1 66 SCIENTIFIC APPARATUS. 



dorff. A point of chief importance in the construction of galvano- 

 scopes is that the magnetic force due to the current at the place 

 where the needle is hung should be as great as possible. The 

 arrangement of the conducting wire required to fulfil this condition 

 has been investigated by W. Weber, Sir W. Thomson, and H. 

 Weber. In instruments intended to show rapid changes in the 

 strength or direction of a current it is essential that the 

 moving parts should be of small inertia. In this respect the re- 

 flecting galvanometers introduced by Sir W. Thomson are a great 

 advance upon previous forms. The small size of the magnet, which 

 enables a comparatively short length of wire to convey the current 

 many times round it, also causes a great increase of sensitiveness 

 even in the case of currents of constant strength. 



Besides galvanoscopes in which a fixed conductor .acts on a 

 movable magnet, instruments have been constructed in which a 

 fixed magnet acts on a movable conductor. The earliest of these 

 appears to have been the gold-leaf galvanoscope, an exceedingly 

 sensitive instrument, described by Gumming (see his " Electro- 

 dynamics," p. 177 : 1827), which consisted of a strip of gold-leaf 

 suspended between the poles of a horse-shoe magnet and made a 

 part of the circuit in which the current was to be detected. More 

 recently, the same general principle has been applied by Sir W. 

 Thomson in his " Siphon Recorder." 



The principle usually adopted in the construction of galvano- 

 meters for measuring the strength of electric currents, is to place a 

 measured length of the circuit in an accurately defined position 

 with respect to a magnetic needle, and to estimate the force exerted 

 by the current on the magnet from observations of the angular 

 deflection of the magnet from its position of equilibrium, com- 

 bined with a knowledge of the intensity of that component of the 

 earth's magnetic force which is effective upon the magnet. The 

 best-known types of such instruments are the tangent-galvanometer 

 and sine-galvanometer of Pouillet (1837). In Ritchie's torsion- 

 galvanometer, and in some instruments since constructed on the 



