y 



96 



position. The same effect resulted from the simple introduction 

 of the iron cylinder into, or its removal out of, the helix ; evidently 

 in consequence of the magnetism acquired by position with relation 

 to the magnetic poles of the earth. When a cylindric magnet was 

 substituted for the soft iron, the same phenomena, obviously ascriba- 

 ble to terrestrial induction, were in either case observed. Similar 

 but more feeble indications of the same effect were obtained by in- 

 verting the helix alone, without its association with any ferruginous 

 body whatever. 



The influence of terrestrial magnetism in eliciting electricity from 

 revolving metallic bodies was next made the subject of investigation. 

 A copper plate was made to rotate in a horizontal plane, one of the 

 wires of the galvanometer being brought into contact with its axis, 

 and the other attached to a leaden conductor pressing against the 

 amalgamated edge of the plate. The needle of the galvanometer 

 was immediately deflected, either to the east or west, according to 

 the direction of the rotation ; and by successively changing this 

 direction, and accommodating it to the oscillations of the needle, 

 the arc of its vibrations was soon made to extend to 50° or 60°. 

 When the plane of rotation passed through the line of dip, the 

 galvanometer was not affected. A copper plate, revolving in a plane 

 perpendicular to the line of dip, thus composes a new electrical 

 machine, differing from the common plate machine in the circum- 

 stance of the material of which it is formed being the most perfect 

 conductor, while in the latter it. is the most perfect non-conductor. 

 Insulation, which is essentially required in the latter, is fatal to the 

 efficacy of the former. The quantity of electricity produced by the 

 metallic machine does not appear to be inferior to that evolved by 

 the glass machine, although differing much in its intensity. On 

 employing copper wires of greater thickness, more powerful effects 

 were obtained. The author expects that the effects may be still 

 further increased by certain combinations of superposed plates, 

 alternately revolving in opposite directions. 



The author proceeds to show by experiment the manner in which 

 terrestrial magneto-electric induction produces phenomena similar 

 to those observed by Messrs. Barlow and Christie when ferruginous 

 bodies are in rapid rotation, and which have been ascribed to a 

 change in the ordinary disposition of the magnetism of the ball. 

 He found that the rotation of a copper ball on an axis either hori- 

 zontal or otherwise inclined to the line of dip, gave rise to a circu- 

 lation of electric currents in a plane perpendicular to that of revo- 

 lution, and inexact conformity with the law already deduced by the 

 author in the first part of this paper. This law is illustrated in a 

 very simple manner by the following experiment. — A copper wire, 

 eight feet long, had its two ends fastened, each respectively to the 

 ends of the wires of the galvanometer, so as to form with them one 

 uninterrupted circuit. Upon moving the copper wire to and fro over 

 the galvanometer, whilst the lower part remained steady, the mag- 

 netic needle was immediately deflected, in a direction regulated by 

 the relative position of the wire and the direction of its motion. 



