97 



It is a further consequence of this law, that the rotation of the 

 globe of the earth itself must tend to induce electric currents in its 

 own mass, passing in each hemisphere from the equatorial to the 

 polar regions ; so that if one set of conductors could be applied at 

 the equator, and another at the poles, negative electricity would be 

 collected by the former, and positive electricity by the latter. The 

 electricity of metalliferous veins in the mines of Cornwall (of which 

 an account has been given by Mr. Fox, in a paper lately published 

 in the Philosophical Transactions,) does not appear, however, to be 

 referrible to magneto-electric induction. It may be a question 

 whether the phenomena of the Aurora Borealis and Australis may 

 not arise from the discharge of this induced electricity consequent 

 on the earth's rotation. 



As it appeared probable that there exists some natural difference 

 in the intensity of these electric currents induced by magnetism in 

 different conducting bodies, the author endeavoured to determine 

 what effects might arise from this difference in the case of iron and 

 copper. For this purpose he joined together the ends of wires of 

 each of these metals, each 120 feet in length, extended in the direc- 

 tion of the magnetic meridian. The copper wire was then divided 

 in the middle and examined by a delicate galvanometer, but no 

 evidence of any electrical current was obtained. The same negative 

 result attended trials with wires of these metals, twisted together, 

 and passed between the poles of a powerful magnetic battery. 

 Similar experiments tried with other metals, and also with a circuit 

 composed of copper and sulphuric acid, afforded in like manner no 

 indications of electric currents. Hence it appears that when metals 

 or other conductors of different kinds are equally subjected to 

 magneto-electric induction, they exhibit equal powers with respect 

 to the currents induced in them. 



By another experiment the author shows that these effects of 

 magneto-electric induction are not owing to the motion of the 

 magnet and conductor relatively to each other, but that they take 

 place to an equal degree when the two are united so as to revolve 

 together, and when, consequently, they are relatively at rest. 

 Electric currents are produced also in the substance of the magnet 

 itself, simply by revolution on its own axis while floating on mercurv, 

 and the circuit completed by wires making a communication be- 

 tween the mercury and the axis of the magnet. 



The author has comprised the phenomena here related in the 

 following general formula. Referring to the pole of the magnet as 

 the centre of action, if all the parts of the metallic conductor move 

 in the same direction and with the same angular velocity, no electric 

 currents are produced : but if one part cut the magnetic curves 

 while another part is stationary, or if the motion of the whole be 

 in one direction, but its angular velocity relatively to the pole of 

 the magnet be different, then, in either case, currents will be pro- 

 duced ; the maximum effect taking place when different parts move 

 in different directions across the magnetic curves. 



