PHYSIOLOGIC APPARATUS 773 



comes to rest after deflection. This is accomplished by the development of 

 induction currents in the copper cylinder, the direction of which is opposite 

 to that of the movement of the needle. The instrument, therefore, is 

 aperiodic that is to say, when the needle is influenced by a current it 

 moves comparatively slowly until the maximum deflection is reached, when 

 it comes to rest without oscillations. When the circuit is broken the needle 

 swings slowly back to zero, and again comes to rest without oscillations. 



Inasmuch as the needle is not astatic, it is rendered so by the use of an 

 accessory magnet the so-called Hauy's bar. This magnet, supported by a 

 rod directed perpendicular to the coils, is placed in the magnetic meridian, 

 horizontal to the needle, with its north pole pointing north. By sliding the 

 magnet toward the needle the directive influence of the earth's magnetism 

 is gradually diminished, and when it is reduced to a minimum the needle 

 acquires its highest degree of instability. By means of a pulley an angular 

 movement can be imparted to the end of the accessory magnet in the direc- 

 tion 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 astro- 

 nomic 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 number 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. Not withstanding the extreme sensi- 

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

 vestigation of many physiologic processes, to possess some means which 

 will respond even more promptly to slight variations in electro-motive 

 force. This has been realized in the construction by Lippmann of the capil- 

 lary electrometer. The principle of this apparatus rests upon the fact that 

 the capillary constant or the surface-tension of mercury undergoes a change 

 upon the passage of an electric current, in consequence of a polarization by hy- 

 drogen taking place at its surface. If a capillary glass tube be filled with mer- 

 cury and its lower end inserted into a solution of sulphuric 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 negative electrode, in conse- 

 quence 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 constructed the capillary electrometer, a con- 

 venient modification of which devised by M. v. Frey, is shown in Fig. 352. 

 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 



