Fig. 16.
Fig. 17.
Fig. 18.
The latter is left perfectly free to expand; and yet, owing to its particular shape, the pressure in the cylinder during the compression and explosion stroke only tends to make the joint between the tube and washer more secure. The action of this ignition device depends upon the tube heater H, which is merely a small bunsen burner, the flame of which impinges on the tube at one particular spot, raising it to a very high temperature—almost white heat. Most of my readers will know the formation of the bunsen flame. It in composed of two distinct zones. The inner one, marked A in fig. 18, is a perfectly cold part of the flame, and appears to be a pale-blue coloured cone.
It is the outer zone which is the hot portion of the flame, hence this part only must be allowed to play on the tube. The tip of the blue cone A must be kept about 1⁄4 in. below the tube, in order to ensure the hottest part of the flame impinging precisely where the heat is required.