376 THE EARTHS INDUCTIVE POWEE. [SECT. xxxv. 



SECTION XXXV. 



The Action of Terrestrial Magnetism upon Electric Currents Induction of 

 Electric Currents by Terrestrial Magnetism The Earth Magnetic by In- 

 duction Mr. Barlow's Experiment of an Artificial Sphere The Heat of 

 the Sun the Probable Cause of Electric Currents in the Crust of the Earth, 

 and of the Variations in Terrestrial Magnetism Terrestrial Magnetism 

 possibly owing to Rotation Magnetic Properties of the Celestial Bodies- 

 Identity of the five kinds of Electricity Connexion between Light, Heat, 

 and Electricity or Magnetism. 



IN all the experiments hitherto described, artificial magnets 

 alone were used ; but it is obvious that the magnetism of 

 the terrestrial spheroid, which has so powerful an influence 

 on the mariner's compass, must also aflect electrical currents. 

 It consequently appears that a piece of copper wire bent into 

 a rectangle, and free to revolve on a vertical axis, arranges 

 itself with its plane at right angles to the magnetic meridian, 

 as soon as a stream of electricity is sent through it. Under 

 the same circumstances a similar rectangle, suspended on a 

 horizontal axis at right angles to the magnetic meridian, 

 assumes the same inclination with the dipping needle ; so 

 that terrestrial magnetism has the same influence on electrical 

 currents as an artificial magnet. But the magnetic action of 

 the earth also induces electric currents. When a hollow 

 helix of copper wire, whose extremities are connected with 

 the galvanometer, is placed in the magnetic dip, and suddenly 

 inverted several times, accommodating the motion to the 

 oscillations of the needle, the latter is soon made to vibrate 

 through an arc of 80 or 90. Hence it is evident that, 

 whatever may be the cause of terrestrial magnetism, it pro- 

 duces currents of electricity by its direct inductive power 

 upon a metal not capable of exhibiting any of the ordinary 



