CHAP, v.] ELECTRICITY AND MAGNETISM. 297 



and which forms part of the circuit. The arrows show 

 the direction of the currents. The currents in the 

 circular coils constitute a magnetic shell, whose N. -seek- 

 ing face is uppermost. The lines of force due to this 

 shell therefore run vertically in an upward direction. 

 According to the converse to Ampere's Rule (Art. 186), 

 a man swimming in one of the horizontal branches 

 from the centre a outwards, and looking along the lines 

 of force, i.e. turned on to his back, so as to look upwards, 

 will be carried, along with the conductor, toward his left 

 hand. And the pivoted conductor as seen from above 

 will rotate continuously in the same sense as the hands 

 of a clock around the centre a. A pole of a magnet 

 can also be made to rotate round a current ; and if a 

 vertical magnet be pivoted so as to turn around its 

 own axis it will rotate when a current is led into its 

 middle region and out at either end. If the current is 

 led in at one end and out at the other there will be no 

 rotation, since the two poles will thus be urged to rotate 

 in opposite ways, which is impossible. Liquid con- 

 ductors too, can exhibit electromagnetic rotations. Let a 

 cylindrical metallic vessel connected to one pole of a 

 battery be filled with mercury or dilute acid, and let 

 a wire from the other pole dip into its middle, so that 

 a current may flow radially from the centre to the 

 circumference, or vice versa; then, if this be placed 

 upon the pole of a powerful magnet, or if a magnet 

 be held vertically over it, the liquid may be seen to 

 rotate. 



336. Electrodyimmometer. Weber devised an 

 instrument known as an electrodynamometer for measur- 

 ing the strength of currents by means of the electro- 

 dynamic action of one part of the circuit upon another part. 

 It is in fact a sort of galvanometer, in which, instead of 

 a needle, there is a small coil suspended. One form of 

 this instrument, in which both the large outer and small 

 inner coils consist of two parallel coils of many turns, is 



