the Magnet on Electric Currents. 221 



in series and five abreast. The strength of the magnetic field 

 in which the coil was placed was probably fifteen or twenty 

 thousand times IT, the horizontal intensity of the earth's 

 magnetism. 



Making the spiral one arm of a Wheatstone's bridge, and 

 using a low-resistance Thomson galvanometer, so delicately 

 adjusted as to betray a change of about one part in a million 

 in the resistance of the spiral, I made, from October 7th to 

 October 11th inclusive, thirteen series of observations, each 

 of forty readings. A reading would first be made with the 

 magnet active in a certain direction, then a reading with the 

 magnet inactive, then one with the magnet active in tbe 

 direction opposite to the first, then with the magnet inactive^ 

 and so on till the series of forty readings was completed. 



Some of the series seemed to show a slight increase of re- 

 sistance due to the action of the magnet, some a slight decrease, 

 the greatest change indicated by any complete series being 

 a decrease of about one part in a hundred and fifty thousand. 

 Nearly all the other series indicated a very much smaller 

 change, the average change shown by the thirteen series being 

 a decrease of about one part in five millions. 



Apparently, then, the magnet's action caused no change 

 in the resistance of the coil. 



But though conclusive, apparently, in respect to any change 

 of resistance, the above experiments are not sufficient to 

 prove that a magnet cannot affect an electric current. If 

 electricity is assumed to be an incompressible fluid, as some 

 suspect it to be, we may conceive that the current of elec- 

 tricity flowing in a wire cannot be forced into one side of the 

 wire or made to flow in any but a symmetrical manner. The 

 magnet may tend to deflect the current without being able to 

 do so. It is evident, however, that in this case there would 

 exist a state of stress in the conductor, the electricity pressing, 

 as it were, toward one side of the wire. Reasoning thus, I 

 thought it necessary, in order to make a thorough investi- 

 gation of the matter, to test for a difference of potential 

 between points on opposite sides of the conductor. 



This could be done by repeating the experiment formerly 

 made by Prof. Rowland, and which was the following : — A 

 disk or strip of metal, forming part of an electric circuit, 

 was placed between the poles of an electromagnet, the disk 

 cutting across the lines of force. The two poles of a sensitive 

 galvanometer were then placed in connexion with different 

 parts of the disk, through which an electric current was 

 passing, until two nearly equipotential points were found. 

 The magnet-current was then turned on and the galvanometer 



