The coil of piping at P is 3⁄8 in. diameter copper tube coiled round to the shape shown, the lower end being tightly screwed into the top of the cock. The opposite end is screwed for a short length of 3⁄8-in. gas thread, and very tightly fitted with a screw plug such as that used by plumbers for shutting off a portion of gas piping. Before screwing this on, a small hole about 1⁄32 in. in diameter should be drilled in the middle of same; this is the nipple for the exit of gas to the burner.
The 3⁄8-in. copper tubing should not require filling with anything before bending; this operation is best done round a mandrel of wood. No heating is necessary. A certain amount of flattening of the tube will no doubt occur, but this is immaterial. The end of the flame tube is, of course, open, otherwise the flame could not emerge. The nipple end of the flame tube is also quite open, with the nipple end of the coil just projecting inside. The portion of tubing shown dotted in [Fig. 70] takes the vapour from the coils to the nipple; it does not pass through the coils, but at the back of them, and bends round as shown.
The lid of the container should now be carefully soldered down all round, and then the 3⁄16-in. brass stay rod shown at S must be fitted and both ends sweated over. It is essential that this stay is not omitted, as otherwise the pressure to which the container is subjected would bulge out the ends.
The flame tube A should now be made of 1⁄32-in. sheet-iron, being held in place by two or three clips riveted on and bent over the coil ends as shown at C, only one of which is shown for clearness. The flame tube does not taper, nor is the back end closed up. Its diameter should be 11⁄4 in. and length 31⁄2 in., but this depends on the size of the hole in the nipple. No holes are required in this tube, as all the air is drawn in at the end. The nipple has a gas thread cut inside it, and it is screwed tightly on to the end of the copper coil. The length of the pipe from tap to coil is not important; about 9 in. will be satisfactory.
The best means of joining the nipple to the tube is to screw the end of the tube before bending it round at the end, and then to screw the nipple on tightly. The pipe can afterwards be bent as shown.
The jet hole in the nipple should be about 1⁄32 in. bare, and should be so drilled that the issuing gas passes through the centre of the flame tubes. The tube is open at both ends, to allow air to be drawn down and complete the combustion of the paraffin vapour, the mixture igniting and burning properly when the vapour passes out at the other end and comes into contact with the atmosphere.
To use the lamp, it should be filled about two-thirds or one-half full of paraffin oil, and the container cap then screwed hard down with a leather cap as a washer. A cycle pump should then be connected to the valve V, and a few strokes given, the cock D being meanwhile closed. The copper coil should next be put into a gas flame or the fire for a few minutes until nearly red hot, and then the cock D should be opened a shade, which will allow the paraffin to rise up the pipe I and enter the coils P, where it will become vaporised, and the gas will then issue from the nozzle N, and burn at the mouth of the coils.
The lamp may then be applied to the job, and five or six more strokes given to the pump, when the flame should burn with an intense heat and give out a roaring noise.
The theory of action of lamps of this character is as follows: The pressure of air in the container forces the paraffin up the copper tube into the coil, where it is vaporised by the red-hot piping. The vapour then issues at some pressure from the nipple hole and, passing down the flame tube, an ejector action is caused which draws air in with it. This air mingles with the paraffin vapour, and when the mixture issues at the other end, and comes into contact with more air, combustion takes place.
The heat produced by the lamp should be quite sufficient to braze the bottom bracket of a motor-cycle frame and other similar jobs. The size of nipple with relation to the size of the flame tube is most important. If the flame tube be too large in proportion, the result is an excess of air, which cools the gas unduly and the flame dies out. A candle is put out in a draught from the same cause. On the other hand, if the flame tube is too small, or the nipple hole too large, the result is an excess of gas, causing incomplete combustion and a partly luminous and smoky flame, with less heating power. To ascertain if the nipple hole is too small, take a strip of tin and, whilst the lamp is burning, hold the tin partly over the end of the flame tube, so as to restrict the amount of air entering, and note the result.