of exhaustion, may be described and shown here, as it presents a
feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4
inches diameter were taken, and in the centre of each was mounted an
equal length of an ordinary incandescent lamp filament of uniform
thickness. In each bulb the piece of filament was fastened to the
leading-in wire of platinum, contained in a glass stem sealed in the
bulb; care being taken, of course, to make everything as nearly alike
as possible. On each glass stem in the inside of the bulb was slipped
a highly polished tube made of aluminium sheet, which fitted the stem
and was held on it by spring pressure. The function of this aluminium
tube will be explained subsequently. In each bulb an equal length of
filament protruded above the metal tube. It is sufficient to say now
that under these conditions equal lengths of filament of the same
thickness--in other words, bodies of equal bulk--were brought to
incandescence. The three bulbs were sealed to a glass tube, which was
connected to a Sprengel pump. When a high vacuum had been reached, the
glass tube carrying the bulbs was sealed off. A current was then
turned on successively on each bulb, and it was found that the
filaments came to about the same brightness, and, if anything, the
smallest bulb, which was placed midway between the two larger ones,
may have been slightly brighter. This result was expected, for when
either of the bulbs was connected to the coil the luminosity spread
through the other two, hence the three bulbs constituted really one
vessel. When all the three bulbs were connected in multiple arc to the
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