[Illustration:  Fig. 68.—­Wheel for steam turbine, showing one vane twisted into final position.]

A spindle is made out of a large pin, beheaded, the rough end of which must be ground or filed to a sharp point.  Next, just break through the metal of the disc at the centre with a sharpened wire nail, and push the spindle through till it projects a quarter of an inch or so.  Soldering the disc to the spindle is most easily effected with a blowpipe or small blow-lamp.

The Boiler.—­In the centre of the boiler make a dent, to act as bottom bearing for the spindle.  From this centre describe a circle of 5/8-inch radius.  On this circle must be made the steam port or ports.  Two ports, at opposite ends of a diameter, give better results than a single port, as equalizing the pressure on the vanes, so that the spindle is relieved of bending strains.  Their combined area must not, however, exceed that of the single port, if one only be used.  It is important to keep in mind that for a turbine of this kind velocity of steam is everything, and that nothing is gained by increasing the number or size of ports if it causes a fall in the boiler pressure.

The holes are best made with a tiny Morse twist drill.  As the metal is thin, drill squarely, so that the steam shall emerge vertically.

For the upper bearing bend a piece of tin into the shape shown in Fig. 67.  The vertical parts should be as nearly as possible of the same length as the spindle.  In the centre of the underside of the standard make a deep dent, supporting the metal on hard wood or lead, so that it shall not be pierced.  If this accident occurs the piece is useless.

Place the wheel in position, the longer part of the spindle upwards, and move the standard about until the spindle is vertical in all directions.  Scratch round the feet of the standard to mark their exact position, and solder the standard to the boiler.  The top of the standard must now be bent slightly upwards or downwards until the spindle is held securely without being pinched.

A 3/16-inch brass nut and screw, the first soldered to the boiler round a hole of the same size as its internal diameter, make a convenient “filler;” but a plain hole plugged with a tapered piece of wood, such as the end of a penholder, will serve.

Half fill the boiler by immersion in hot water, the large hole being kept lowermost, and one of the steam vents above water to allow the air to escape.

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.