168 MR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
force of those already obtained ; being assured that the latter would find their 
full development hereafter. 
160. I referred in my former paper to the probable influence of terrestrial 
magneto-electric induction (137.) in producing, either altogether or in part, 
the phenomena observed by Messrs. Christie and Barlow *, whilst revolving 
ferruginous bodies; and especially those observed by the latter when rapidly 
rotating an iron shell, and which were by that philosopher referred to a change 
in the ordinary disposition of the magnetism of the ball. I suggested also that 
the rotation of a copper globe would probably insulate the effects due to elec- 
tric currents from those due to mere derangement of magnetism, and throw 
light upon the true nature of the phenomena. 
161. Upon considering the law already referred to (114.), it appeared im- 
possible that a metallic globe could revolve under natural circumstances, with- 
out having electric currents produced within it, circulating round the revolving 
globe in a plane at right angles to the plane of revolution, provided its axis of ro- 
tation did not coincide with the dip ; and it appeared that the current would be 
most powerful when the axis of revolution was perpendicular to the dip of the 
needle : for then all those parts of the ball below a plane passing through its 
centre and perpendicular to the dip, would in moving cut the magnetic curves 
in one direction, whilst all those parts above that plane would cut them in the 
other direction : currents therefore would exist in these moving parts, pro- 
ceeding from one pole of rotation to the other; but the currents above would 
be in the reverse direction to those below, and in conjunction with them would 
produce a continued circulation of electricity. 
162. As the electric currents are nowhere interrupted in the ball, powerful 
effects were expected, and I endeavoured to obtain them with simple apparatus. 
The ball I used was of brass ; it had belonged to an old electrical machine, 
was hollow, thin (too thin), and four inches in diameter ; a brass wire was 
screwed into it, and the ball either turned in the hand by the wire, or some- 
times, to render it more steady, supported by its wire in a notched piece of 
wood, and motion again given by the hand. The ball gave no signs of mag- 
netism when at rest. 
163. A compound magnetic needle was used to detect the currents. It was 
* Christie, Phil. Trans. 1825. pp. 58. 347, &c. Barlow, Phil. Trans. 1825. p. 317. 
