138 Mr. §. Tolver Preston on some Electromagnetic 
dorff’s Annalen before referred to. I will only mention one 
point. In one of Pliicker’s experiments he used an apparatus 
consisting of a copper disk, d (fig. 5), in which six round 
bar-magnets were fixed vertically, as the diagram shows. The 
whole was rotated about the central axis. It was then thought 
(according to the view adopted by him and previously held 
by Faraday) that the lines of force did not share the rota- 
tion* of each bar-magnet about its axis, although they did 
share the rotation, 7. e. translation (in a circle), of each bar- 
magnet about the avis of the disk when the latter revolved. 
Hence it was believed that when the whole system revolved, 
the lines of force of each magnet remained awially in absolute 
rest, and therefore (virtually) revolved backwards, relatively 
to the direction of revolution of the disk, thereby intersecting 
the disk and themselves, and charging up the disk inductively, 
as it was thought. One sees to what a complicated result 
this hypothesis leads. The inductive charge of the disk was 
thought to be proved by touching the axis with one galvano- 
meter wire, and the periphery of the disk with the other, 
when a deflection was observed. It was overlooked that 
this deflection is attributable to the passing of the magnets 
through the galvanometer-loop or circuit as the disk revolves, 
whereby the lines of force of the magnets successively inter- 
sect the wires. 
If one wished to illustrate this subject further experimen- 
tally, it would be necessary to be on one’s guard against a spe- 
cial case, which otherwise might ledd to deceptions. This may 
be best seen by taking a particular example. We will suppose 
that a magnet, m (fig. 6), taken for simplicity the shape of a 
sphere, is surrounded by a metallic covering, or shell, h, with 
an air-space between. At the poles of the spherical magnet, 
projections ns reach through holes in the shell, by which the 
magnet can be put in rotation independently—or (when neces- 
sary) connected to earth, or to an electrometer. Then, by 
the rotation of the magnet-sphere about its magnetic axis ns, 
the lines of force (partaking of the motion, on my view) will 
intersect the metallic shell h, and therefore strive to generate 
a current there, such that the parts of the shell opposite the 
equator and poles of the magnet will be statically charged 
with the opposite electricities, whose sign will depend on the 
direction of rotation. The charge on the metallic shell is 
accordingly just the same as if the magnet had been fixed and 
* The lines of force were supposed to behave in this respect as mag- 
netic needles would do, if balanced on pins projecting from the axis or 
each magnet; then when the whole revolves, the needles (pointing 
steadily north and south) would witually revolve backwards, or remain 
axvally at rest in space. 
