420 Mr. H. H. Hall on the Rotation of the 
hard and stiff as to be quite brittle at the ordinary temperature 
of the air. 
The plate of glass bearing the strip of metal so imbedded 
was, when about to be tested, placed with B, B, vertical in the 
narrow part of a tank whose horizontal section is shown in 
fig. 2. This tank, TT, containing the plate of glass with the 
metal strip was placed between the poles P, P, of the electro- 
magnet. The tank was filled with water, which was sometimes 
at rest and sometimes flowing. Bv this means the tempera- 
ture of the strip of metal was under tolerable control, and the 
inconvenience from thermoelectric effects at a and a con- 
siderably lessened. The diameter of the plane circular ends 
of the pole-pieces P, P, is about 3°7 centim. 
The general method of mest of the experiments to be men- 
tioned did not differ much from that described in the Philo- 
sophical Magazine for November 1880. The intensity of the 
magnetic field was estimated as before, by the impulse given 
to a galvanometer-needle when a small coil in connection with 
the galvanometer was suddenly removed from the field. This 
impulse was compared with that given to the same needle by 
the current obtained by turning an earth-inductor of known 
dimensions. 
The direct current through the strip under examination 
was measured by means of a tangent galvanometer. The 
transverse current was measured by means of a Thomson 
galvanometer, the reduction-factor of this instrument usually 
being determined by passing through it a current of known 
strength a few minutes before and a few minutes after each 
set of observations on the transverse current. 
The rotational powers will then be given in ostensibly 
absolute measure ; but an uncertainty of several per cent. 
attaches to the values given owing to uncertainty in regard 
to the following quantities :—I1st, the thickness of the strips 
examined ; 2nd, the dimensions of the small test-coil used for 
getting strength of magnetic field; 3rd, the horizontal in- 
tensity of the earth’s magnetism ; 4th, the reduction factors 
of the tangent-galvanometer used ; 5th, the magnitude of the 
direct effect exerted by the electromagnet upon the Thomson 
galvanometer at a distance of about 50 feet. 
One of the most troublesome operations in these experi- 
ments is that of determining the thickness of the metal strips 
examined. Owing to the inevitable slight roughness of the 
surface, direct measurements with calipers is likely to give 
too great a thickness. On the other hand, the density of 
many specimens is subject to considerable uncertainty, and 
therefore the indirect method by means of weight and density, 
