304 Mr. E. H. Hall on the new Action of 



be obtained by reversal of the magnet current. Leaving now 

 the distance between the poles very nearly the same as before, 

 and using, both in the magnet and the gold strip, as nearly as 

 possible the same strength of current which had just been em- 

 ployed in the previous trial, the plate was turned into the 

 position indicated by the dotted lines in fig. 2. With this 

 second arrangement no action of the kind previously seen was 

 detected, or at least none that could with certainty be distin- 

 guished from the direct action of the magnet on the Thomson 

 galvanometer. This latter effect produced a deflection of only 

 a few millimetres, and could not have masked any considerable 

 action of the kind looked for. 



The first part of this experiment, then, shows our main fact, 

 viz. that, in a conductor subjected to the given conditions, a 

 permanent electromotive force is at once established, which 

 has a direction perpendicular to the direction of magnetic force 

 and perpendicular to the direction of the primary current in 

 the conductor. The second part of the experiment shows that, 

 under similar conditions, no electromotive force is set up in 

 the direction of the magnetic force, or at least none of the 

 same order of magnitude as that described above. 



The third experiment to be described was made at the sug- 

 gestion and desire of Professor Rowland. It was to test for 

 an action of the magnet on the lines of static induction in 

 glass. A thick piece of plate glass about 4 centims. square 

 was taken, and a hole about 4 millims. in diameter was drilled 

 through each of the four lateral faces. These four holes were 

 all directed toward the centre of the glass ; and each extended 

 to within about 7 millims. of this point. If the holes had 

 met, they would have formed two cylindrical channels at right 

 angles to each other, and extending straight through the glass 

 from lateral face to lateral face. In each hole a loosely fitting 

 plug of brass several millims. long was placed, and securely 

 fastened with a cement of insulating material. Leading out 

 from each plug was a wire, which was insulated for some cen- 

 tims. by being surrounded with a glass tube. The piece of 

 plate glass thus prepared was placed between the poles of the 

 magnet, precisely as a plate bearing a strip of gold would be. 

 One of the brass plugs was placed in connexion with the inner 

 coating of a battery of Leyden jars charged by means of a 

 Holtz machine, the opposite plug being in connexion with the 

 outer coating of the jars and with the earth. The other two 

 plugs were placed in connexion with separate quadrants of a 

 Thomson electrometer. The quadrants were both insulated 

 from the earth. The electrometer was sufficiently sensitive to 

 deflect the spot of light about 170 millims. for the electromo- 



