Discharge in a Magnetic Field. 255 



The question naturally arises whether in the case of solid 

 conductors the current becomes displaced when such con- 

 ductors are placed in the magnetic field. It is well known 

 that metals, bismuth in particular, offer considerably more 

 resistance to the passage of the current in the magnetic field, 

 everything else being equal. It occurred to me that this 

 increased resistance is possibly spurious, and due simply to 

 the displacement of the current, the displacement being easier 

 to effect in some metals than in others. I made a large 

 number of experiments in regard to this question, and found 

 that this displacement does take place, although in a very 

 small degree when compared with that which results in the 

 case of a conductor consisting of rarefied gas. A few years 

 ago the displacement was noticed by Mr. Hall, and is gene- 

 rally known under the name of the " Hall effect;" but the 

 phenomenon had been observed by myself many years pre- 

 viously, although 1 did not publish it, believing it to be a 

 known fact. 



1 made a large number of experiments with galvanometers 

 built on the D'Arsonval-Deprez type, and obtained very vary- 

 ing results by modifying the magnetic field. By increasing 

 the field a maximum sensibility was reached, which decreased 

 on further increasing the field. 



The various experiments described no doubt indicate the 

 cause of this, viz. that the field being made too powerful, less 

 current passes through the coil, and the sensibility begins to 

 fall. I had a special galvanometer-apparatus made to fit my 

 large magnet, converting it probably into the largest galvano- 

 meter of the type extant ; but the sensibility is exceedingly 

 small when the magnet is fully excited, and increases rapidly 

 when the excitation is somewhat diminished. 



A pretty way to illustrate the sensibility is the following. 

 A small current is passed through the galvanometer- coil when 

 the magnet is excited, and the coil-current increased until a 

 small deflexion is produced. The exciting current is then 

 turned off to permit the magnetism to fall gradually. The 

 deflexion will then become greater and greater, until the dot 

 of light passes the end of the scale, after which the sensibility 

 again decreases as the magnetism falls. 



From all that has been said up to this point, I think it is 

 worth while for a course of experiments to be made on the 

 varying resistances of different metals in the magnetic field with 

 varying currents. It is probable that it will be found that 

 there is no constant for any given metal ; and if this should 

 be shown to be the case, my view that the increased resistance 

 of the metal in the magnetic field is spurious will be proved. 



