A spirit lamp supplies the necessary heat. Or the boiler may be held in a wire cradle over the fire, near enough to make the wheel hum. Be careful not to over-drive the boiler. As a wooden plug will probably be driven out before the pressure can become dangerous, this is a point in favour of using one. Corrosion of the boiler will be lessened if the boiler is kept quite full of water when not in use.
A Practical Steam Turbine.
The next step takes us to the construction of a small turbine capable of doing some useful work. It is shown in cross section and elevation in Fig. 69.
[Illustration: FIG. 69.—Model steam turbine, showing vertical cross section (left) and external steam pipe (right).]
The rotor in this instance is enclosed in a case made up of two stout brass discs, D and E, and a 3/4-inch length of brass tubing. The plates should be 1/2-inch larger in diameter than the ring, if the bolts are to go outside. The stouter the parts, within reason, the better. Thick discs are not so liable to cockle as thin ones, and a stout ring will make it possible to get steam-tight joints with brown-paper packing.
The wheel is a disc of brass, say, 1/25 inch thick and 4 inches in diameter; the spindle is 3/16 inch, of silver steel rod; the bearings, brass tubing, making a close fit on the rod.
If you cannot get the ring ends turned up true in a lathe—a matter of but a few minutes’ work—rub them down on a piece of emery cloth supported on a true surface, such as a piece of thick glass.
Now mark out accurately the centres of the discs on both sides, and make marks to show which face of each disc is to be outside.
On the outside of both scribe circles of the size of the bearing tubes, and other circles at the proper radius for the bolt hole centres.
On the outside of D scribe two circles of 2-inch and 1-11/16-inch radius, between which the steam pipe will lie.