The total length of the whole flame is, to a certain extent, immaterial; but, generally speaking, it should be adjusted so that the length of the inner cone A is about 1 in. or 1-1⁄4 in. The same methods which we described in the early part of this chapter can be employed in the adjustment of this burner, but some care should be exercised to get the correct flame length.
The result of allowing the cold part of the flame to impinge on the tube is observable in fig. 18. The black spot indicated on the drawing actually appears as a black or sooty spot when looking at the tube under these conditions; but in reality no discoloration whatever takes place, the spot disappearing immediately the cone A is made shorter, or the burner H lowered in the chimney B, so that the tip of A is just below, and does not touch the tube at all.
The adjustment of the length of cone A may be accomplished in two ways—(1) by keeping the supply of gas constant, and varying the amount of air admitted at aperture K, fig. 18; (2) by keeping the supply of air constant, and varying the amount of gas admitted through nipple N. The first method is to be preferred when it is necessary to make any slight adjustment due to the variation of gas pressure during the day, and may be accomplished by fitting a small sliding shield G, as shown in the figs. 16 and 17, and moving it round so that it covers, more or less, the aperture K. Thus the length of cone A may be adjusted to a nicety in a very few seconds. This shield keeps all draughts and puffs of wind from the fly-wheel away from the aperture, and helps the flame to burn very steadily. In the first place, of course, the flame will be regulated by opening out or tapping up the nipple N (an enlarged sketch of which is given in fig. 14), so that cone A is just about 1-1⁄4 in. long when air aperture is full open; but once this is done, any future adjustment can be made by throttling the air-supply, or raising or lowering the burner bodily, the set screw keeping it in any desired position (see fig. 17).
From the foregoing remarks it will be seen that the most noteworthy features of this form of ignition are the ease and certainty with which the tube can be fixed in a few moments; that when the two nuts on the studs SS have been tightened up there is no likelihood of the joints being "blown," for, as we said before, only the metal washer is clamped up, the porcelain tube itself being as free to expand as it was before. It is also at once obvious when any adjustment of the flame is necessary; there need be no uncertainty as to whether the tube is hot enough or not.
CHAPTER V
MAGNETO IGNITION
The third form of ignition we have to deal with is the electric.
There are a great number of different types made and used, but for gas-engine use perhaps that known as the magneto ignition is the most satisfactory. With this form, neither accumulators, dry batteries, or spark coils are required, and consequently a greater simplicity is arrived at than would otherwise be the case.