PHYSICS. 609 



and the author regards the result as a remarkable verification of the fact 

 that to certain eyes a faint luminosity accompanies the creation of a 

 powerful magnetic field. (Phil. Mag., April, 1883, V, xv, 270.) 



2. Electromotors. 



Bichat and Blondlot have made a series of experiments to determine 

 the influence of pressure on the contact-potential between a metal and 

 the liquid in which it is immersed. Two electrodes of different metals 

 were used, immersed in a solution of a salt of one of them. The results 

 obtained put beyond question the influence of pressure on the electrical 

 difference between a liquid and a metal, amounting to as much as a 

 thousandth of a volt for one hundred atmospheres. (J. Phys., Novem- 

 ber, 1883, II, II, 503.) 



The same authors have extended their investigations and have sub- 

 sequently measured the difference of potential between two liquids hi 

 contact. This difference was measured by means of a Thomson-Mascart 

 electrometer, an apparatus similar to Thomson's water-dropping col- 

 lector being employed to equalize the potentials of the layers of air 

 which covered the liquids in the two vessels. Between water acidulated 

 with ten per cent, sulphuric acid and uitric acid, the difference was 

 0.48 Daniell. (J. Phys., December, 1883, II, II, 533.) 



Blake has experimentally examined, in Helmholtz's laboratory, the 

 two evaporation hypotheses for the production of atmospheric electri- 

 city. The first of these supposes the electricity to be produced simply 

 by the evaporation of the liquid ; the second that a convection of the 

 electricity occurs by means of the vapor arising from the surface wben 

 the liquid is electrified. The results of his experiments are conclusive 

 apparently in showing that no electrification whatever is produced, 

 directly or indirectly, from the evaporation of liquids. (Phil.. Mag., 

 September, 1883, V, xvi, 211.) 



The phenomenon of Hall, discovered in 1880, has been the subject of 

 numerous investigations. He observed that if a thin leaf of metal 

 conveying a current be placed on the pole of a powerful electro-magnet 

 perpendicular to the lines of force, a new electromotive force is de- 

 veloped normal to the lines of force and to the direction of the current 

 through the metal, tending to produce a transverse current, which, for 

 most of the metals, is in a contrary direction to the displacement which 

 a movable conductor would experience under the same conditions, but 

 for the strongly magnetic metals, as iron, cobalt, and zinc, is in the 

 same direction. He gives the name "rotational coefficient" to the quo- 

 tient of E by V, in which E is the difference of potential per centimeter 

 of width, produced in the film, and V the current intensity per unit 

 section traversed. Bighi has modified the form of the film, using three 

 electrodes instead of four. He lias also found that the effect is more 

 marked with bismuth than with any other metal, so much so that he 

 believes he can show the phenomenon with the earth's maguetism. 

 H. Mis. G9 39 



