To extinguish the jet, first draw away the pastille, and then close successively the taps of the acetylene and oxygen.
791. Precautions to be Taken in the Manipu lation of Oxygen Cylinders. Oxygen is supplie d under a pressure of 1,700 lb. per sq. in., in cylinders of forged steal- In order to ensure regular supply of the gas at a pressure slightly greater than that of the atmosphere, the cylinder is fitted with a regulator valve, shown in section (Fig. 198). When the metal diaphragm of this valve is subjected to a pressure greater than that fixed by the regulator, the passage between the gas tube and the pressure reduction chamber is automatically closed until the pressure has fallen below the desired point.
Serious and sometimes fatal accidents can be caused by the presence in the joints of organic matter (leather, rubber, fatty matter, etc.), which may ignite spontaneously and cause the cylinder to explode.
Before fitting a regulator valve to a loaded cylinder, the tap of the latter must be slightly unscrewed to let a little oxygen escape, and it must then be closed without jamming too hard. Carefully clean by dry rubbing the outlet of the cylinder and the cup which forms the end of the threaded tube of the regulator valve, so as to remove any dust which may give rise to a leak at this joint. Place the nut of the valve into the connection on the tube and screw it half-way. Then screw the valve fully into the nut, tightening and loosening the latter so as to give the valve the desired position and then screw the nut home, using a special key if necessary.2 After fitting the valve, screw home the regu lating screw (usually ending in a winged key) in the axis of the valve-box. Open fully, by unscrewing, the supply tap of the valve, and then slightly open the tap of the cylinder. As soon as the gas flows, adjust the pressure of the outflow (seen on one of the gauges, the other indicating the pressure of gas in the cylinder') by turning the regulating screw, and then close the tap if the oxygen is not immediately required.
To stop the flow after using the lantern, first close the tap of the cylinder without jamming it, and let the gas escape from the pressure reduction chamber. When the needles of both
gauges have returned to zero, close the supply tap. The needles of both gauges must remain at zero.
792. Electric Light. The incandescent lamps used in projection lanterns are the same as described for enlarging lanterns (§ 758), so that it will suffice here to deal with arc lamps.
To avoid misunderstanding, it is well to state here that arc lamps of the mirror type, the comparatively recent introduction of which marks an appreciable progress in cinematograph projection, cannot be used satisfactorily for the " still " projection of slides of the ordinary The efficiency of an arc in a projector depends on two factors : the intrinsic brightness of the crater (itself proportional to the square root of the density of current, quotient of the number of amperes by the area of the crater), and the useful area of the crater, measured in a plane perpen dicular to the optical axis, which area is smaller as the carbon is more inclined on the axis. The brightness of the positive crater of an arc with ordinary carbons is about three times that of fused tungsten, which latter is obviously the maximum brightness of an incandescent lamp. The maximum brightness of the crater is determined by the temperature of volatiliza tion of carbon. To increase the steadiness of the arc, it is usual to use carbons with a core of softer carbon occupying the position of the lead in a pencil, but as the carbon of the core is more volatile, the intrinsic brightness of the arc is lowered. This drawback is overcome by using carbons with a mineralized core (carbon paste mixed with the fluorides of various metals, such as calcium, barium, etc.). The arc is then steadier, longer, and provides the major part of the light (about 85 per cent), the remainder being supplied by the positive crater. Further more the flame arc gives, according to the salts used in the mineralization, various coloured lights or white and the consumption of current is considerably reduced.