147.--Apparatus for illustrating an apparent
scientific paradox.]
Press the coin against the reel and blow hard through the tube. One would
expect the coin to fall; but, on the contrary, the harder you blow the
tighter will it stick, even if the reel be pointed downwards. Only when you
stop blowing will it fall to the floor.
This is a very interesting experiment, and will mystify onlookers who do
not understand the reason for the apparent paradox, which is this. The air
blown through the reel strikes a very limited part of the nearer side of
the halfpenny. In order to escape, it has to make a right-angle turn and
pass between coin and reel, and, while travelling in this direction, loses
most of its repulsive force. The result is that the total pressure on the
underside of the coin, plus the effect of gravity, is exactly balanced by
the atmospheric pressure on the outside, and the coin remains at that
distance from the reel which gives equilibrium of forces. When one stops
blowing, the air pressure on both sides is the same, and gravity makes the
coin fall away.
The function of the pins is merely to keep the halfpenny centred on the
hole.
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