An electrical exhibition on a comparatively small scale was opened in
Paris, March 22, 1885, with considerable eclat, the President of
the Republic being present. Engines to the extent of 200 H.P. are
employed to work the lights. Among the exhibits is the Cruto
light. _Engineering_ says: At the first glance it presents the same
appearance as an Edison lamp, having the same form of globe, and
apparently a similar luminous filament. But this latter is made in
an entirely different manner. A platinum wire is employed, 1/100 of
a millimeter in diameter. This is obtained by the Wollaston process,
that is to say, a piece of coarse platinum wire is covered with a
stout coating of silver, and drawn down till the outside diameter
is 1/10 millimeter. The silver coating is then dissolved in a bath of
nitric acid, and the platinum wire is left behind. This wire is then
cut into lengths, bent into a U form, and placed in a glass globe, in
which circulates a current of bicarbonated hydrogen obtained by the
action of sulphuric acid on alcohol. This gas, previously purified,
circulates around the platinum filament, through which an electric
current is passed sufficient to bring it to a red heat. This
decomposes the gas, and a thin coating of absolutely pure carbon is
deposited on the wire. The operation is continued until a sufficient
thickness of carbon has been deposited for each type of lamp, and the
method of regulating the amount of deposit is effected very
simply, and, in fact, almost automatically. Indeed, one of the most
interesting features of the process is its great simplicity, although
it is somewhat more costly than the ordinary methods of producing
incandescence lamps. After having been subjected to the action of
the gas for two or three hours, the filament is taken from the glass
globe, its diameter is carefully measured, the length is calibrated,
and it is set on a platinum support, to which it is soldered by a very
ingenious process. The filament is then introduced into a second
glass globe charged with bicarbonate of hydrogen; it is placed between
pincers that hold the carbon near its union with the platinum, and the
platinum some millimeters below. These pincers are then thrown into
circuit, and a powerful current is passed through the part which is
to be soldered. The platinum and carbon become incandescent, the
bicarbonate is decomposed, and a fresh deposit of carbon solders the
filament to its support. The system thus mounted is placed within the
permanent globe, and a vacuum is obtained in the ordinary way, while
the testing and finishing details present nothing of special interest.
The finished lamp is then photometrically tested, and placed on a
support something like the Edison mounting. Upon it are engraved
the working constants. As an ordinary practical result, these lamps,
working with 50 volts and 1.15 amperes, give a luminous intensity of
20 candles, or the equivalent in luminous spherical intensity of 1.1
Edison A lamps. This result is interesting, especially as the life
of the lamp ranges from 900 to 1,100 hours, as was demonstrated by
various careful tests made with some 250 lamps; the most valuable
trials having taken place at the Turin Exhibition. After prolonged
use, a diminution in the fall of potential is produced, to a more
marked degree than in the Edison lamp, and the light can be maintained
constant by increasing the strength of the current in a proportion
that can be determined by means of resistances. The Cruto filament
examined under the microscope appears to be uniformly magnetic, and
is very regular, except at the curved parts where the diameter is
slightly diminished, and it is here that rupture generally takes
place. The great structural regularity of the filament probably
accounts for its high durability, and from the fact that it may
be worked with a higher current than probably any other form of
incandescence lamp. M. Desroziers in a series of experiments obtained
as much as 250 carcel spherical luminous value per horse-power; this
characteristic is one likely to be of great value in electric lighting
by incandescence of high intensity. At present only 20-candle lamps
are made on the Cruto system. The carbon filament, when properly
prepared, is gray in hue and of metallic appearance; it is built up
in very fine laminæ indicating the mode of manufacture. The results
obtained with these lamps vary as much as 25 per cent., according to
the care bestowed in producing the filament. If traces of air exist in
the globe, they very quickly manifest themselves by the surface of
the glass becoming blackened, while an increased energy is required to
maintain the brightness of the light.
In the early days of this lamp it was thought necessary to remove
the delicate platinum wire which forms the core of the filament, by
raising the strength of the current sufficiently to destroy it in the
course of manufacture. This, however, was given up, and the platinum
now remains either as a continuous wire or as a series of small
separated granules.
* * * * *