no PHYSIOLOGICAL PHYSICS. [Chap. xi. 



The advantages of Wiedem aim's boussole 

 are, that by the copper damper, the arrangement of 

 the coils, and the accessory magnet, the needle is made 

 quite " aperiodic," or " dead beat." In other words, 

 when affected by a current, it swings round with com- 

 parative slowness till the maximum deflection is ob- 

 tained, which it reaches, and at which it rests, without 

 oscillation. When the current is withdrawn, it swings 

 back and stops, again without oscillation, at the zero 

 point ; if it should pass the zero point, a current in 

 the contrary direction would be indicated. 



CHAPTER XI. 



THE USE OF THE GALVANOMETER IN PHYSIOLOGY. 



THE great purpose for which the galvanometer is 

 employed is the detection of electrical currents in 

 living tissues ; the object, indeed, of all the search for 

 means of obtaining sensitive instruments, and the 

 indirect cause of the discovery of galvanic electricity 

 and all its subsequent developments. For an account 

 of the history and theories of animal electricity, and 

 for a discussion of the electrical currents detected in 

 muscle, nerve, and other textures, reference must be 

 made to the ordinary physiological text-books. What 

 will be described in this chapter are the apparatus 

 employed and the arrangements made for detecting 

 the currents and estimating their amount, and for 

 other similar experiments. 



At the outset, however, it is evident that the 

 extreme sensitiveness of the galvanometer throws 

 great difficulty in the way. For it is evident that 

 very slight changes in the arrangements by which 



