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times H, tlie horizontal intensity of the Earth’s magnetism. Making the 
spiral one arm of a Wheatstone’s bridge, and using a low resistance Thomson’s 
galvanometer, so adjusted as to betray a change of one-millionth in the 
resistance of the spiral, Mr. Ha, 11 made thirteen series of observations, each 
of forty readings, with the magnet excited, inactive, and reversed, suc- 
cessively. The results were all but negative. It was not, however, com- 
pletely shown that a magnet does not tend unsuccessfully to deflect a 
current. To test this point, a disc or strip of metal, forming part of an 
electric circuit, was placed between the poles of an electro-magnet, cutting 
across the lines of force. The two poles of a sensitive galvanometer were 
then placed in connexion with different parts of the disc, until two nearly 
equipotential points were found. The magnet current was then turned on, 
and the galvanometer was observed without result. The experiment was 
repeated, using gold-leaf mounted on a plate of glass or a metal strip. A 
decided deflection of the galvanometer was thus obtained. It was permanent, 
and therefore not accounted for by induction. It was reversed when the magnet 
was reversed ; but not by transferring the poles of the galvanometer on the strip. 
It was just what would be expected if the current were pressed, but not moved, 
towards one side of the conductor. In regard to direction, if we suppose the 
current a single stream, flowing from positive to negative poles, i. e., from 
carbon to zinc, the phenomena indicate that two parallel currents in the 
same direction tend to repel each other. By the opposite supposition they 
attract. It is well known that two conductors bearing parallel currents in 
the same direction, are drawn towards each other. It is early to decide 
whether these facts have any bearing on the absolute direction of the current. 
Quantitative determinations on the subject were also commenced. ‘ It is, 
perhaps, allowable,’ says the writer, 1 to speak of the action of the magnet, as 
setting up in the strip of gold-leaf a new electromotive force, at right angles 
to the primary force.’ 
Hall's Discovery of a New Action of Maynetism on Electric Currents is 
stated by Professor Rowland as follows : — Whenever a substance transmit- 
ting an electric current is placed in a magnetic field, besides the ordinary 
electromotive force in the medium, we have another acting at right angles to 
the current, and to the magnetic lines of force. Whether there may not be 
also an electromotive force in the direction of the current, has not yet been 
determined with accuracy ; but it has been proved, within the limits of ex- 
periment, that no electromotive force exists in the direction of the lines of 
magnetic force. This electromotive force in a given medium, is proportional 
to the strength of the current and to magnetic intensity, being reversed when 
either of these is reversed. It has also been found to differ in direction in 
iron from that in gold or silver. In gold the effects are such as would hap- 
pen were the electric current to be rotated in a fixed direction with respect 
to the lines of magnetic force, to an amount depending only on the magnetic 
force, and not on the current. This fact seems to point to another very im- 
portant case of rotation, namely, that of the plane of polarized light. By 
Maxwell’s theory, light is an electrical phenomenon, and consists of waves of 
electrical displacement, the currents of displacement being at right angles to 
the direction of propagation. If the action takes place in dielectrics, the ro- 
tation of the plane of polarization of light is explained. 
