Page:Elementary Text-book of Physics (Anthony, 1897).djvu/370

356 of these tubes is supposed to be connected with the peculiar motions which characterize the molecule of the magnetic body. Diamagnetism would then be explained by supposing a similar motion enforced upon the molecules of the other bodies in the field to an extent in each which depends upon the nature of the body.

300. The Hall Effect.—Hitherto it has been assumed that when currents interact, it is their conductors alone which are affected, and that the currents in the conductors are not in any way altered. Hall has, however, discovered a fact which seems to show that currents may be displaced in their conductors. If the two poles of a voltaic battery be joined to two opposite arms of a cross of gold-foil mounted on a glass plate, and if a galvanometer be joined to the other two arms at such points that no current flows through it, and if a magnet pole be brought opposite the face of the cross, a permanent current will be indicated by the galvanometer. The same effect appears in the case of other metals. The direction of the permanent current and its amount differ under the same circumstances for different metals. The coefficient which represents the amount of the Hall effect in any metal is called the rotational coefficient of that metal.

Since the rotational coefficients of such metals as have been tested agree in sign and in relative magnitude with their thermoelectric powers (§ 316), it is argued by Bidwell, v. Ettingshausen, and others that the Hall effect is due to thermoelectric action.

301. Currents in a Magnetic Field Due to Inequalities of Temperature.—If a thin strip of bismuth be placed in a magnetic field so that the magnetic force is normal to its surface, and if ono of the edges of the strip be kept at a higher temperature than the other and the two ends of the strip joined by a wire in which a galvanometer is inserted, a continuous current will flow through the circuit. The direction of this current changes when the direction of the flow of heat changes or when the magnetic field is reversed. The strength of the current is different in different metals. These facts were discovered by v. Ettingshausen. Conversely, if a current be sent through the strip of bismuth placed in the magnetic field, there