PHYSICS. 373 



C. G. S. units. He proposes to use this variation in the resistance for 

 the purpose of measuring the intensity of a magnetic field. Thi«increase 

 of resistance is in part due to the deviation of the equipoteutial lines, 

 and hence the resistance shoukl be divided by the cosine of this devia- 

 tion. But since this coefficient does not exceed 1.005, the explanation 

 of the greater part of the phenomenon is to be sought in the change of 

 structure of the metal, which produces a deformation in the electric field 

 of the metallic plate experimented on. (C. E., February, 1886, cii, 358.) 



It has long been a question whether the intensity of terrestrial mag- 

 netism varies with the altitude; Humboldt, Kupfler, Forbes, Bravais, 

 and Martin maintaining that it diminishes as the altitude increases, and 

 Quetelet and Hartz holding the opposite view. Maurer has discussed 

 a series of observations made simultaneously on tiie summit of the 

 Sentis, 2,465 meters, and at the Polytechnicum in Zurich by himself 

 and Bayer, lasting seventy-nine days, and has failed to find any sensible 

 difference. Hence either the magnetic force is constant with altitude 

 or the variation is very small. (Arch. Sci. Phys., Gen., xxii, 339; J. 

 Phys., June, 1886, II, v, 290.) 



Balfour, Stewart, and Schuster have discussed the phenomena of 

 terrestrial magnetism, the former in a paper entitled " On thegoause of 

 the Solar-diurnal Variations of Terrestrial Magnetism," and tl 8 latter 

 in a paper "On the Diurnal Period of Terrestrial Magnetism." Both 

 these investigators conclude that the cause of these variations lies in 

 the upper atmospheric regions; and we can not imagine, says Stewart, 

 such a cause to exist in any other form than that of a system of electric 

 currents. (Phil. Mag., April, May, 1886, V, xxi, 349, 435 ; Nature, 

 April, 1886, xxxiii, 614, 620.) 



Krueger has proposed to determine directly the vertical intensity of 

 a magnetic field by measuring the deflection which a disk suspended 

 horizontally by means of a verticfil wire in a solution of copper sulphate 

 experiences when traversed in a radial direction by a current. The 

 vertical intensity of the earth's magnetism at Gottingen thus deter- 

 mined was 2.2903 times the horizontal intensity ; a value correct to the 

 570 part. (Wied. Ann., xxviii, 613 ; Phil. Mag., September, 1886, 

 V, XXII, 311). 



Nichols, following Eemsen, has experimented to determine the chem- 

 ical behavior of iron in the magnetic field. A definite quantity of pow- 

 dered iron was introduced into a known weight of acid contained in a 

 beaker placed between the poles of an electro magnet, and the rise of 

 temperature noted in alternate experiments with the current off and on. 

 Nitric, hydrochloric, and sulphuric acids were used, and also aqua regia. 

 In all these both the speed of the reaction and the heat produced was 

 greater in the magnetic field except in the case of sulphuric acid, where 

 the heat was decreased. The prompt destruction of the so-called pas- 

 sivity of iron in the magnetic field was distinctly observed. (Am. J. 

 Sci., April, 1886, III, xxxi, 272). 



