688 



TEXT-BOOK OF PHYSIOLOGY. 



making the magnet ring-shaped is to increase its strength in proportion to 

 its size, and to get rid of the central inactive part. Connected with and 

 passing upward from the magnetized ring through the copper cylinder is 

 an aluminium rod, surmounted by a circular plane mirror. Above the 

 mirror rises a glass tube, which carries on top, on an ebonite support, a 

 little windlass, capable of being centered by three small screws. On the 

 windlass is wound a single filament of silk, which passes down the tube 

 and is attached to the mirror. The magnet can, by this contrivance, be 

 raised or lowered and centered in the copper chamber. Deflections of 

 the mirror from currents of air are prevented by inclosing it with a brass 

 cover provided with a glass window. The coils are placed on each side 

 of the copper chamber, and supported by a rod, on which they slide. By 

 this arrangement they can be approximated until they meet and completely 



conceal the cylin- 

 der. By varying 

 the position of the 

 coils the influence 

 of the current 

 upon the needle 

 can be increased 

 or diminished. 

 An advantage 

 which this galvan- 

 ometer possesses 

 is the damping of 

 the oscillation of 

 the needle, so that 

 it quickly comes to 

 rest after deflec- 

 tion. This is ac- 

 complished by the 

 development of 

 induction currents 

 in the copper cyl- 

 inder, the direc- 

 tion 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 deflec- 

 tion 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 



FIG. 350. WIEDEMANN'S BOUSSOLE. 



