Pa 
218 £E. H. Hall—Rotational Coefficients of Metals. 
temperature, the effect being an increase of perhaps two-thirds 
of one per cent for a rise of 1°C. Possibly a comparison o 
the effect of change of temperature upon the magnetic permea- 
bility of iron, nickel and cobalt, with the effect of the same 
change upon the rotational coefficient, will be of value when 
both effects shall have been more fully studied. 
Leaving the matter of effect of change of temperature and 
referring again to the article on nickel and cobalt (Phil. Mag., 
Sept., 1881), we see that the rotational coefficient in nicke 
decreases as we increase the strength of the magnetic field; 
i. e. the rotational effect, other things being equal, increases less 
rapidly than the intensity of the magnetic field. : 
Experiments were made for the purpose of determining 
whether a similar relation would hold in iron. The iro 
nor must it be forgotten that it is by no means proved, as yet, 
that the non-magnetic metals will show a constant rotational 
piece of steel was firmly imbedded upon a plate of glass in 3 
layer of cement made of melted beeswax and resin. his 
flowing through it, was placed in the usual position betwee? 
the poles of the electro-magnet; the magnet current was turned 
again interrupted, the plate was again removed from the fiel 
and another reading of the Thomson galvanometer was made. 
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