PHYSIOLOGIC APPARATUS. 



689 



an angular movement can be imparted to the end of the accessory magnet 

 in the direction of the magnetic meridian, which serves to keep the needle 

 on the zero of the scale. The deflections of the needle are observed by 

 means of an astronomic telescope, above which is placed a scale divided 

 into centimeters and millimeters, and distant from the galvanometer about 

 six or eight feet. As the numbers on the scale are reversed, they will be 

 seen in the mirror in their natural position, and with the deflection of the 

 needle the numbers will appear as if drawn across the mirror. The extent 

 of the deflection is readily determined when the needle comes to rest. 



The reflecting galvanometer of Sir William Thompson is also used for 

 the same purposes. 



The Capillary Electrometer. Notwithstanding the extreme sensi- 

 tiveness of the modern galvanometer, it has been found desirable, in the 

 investigation of many 

 physiologic processes, to 

 possess some means 

 which will respond even 

 more promptly to slight 

 variations in electromo- 

 tive force. This has 

 been realized in the con- 

 struction by Lippmann 

 of the capillary electro- 

 meter. The principle 

 of this apparatus rests 

 upon the fact that the 

 capillary constant or the 

 surface-tension of mer- 

 cury undergoes a change 

 upon the passage of an 

 electric current, in con- 

 sequence of a polariza- 

 tion by hydrogen taking 

 place at its surface. If 

 a capillary glass tube be 

 filled with mercury and 

 its lower end inserted 

 into a solution of sul- 

 phuric acid, and the 



former connected with the positive and the latter with the negative 

 electrode, it will be observed, upon the passage of the current, that a 

 definite movement of the mercury takes place, in the direction of the nega- 

 tive electrode, in consequence of the diminution of its capillary constant or 

 the tension of its surface in contact with the acid. As a reverse movement 

 follows a cessation of the current, a series of oscillations will follow a rapid 

 making and breaking of the current. If the direction of the current is 

 reversed, the capillary constant is increased and the mercury ascends the 

 tube toward the negative pole. From facts such as these Lippmann con- 

 structed the capillary electrometer, a convenient modification of which, 



FIG. 351. VON FREY'S CAPILLARY ELECTROMETER. 



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