322 Models of the Aether. 



An interesting application of Hall's discovery was made in 

 the same year by Boltzmann,* who remarked that it offered a 

 prospect of determining the absolute velocity of the electric 

 charges which carry the current in the strip. For if it is 

 supposed that only one kind (vitreous or resinous) of electricity 

 is in motion, the force on one of the charges tending to drive it 

 to one side of the strip will be proportional to the vector- 

 product of its velocity and the magnetic intensity. Assuming 

 that Hall's phenomenon is a consequence of this tendency of 

 charges to move to one side of the strip, it is evident that the 

 velocity in question must be proportional to the magnitude of 

 the Hall electromotive force due to a unit magnetic field. On 

 the basis of this reasoning, A. von Ettingshausenf found for the 

 current sent by one or two Daniell's cells through a gold strip 

 a velocity of the order of 0*1 cm. per second. It is clear, however, 

 that, if the current consists of both vitreous and resinous charges 

 in motion in opposite directions, Boltzmann's argument fails ; 

 for the two kinds of electricity would give opposite directions 

 to the current in Hall's phenomenon. 



In the year following his discovery, Hall} extended his 

 researches in another direction, by investigating whether a 

 magnetic field disturbs the distribution of equipotential lines in 

 a dielectric which is in an electric field ; but no effect could be 

 observed. Such an effect, indeed,|| was not to be expected on 

 theoretical grounds; for when, in a material system, all the 

 velocities are reversed, the motion is reversed, it being 

 understood that, in the application of this theorem to electrical 

 theory, an electrostatic state is to be regarded as one of rest, and 

 a current as a phenomenon of motion ; and if such a reversal be 



* Wien Anz., 1880, p. 12. Phil. Mag. ix (1880), p. 307. 



t Ann. d. Phys. xi (1880), pp. 432, 1044. 



1 Am. Jour. Sci. xx (1880), p. 164. 



In 1885-6 E. van Aubel, Bull, de 1'Acad. Roy. de Belgique (3) x, p. 609 ; 

 xii, p. 280, repeated the investigation in an improved form, and confirmed the 

 result that a magnetic field has no influence on the electrostatic polarization of 

 dielectrics. 



|| H. A. Lorentz, Arch. Neerl. xix (1884), p. 123. 



