128 Mr. G. Gore on the Rotations of Bar-magnets, tyc. [Dec. 9, 



result proves that the rotations are not due to terrestrial magnetic 

 influence. 



15. As the directions of magnetic polarity, electric current, and rota- 

 tion agree with those in the different forms of Ampere's experiment, 

 and as in most, if not all, of the previously known cases of rotation of a 

 bar-magnet or conducting-wire on its axis an electric current passes 

 through the end of the bar or wire, it is evident that those rotations 

 were due, not only to the portions of current in the mercury and fixed 

 conductors connected with it near the middle of the magnet or wire, but 

 also to the influence of the currents in the fixed conductors near the ends 

 of the magnet or wire. 



[Note added September 1875. — It having been suggested by Pro- 

 fessors Maxwell and Stokes that the rotation in the foregoing experi- 

 ments was due to the influence of the portions of current in the cups 

 of mercury or in the fixed conductors near the ends of the movable wire 

 or magnet upon the movable magnet, I diminished the internal diameter, 

 both of the upper and lower cups, from 4 millims. to 1*75 millim., and 

 arranged the following apparatus and experiment. 



The fixed upper wire was of brass, 2-5 millims. diameter and 60 millims. 

 long ; it had no coil upon it, and was used as a conductor only ; its lower 

 end terminated in a fine point of a steel needle projecting 6 millims. 

 The lower fixed wire (also used as a conductor only) was of platinum, to 

 resist the action of the mercury ; it was 2-3 millims. diameter and 75 

 millims. long, with a cavity in its upper end 3*5 millims. deep and 

 1*75 millim. diameter, and containing a thin plate of ruby in its lower 

 part, with a minute hole in the centre for the needle-point to rotate in. 

 The movable wire was 2*5 millims. diameter and 125 millims. long, its 

 upper half being composed of soft iron and its lower half of brass ; its 

 lower end terminated in a needle-point like that of the upper fixed wire, 

 and its upper end had a cavity and perforated ruby plate like that in the 

 lower fixed wire. A voltaic coil 60 millims. long and 7 millims. internal 

 diameter, composed of four layers of cotton covered with stout copper 

 wire, was used to magnetize the iron half of the movable wire, and 

 fixed by means of a separate support in a proper vertical position before- 

 hand, so as to enclose in its axis the iron wire portion only. The little 

 cups were also each half filled with a minute globule of mercury before 

 putting the movable wire into its place. 



After adjusting the wire so as to make rotation easy, a current from 

 6 Grove's elements of one-pint capacity arranged as a series of 6, also 

 as a double series of 3, was passed through the coil and vertical wires, 

 and the direction of the portion of the current in the coil alone, also in 

 the vertical wires alone, was varied ; but notwithstanding that plenty of 

 current passed, no signs of rotation could be detected. These results, 

 therefore, strongly support the opinion that the rotation in the. ex- 

 periments was due to the action of the portions of the current in the 

 cups of mercury.] 



