342 CURVED LINES OF MAGNETIC FORCE. SECT. XXXIV. 



earth's surface where there is sensible dip, and bending round 

 enter the earth again at the magnetic equator. They induce 

 electric currents in conducting-wires, moving across them exactly 

 the same as in artificial magnets ; and when a hollow helix, or 

 coil of copper wire, whose extremities are connected with a gal- 

 vanometer, is placed in the magnetic dip, and suddenly moved 

 across the lines of force, the needle of the galvanometer will 

 vibrate through an arc of 80 or 90, in consequence of the 

 electric current induced by these lines of magnetic force in the 

 wire, and the action is greater when a core of soft iron is placed 

 in the helix, which becomes a temporary magnet by induction. 

 Again, if a copper plate be connected with a galvanometer by two 

 copper wires, one from the centre, and another from the cir- 

 cumference, in order to collect and convey the electricity, it is 

 found that, when the plate is made to revolve in a plane passing 

 through the line of the dip, the galvanometer is not affected. 

 But as soon as the plate is inclined to that plane, electricity 

 begins to be developed by its motion across the lines of magnetic 

 force ; it becomes more powerful as the inclination increases, and 

 arrives at a maximum when the plate revolves at right angles to 

 the line of dip. When the revolution is in the same direction 

 with that of the hands of a watch, the current of electricity flows 

 from its centre to the circumference ; and when the rotation is 

 in a contrary direction, the current sets the opposite way. Thus 

 a copper plate, revolving at right angles to the line of the dip, 

 becomes a new electrical machine, differing from the common 

 plate-glass machine by the copper being the most perfect con- 

 ductor, whereas glass is the most perfect non-conductor; besides 

 insulation, which is essential to the glass machine, is fatal to the 

 copper one. The quantity of electricity evolved by the metal 

 does not appear, to be inferior to that devolved by the glass, 

 though very different in intensity. Even a ship crossing the 

 lines of force must have electric currents running through her. 

 Dr. Faraday observes that such is the facility with which elec- 

 tricity is generated by the magnetic lines of force, that scarcely 

 any piece of metal can be moved without a development of it ; 

 consequently, among the arrangements of steam-engines and 

 metallic machinery, curious electro-magnetic combinations pro- 

 bably exist which have never yet been noticed. Thus magnetic 

 lines of force certainly issue from the surface of the globe. 



No doubt the earth is a magnet on a vast scale, but it differs 



