Strong Fields at Different Temperatures. 299 



§§2, 16). Small disks M were turned out of the same 

 material as that above defined (§ 3), and polished on one 

 side. These mirrors were fixed to one pole P : of a Ruhm- 

 korff electromagnet (see PI. VIII. fig. 3, full size). On its 

 way towards the reflecting surface and back the light passed 

 through the narrow bore of the other pole Pg, which thus 

 screened off all but a small central patch of the mirror. 

 Here the lines of induction evidently issue normally from the 

 metal for reasons of symmetry. P 2 and M could be kept 

 very nearly at 100° by means of a steam-jacket JJ. 



§ 10. A very thin standard glass-plate Gr, silvered (S) at 

 the back, could be placed immediately in front of the mirror. 

 The light then being reflected in it suffers a double magnetic 

 rotation in the glass ; this affords a measure for the field ,g 

 directly in front of the metal, equal, by the principle of 

 continuity alluded to, to the induction £)' immediately inside 

 it. The glass plate had previously been magneto-optically 

 standardized by comparison with bisulphide of carbon, whose 

 Verdet's constant is known from the determinations of Lord 

 Rayleigh and Koepsel ; such standards are very convenient. 

 Owing to Ruhmkorff's well-known construction the ray has 

 to pass through one of the coils on its way to and from M 

 through P 2 . The ensuing very slight double rotation in the 

 magnetized air must be proportional to the current, and may 

 be calculated from the data of H. Becquerel, Kundt and 

 Rontgen. It was in every case corrected for, the maximum 

 value being -f *3' ; I have actually observed about this rota- 

 tion with a silver mirror. 



§ 11. Results. — The first thing experimentally obtained 

 was the curve e = funct. 08'), whose ordinates must be and 

 were actually found proportional to those of 3 = funct. (£*') ;* 

 the latter being calculated from g = funct. (.9'), as far as this 

 goes (see § 6). In order further to exemplify the somewhat 

 complicated mode of calculation the numbers for cobalt and 

 nickel are given in Table I. Here the italics are the three 

 highest corresponding values of 3 and £$', indirectly given 

 by the magnetometric method ; the ordinary type refers to 

 the five highest corresponding values of e and $)', obtained 

 magneto-optically with all possible accuracy !• The value of 

 3 marked * is interpolated from those above and below ; it 

 corresponds to the rotation e printed to its left ; the ratio of 



t I. e. from 30 observations of the analyser's azimuth ; the smaller 

 values of e were based on 4 observations and measured only in order to 

 approximately check the whole method by the proportionality of ordinates 

 above alluded to. 



