 

 ing a l<> per cent, solution of sulphuric acid. The 

 upper part of the tube is connected by a thick- 

 walled rubber tube with a pressure-bulb containing 

 mercury (<■). As the pressure-bulb is raised, the 

 mercury is driven into the capillary, the flow being 

 opposed by the capillary resistance. By a suffi- 

 ciently great pressure, mercury may he driven to the 

 extremity of the capillary and all the air expelled. 

 When the pressure is relieved the mercury rises 

 again in the tube, drawing the sulphuric acid after 

 it. The column of mercury will come to rest at a 

 point where the pressure and the capillary force just 

 balance. Seen through the microscope (e), the end 

 of the column of mercury, where it is in contact 

 with the sulphuric acid appears as a convex menis- 

 cus (</). Any alteration of the surface tension of 

 the meniscus causes the mercury to move with 

 great rapidity in one direction or the other along 



the tube; and a very slight difference of electric 



Fig. 63.— Schema of capillary electrometer. , . , re <. i, „ •„ „ r„„„ *^„ 



1 ' potential suffices to cause a change in surface ten- 



sion of the mercury-sulphuric acid meniscus. A platinum wire fused into the glass tube 

 (a), and another dipped into a little mercury at the bottom of the cup holding the acid, 

 permit the mercury in the capillary and the acid to be connected with the body the elec- 

 tric condition of which is to be examined. If the mercury and acid be connected with 

 two points of different electric potential, as g and h of muscle M, the mercury will instantly 

 move from the direction of greater to that of lesser tension, descending deeper into the 

 tube if the tension be raised on the mercury side, or lowered on the acid side, and vice 

 versa. As seen through the microscope the picture is reversed (d ), and the movements 

 of the mercury appear to be in the opposite direction to that stated. The extent of the 

 movements of the mercury column can be estimated by a scale in the eyepiece. More- 

 over, the movement of the mercury can he recorded photographically, by placing a strong 

 light behind the column of mercury, and letting its shadow fall through a slit in the wall 

 of a dark chamber, upon a sheet of sensitized paper stretched over the surface of a revolv- 

 ing drum or a sensitized plate moved by clockwork or other suitable mechanism. This 

 instrument, of which there are a number of different forms besides that originally devised 

 by Lippmann, is very delicate, recording exceedingly slight differences in electrical poten- 

 tial. 



The movements of a galvanometer may he recorded photographically by letting the 

 beam of light reflected from the mirror fall through a horizontal slit on a photographic 

 plate. 1 1' the plate he arranged to descend at a regular rate in a dark chamber behind the 

 screen holding the slit, the movements of the galvanometer magnet will be pictured as 

 black lines on a white ground. 



The movements of the mercury column of a capillary electrometer may he recorded in 

 a similar manner, by placing the instrument in front of a vertical slit behind which a pho- 

 tographic plate or sheet of sensitized paper moves horizontally. If a strong light falls on 

 1 Bernstein : Pjliirjer's Archiv, 1898, Bd. lxxiii. S. 376. 



