12 



NATURE 



[Nov. I, 1888 



which it is submitted, can show no Kerr's effect. Free 

 space appears to be of this nature ; and gases approach 

 it very nearly, but not quite. 



In iron, ^ is greater for an increasing than for a decreas- 

 ing force, as is shown by the loops in Ewing's curves ; hence 

 the circular component agreeing in direction with the 

 magnetizing current will travel slower than the other 

 component, and hence the rotation in iron will be against 

 the direction of the magnetizing current. The same 

 appears to hold in most paramagnetic substances, and 

 the opposite in most diamagnetic, but the mere fact of 

 paramagnetism or diamagnetism is not sufficient to tell 

 us the sign of the effect in any given substance. We 

 must know the mode in which its magnetic permeability 

 is affected by waxing and by waning magnetization 

 respectively. 



Possible Electrical Method 0/ detecting the Faraday Effect. 



Thus far we have considered the rotation of electric 

 displacement by a magnetic field as being examined 

 optically, the displacements being those concerned in 

 light, and the rotation being detected by a polarizing 

 analyzer suitable for determining the direction in which 

 the vibrations occur before and after the passage of light 

 through a magnetized substance. This is the only way 

 in which the effect has at present been observed in 

 transparent bodies. But one ought not to be limited to 

 an optical method of detection. 



Electrical displacements are easily produced in any 

 insulator, and if it be immersed in a strong magnetic 

 field so that the electric and magnetic lines of force are at 

 right angles to each other, every electric disturbance 

 ought to experience a small rotation. A steady strain 

 will not be affected ; it is the variable state only which will 

 experience an effect, but every fresh electric displacement 

 should experience a slight rotatory tendency just like the 

 displacements which occur in light. 



Now to rotate a displacement A P, into the position A c 

 requires the combination with it of a perpendicular dis- 

 placement BC (Fig. 51). Hence the effect of the magnetic 



Fig. si. 



field upon an electric displacement, A B, may be said to be 

 the generation of a small perpendicular E.M.F., BC, which, 

 compounded with the original one, has the resultant 

 effect A c. It will be only a temporary effect, lasting while 

 the displacement is being produced, and ceasing directly 

 a steady state of strain is set up. 



An inverse E.M.F., ad, will be excited by the same 

 magnetic field directly the displacement is reversed. 



'' ■ Il L l l il B 



1) 



Fig. 52. 



And so, if a continual electric oscillation is kept up be- 

 tween A and B in a magnetic field, an accompanying very 

 minute transverse oscillation may be expected, and may 

 be looked for electrically. 



Some such arrangement as that here shown (Fig. 52) 



may be employed. A square of heavy glass, perforated 

 with four holes towards the centre, supplied with elec- 

 trodes ; one pair of electrodes. A, B, to be connected with 

 the poles of some alternating machine, and the other 

 pair, c, D, connected to a telephone or other detector of 

 minute oscillatory disturbance. So soon as a strong 

 steady magnetic field is applied, by placing the glass 

 slab between the poles of a strong magnet, the telephone 

 ought to be slightly affected by the transverse oscillations. 

 This effect has not yet been experimentally observed, but 

 it seems to me a certain consequence of the Faraday 

 rotation of the plane of polarization of light. 



Hall Effect. 



Although the existence of this transverse E.M.F.,. 

 excited by a magnetic field in substances undergoing 

 varying electric displacement, has at present only been 

 detected optically in transparent bodies, i.e. in insulators, 

 yet in conductors the corresponding effect with a steady 

 current has been distinctly observed electrically. By 

 many persons it had been looked for (by the writer and 

 Prof. Carey Foster, among others, though unfortunately 

 they were not sufficiently prepared for its extreme small- 

 ness) ; by Mr. Hall, at Baltimore, was it first successfully 

 observed. 



In conductors it is natural to use a conduction-current 

 instead of a displacement-current. A steady current can 

 be maintained in a square or cross of gold-leaf or other 

 thin sheet of metal between the electrodes A, B, and a 

 minute transverse E.M.F. can be detected, causing a 

 very weak steady current through a galvanometer con- 

 nected to the terminals c, D, so soon as a strong magnetic 

 field is applied perpendicularly to the plate. Fig. 53 will 



Fig. 53. — The direction of the trnnsverse E.M.F. excited by the earth's- 

 vertical magnetic field in this conductor, conveying a current as shown, 

 is CD if it represents gold, DC if it represents iron. 



sufficiently indicate the arrangement. The poles of the 

 magnet are one above and one below the paper. 



In iron it is easy to see which way the transverse 

 E.M.F. ought to be found. It has been shown that a 

 displacement will be rotated in iron against the magnet- 

 izing current ; hence, to rotate the displacement A B to 

 AC (Fig. 51), requires in iron a clockwise magnetizing 

 current. Such a current, or, what is the same thing, a 

 south pole below the paper, a north pole above, excites, in 

 the cross of Fig. 53, E.M.F. in the direction D c, andthis by 

 Ampere's rule is just the direction in which the conductor 

 itself is urged by the magnetic forces acting on the 

 current-conveying substance. Most diamagnetic sub- 

 stances should exhibit a transverse E.M.F. in the oppo- 

 site sense. This transverse E.M.F. excited in conductors 

 conveying a current in a magnetic field is the effect 

 known by the name of Hall. It is, as Prof. Rowland and 

 others have pointed out, intimately connected with the 

 Faraday rotation of light. 



Unfortunately a pure and simple Hall effect is a diffi- 

 cult thing to observe. Magnetism affects the conductivity 

 of metals in a rather complicated manner, and strain 

 affects their thermo-electric properties. Now, a metal 



