If the wick is arranged to turn up and down, the speed of
the top can be regulated.
A Large Top.--The top just described must be light, as the steam driving
it is low-pressure, having free egress from the boiler, and small, as the
steam has comparatively low velocity. The possessor of a high-pressure
boiler may be inclined to make something rather more ambitious--larger,
heavier, and useful for displaying spectrum discs, etc.
The top shown in Fig. 77 is 3 inches in diameter, weighs 1 oz., and was cut
out of sheet-zinc. It stands on a brass disc, round the circumference of
which is soldered a ring of 5/32-inch copper tubing, furnished with a
union for connection with a boiler.
[Illustration: FIG. 77.---Large steam top and base.]
The copper tubing must be well annealed, so as to bend quite easily. Bevel
off one end, and solder this to the plate. Bend a couple of inches to the
curve of the plate, clamp it in position, and solder; and so on until the
circle is completed, bringing the tube snugly against the bevelled end. A
hole should now be drilled through the tube into this end--so that steam
may enter the ring in both directions-and plugged externally.
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