PROGRESS IN NINETEENTH CENTURY 57 



special forms of conductors, by Rayleigh (1871, 1879), Hicks (1883), 

 and others, the discussion of the refraction of lines of flow by Kirch- 

 hoff (1845), and many researches on the limits of accuracy of the 

 law. 



Finally, in regard to the evolution of the modern galvanometer 

 from its invention by Schweigger (1820), we may enumerate in suc- 

 cession Nobili's astatic system (1834), Poggendorffs (1826) and 

 Gauss's (1833) mirror device, the aperiodic systems, Weber's (1862) 

 and Kelvin's critical study of the best condition for galvanometry , 

 so cleverly applied in the instruments of the latter. Kelvin's siphon 

 recorder (1867), reproduced in the Depretz-D'Arsonval system (1882), 

 has adapted the galvanometer to modern conditions in cities. For 

 absolute measurement Pouillet's tangent galvanometer (1837), 

 treated for absolute measurement by Weber (1840), and Weber's 

 dynamometer (1846) have lost little of their original importance. 



Magnetism 



Magnetism, definitely founded by Gilbert (1600) and put on a 

 quantitative basis by Coulomb (1785), was first made the subject 

 of recondite theoretical treatment by Poisson (1824-27). The inter- 

 pretation thus given to the mechanism of two conditionally separable 

 magnetic fluids facilitated discussion and was very generally used 

 in argument, as for instance by Gauss (1833) and others, although 

 Ampere had suggested the permanent molecular current as early 

 as 1820. Weber (1852) introduced the revolvable molecular magnet, 

 a theory which Ewing (1890) afterwards generalized in a way to 

 include magnetic hysteresis. The phenomenon itself was independ- 

 ently discovered by Warburg (1881) and by Ewing (1882), and has 

 since become of special practical importance. 



Faraday in 1852 introduced his invaluable conception of lines of 

 magnetic force, a geometric embodiment of Gauss's (1813, 1839) 

 theorem of force flux, and Maxwell (1855, 1862, et seq.} thereafter 

 gave the rigorous scientific meaning to this conception which per- 

 vades the whole of contemporaneous electromagnetics. 



The phenomenon of magnetic induction, treated hypothetically 

 by Poisson (1824-27) and even by Barlow (1820), has since been 

 attacked by many great thinkers, like F. Neumann (1848), Kirchhoff 

 (1854); but the predominating and most highly elaborated theory 

 is due to Kelvin (1849, et seq.}. This theory is broad enough to be 

 applicable to seolotropic media and to it the greater part of the not- 

 ation in current use throughout the world is due. A new method of 

 attack of great promise has, however, been introduced by Duhem 

 (1888, 1895, et seq.) in his application of the thermodynamic potential 

 to magnetic phenomena. 



