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



suggestion of Professor Rowland, was to employ as the con- 

 ductor to be experimented upon a circular disk of gold leaf, in 

 which the current entering at the centre would radiate to a 

 thick ring at the edge, and so pass off by a wire attached to the 

 ring. In such an apparatus under ordinary conditions the 

 electromotive force, and so the flow of electricity, would be 

 along the radii of the disk ; but if a strong magnetic force 

 were made to act perpendicularly to the face of the disk, a 

 new electromotive force would be set up, which would be always 

 perpendicular to the direction of the magnetic force and to the 

 actual direction of flow of electricity at any instant in every 

 part of the disk. The actual electromotive force under which 

 the electricity would flow would therefore be compounded 

 of two, one of which would in general have the direction of 

 the radii of the disk, while the other would be nearly at right 

 angles to this, though changing its direction constantly as the 

 flow of electricity continually veered from its normal course 

 under the resultant action of the two electromotive forces. The 

 resulting path of the electricity from the centre to the circum- 

 ference of the disk would be, not a straight line as under 

 normal conditions, but a spiral. This path being longer than 

 the straight line, we should expect an increase of electrical 

 resistance in the disk of gold leaf. Before any very extended 

 experiments had been made with this apparatus, however, it 

 was pointed out by Professor Rowland that the increase of 

 resistance which might be looked for in this case would be 

 exceedingly small, probably too small to be detected. This 

 experiment was therefore abandoned, for the time at least. 



The next experiment to be described was a very simple 

 variation upon the main one. And before going further it may 

 be well to give a drawing of such a plate as has been used in 

 making most of the observations to be hereafter recorded. 



In fig. 1, which is about one half the actual size of an ordi- 

 nary plate, g g g g represents the plate of glass upon which 

 the metal strip m m m m is mounted. Contact with this strip 

 is made at the ends by the two thick blocks of brass b b, which 

 are held firmly in place by the four brass clamps worked by 

 means of the screws S, S, S, 8. The main current of electricity 

 enters and leaves the metal strip by means of the binding- 

 screws e e. Punning out from the middle of this strip are two 

 projections which make contact with the clamps C y , C„ worked 

 by the screws S /? S,. From the screws i, i, wires lead to the 

 Thomson galvanometer. The projections from the metal strip 

 just alluded to make the apparatus very easy to adjust; for by 

 scraping off little particles from the proper part of the projec- 

 tions while the current is allowed to run through the metal 



