372 PHYSICS. 



ELECTRICITY. 



1. Magnetism. 



Eowland lias pnblislied a series of important papers on the general 

 equations of electro-magnetic action with application to a new theory of 

 magnetic attractions, and to tlie theory of the magnetic rotation of the 

 plane of polarization of light, in which is contained the mathematical con- 

 sideration of that action of magnetism on electric currents recently dis- 

 covered by Mr. Hall, proving that if Maxwell's theory of light be true, the 

 new action will explain the magnetic rotation of theiilane of polarization. 

 The new theory of magnetism supposes the magnetic field to consist of 

 a perfect fluid whose velocity at any point is represented in magnitude 

 and direction by the magnetic vector-potential at the point. The vortex 

 lines in this fluid are the lines of magnetic induction, and the velocity 

 of angular rotation is proportional to the magnitude of the magnetic 

 force. As 4 tz tim es the electric current is related to magnetic induction in 

 the same way as magnetic induction to the vector-potential, Eowland 

 considers that an electric current consists, as it were, of vortices of vor- 

 tices ; i. c, that certain irregular distribution of the vortices constitutes 

 currents. {Am. J. Math., ii, 334 j iii, 89. See also Mature, June, 1881, 

 xxiv, 204.) 



Trowbridge has made experiments to determine the effect of great 

 cold upon the magnetism of steel, showing that very low temjieratures 

 exercise a far greater influence on the magnetic condition than has 

 hitherto been noticed. Wiedemann has stated that a steel bar magnet- 

 ized at 6^" or 8° C, lost only 4 per cent, on being cooled to — 25^5 but the 

 author finds that a bar magnetized at 20° G. loses at — 00° CO per cent, of 

 its magnetism. {Am. J. Sci., April, 1881, III, xxi, 31G.) 



Pictet has examined a number of steels with reference to their mag- 

 netic power. He finds that this quality depends on the prelseuce of car- 

 bon and on their state of aggregation. One of the two steels giving the 

 best results had ^ of a per cent, of carbon; while samples having l.J to 

 1^ were inferior. German steel made for springs, though of poor quality, 

 made a good magnet. It had little homogenity, and consisted of an in- 

 timate mixture of iron and iron cemented with a small jiroportion of 

 carbon. {Nature, September, 1881, xix, 521.) 



Sir William Thomson has tahen advantage of the fact that the mag- 

 netism of steel changes with the temperature becoming weaker when 

 warmed and recovering its strength on cooling, to construct what he 

 calls a thermo-magnetic thermoscope. Two thin wires of hard steel, 

 each one centimeter long, arranged so as to form a nearly astatic couple, 

 place themselves at right angles to the magnetic meridian. Two other 

 magnets of twice the size, placed one on each side of the astatic couple, 

 act as deflectors. They are laid in a line nearly along the meridian, with 

 their similar poles facing each other, and about two centimeters apart. 



