SECT. XXXV. TERRESTRIAL MAGNETISM. 329 



The principle of thermo-electricity has been employed 

 by MM. Nobili and Melloni for measuring extremely 

 minute quantities of heat in their experiments on the 

 instantaneous transmission of radiant caloric. The 

 thermo-rnultiplier, which they constructed for that pur- 

 pose, consists of a series of alternate bars, or rather fine 

 wires of bismuth and antimony, placed side by side, and 

 the extremities alternately soldered together. When 

 heat is applied to one end of this apparatus, the other 

 remaining at its natural temperature, currents of elec- 

 tricity flow through each pair of bars, which are conveyed 

 by wires to a delicate galvanometer, the needle of which 

 points out the intensity of the electricity conveyed, and 

 consequently that of the heat employed. This instru- 

 ment is so delicate that the comparative warmth of dif- 

 ferent insects has been ascertained by means of it. 



SECTION XXXV. 



The Action of Terrestrial Magnetism upon Electric Currents Induction 

 of Electric Currents by Terrestrial Magnetism The Earth Magnetic by 

 Induction 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 Electricity of 

 Metallic Veins Terrestrial Magnetism possibly owing to Rotation 

 Magnetic Properties of the Celestial Bodies Identity of the Five Kinds 

 of Electricity Connection between Light, Heat, and Electricity or Mag- 

 netism. 



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 affect 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 cur- 



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