XVI. MODEL STEAM TURBINES.

Steam turbines have come very much to the fore during recent years,
especially for marine propulsion. In principle they are far simpler than
cylinder engines, steam being merely directed at a suitable angle on to
specially shaped vanes attached to a revolving drum and shaft. In the
Parsons type of turbine the steam expands as it passes through successive
rings of blades, the diameter of which rings, as well as the length and
number of the blades, increases towards the exhaust end of the casing, so
that the increasing velocity of the expanding steam may be taken full
advantage of. The De Laval turbine includes but a single ring of vanes,
against which the steam issues through nozzles so shaped as to allow the
steam to expand somewhat and its molecules to be moving at enormous
velocity before reaching the vanes. A De Laval wheel revolves at terrific
speeds, the limit being tens of thousands of turns per minute for the
smallest engines. The greatest efficiency is obtained, theoretically, when
the vane velocity is half that of the steam, the latter, after passing
round the curved inside surfaces of the vanes, being robbed of all its
energy and speed. (For a fuller description of the steam turbine, see How
It Works, Chap. III., pp.74-86.)

The turbines to be described work on the De Laval principle, which has been
selected as the easier for the beginner to follow.

A Very Simple Turbine.

We will begin with a very simple contrivance, shown in Fig. 67. As a "power
plant" it is confessedly useless, but the making of it affords amusement
and instruction. For the boiler select a circular tin with a jointless
stamped lid, not less than 4 inches in diameter, so as to give plenty of
heating surface, and at least 2-1/2 inches deep, to ensure a good steam
space and moderately dry steam. A shallow boiler may "prime" badly, if
reasonably full, and fling out a lot of water with the steam.

Clean the metal round the joints, and punch a small hole in the lid, half
an inch from the edge, to give egress to the heated air during the
operation of soldering up the point or joints, which must be rendered
absolutely water-tight.

[Illustration: FIG. 67.--Simple steam turbine.]

For the turbine wheel take a piece of thin sheet iron or brass; flatten it
out, and make a slight dent in it an inch from the two nearest edges. With
this dent as centre are scribed two circles, of 3/4 and 1/2 inch radius
respective