coil, in the largest of them the filament glowed brightest, in the
next smaller it was a little less bright, and in the smallest it only
came to redness. The bulbs were then sealed off and separately tried.
The brightness of the filaments was now such as would have been
expected on the supposition that the energy given off was
proportionate to the surface of the bulb, this surface in each case
representing one of the coatings of a condenser. Accordingly, time
was less difference between the largest and the middle sized than
between the latter and the smallest bulb.
An interesting observation was made in this experiment. The three
bulbs were suspended from a straight bare wire connected to a terminal
of the coil, the largest bulb being placed at the end of the wire, at
some distance from it the smallest bulb, and an equal distance from
the latter the middle-sized one. The carbons glowed then in both the
larger bulbs about as expected, but the smallest did not get its share
by far. This observation led me to exchange the position of the bulbs,
and I then observed that whichever of the bulbs was in the middle it
was by far less bright than it was in any other position. This
mystifying result was, of course, found to be due to the electrostatic
action between the bulbs. When they were placed at a considerable
distance, or when they were attached to the corners of an equilateral
triangle of copper wire, they glowed about in the order determined by
their surfaces.
As to the shape of the vessel, it is also of some importance,
especially at high degrees of exhaustion. Of all the possible
constructions, it seems that a spherical globe with the refractory
body mounted in its centre is the best to employ. In experience it has
been demonstrated that in such a globe a refractory body of a given
bulk is more easily brought to incandescence than when otherwise
shaped bulbs are used. There is also an advantage in giving to the
incandescent body the shape of a sphere, for self-evident reasons. In
any case the body should be mounted in the centre, where the atoms
rebounding from the glass collide. This object is best attained in
the spherical bulb; but it is also attained in a cylindrical vessel
with one or two straight filaments coinciding with its axis, and
possibly also in parabolical or spherical bulbs with the refractory
body or bodies placed in the focus or foci of the same; though the
latter is not probable, as the electrified atoms should in all cases
44