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.

In fig. 19 we show diagrammatically the ordinary form of magneto
machine. Virtually it is a small dynamo which is fixed to the side of
cylinder casting, and is operated in the manner shortly to be described.
As we do not propose to enter into more than a brief explanation of why
and how this apparatus generates current to produce the required spark,
perhaps a simple analogy will make matters most intelligible to any
reader not well acquainted with electrical phenomena. We know that when
a current of electricity is flowing in a wire, and the wire be suddenly
broken, a spark will occur at the point of breakage. This fact may be
observed in an ordinary electric bell when ringing; at the tip of the
contact breaker a number of tiny sparks may be seen to occur, due to the
rapid make and break of the current flowing in the circuit. Precisely
th