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 laminae 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.