The Electrical experimenter

May, 1917 THE ELECTRICAL EXPERIMENTER 25
Simple Apparatus Comprising Box, Candle
and Two Lamp Chimneys for Demonstrating
the Principle of Ventilation.
and become liglitcr and the heavier air at
the top of the room will fall, forcing the
lighter air upward. Thus it is that the air
near the ceiling is always warmer than
that near the floor. This shows the necessity
of opening a window at both the
top and the bottom for best ventilation.
EXPERI.MFXT 19—
Fig. 15 .shows very simple apparatus
which can he made with practically any
material available, for demonstrating the
behavior of air near a source of heat.
C, is a box thru which holes have been
cut to admit tul)es (or glass lamp chimneys)
B. 'A is a lighted candle. The
arrows show the direction of the current
of air.
EXPERIMEXT ^0—
An interesting experiment giving surprising
results and having a simple explanation
can be performed by the use of
a spool and a visiting card. (If no visiting
card is available, the ace from a poker
deck whicli you may have "up your sleeve,"
will do very well.) Place the card up
against the bottom of the spool as in Fig.
16-A and the mouth against t e top of the
spool. Blow vigorously and then let go
of the card. One would naturally expect
that blowing against the card would blow
Experimental Physics
By JOHN J. FURIA, A. B., M. A.
Inatruc tor in Physics and Science Master, Riverdale Country School
LESSON FOUR
and was sucked out; i.e., there is no air
GASES ANU THE ATMOSPHERE
pressure in the tube and the air pressure
(Concluded)
outside can hold up the mercury to a level
rxpands when heated and bc-
of about thirty inches. If now the seal
AIRtnmcs lighter in weight. If we
is broken the air rushes in and the mer-
liave a confined bodv of air such
cury in the tube falls into the cup. (See
as in a room, for example, and
Fig. 18-.\.) The pressure of tlie atmos-
there is a source of heat in the
phere changes from place to place and
room, the air near the source will expand
from time to time. It is, therefore, im-
C/osed/
enc/
Fig- '6
Closed/
end
A Glass Tube, Sealed at One End and Filled
with IVIerctiry, Will Support a 30-inch Column
of Mercury. Owing to Atmospheric Pressure
Acting Against a Vacuum. The Principle
of the Barometer.
it away whereas actually the card stays
fast, close to the, bottom of the spool.
Sometimes, when the conditions are not
just right, the card slides off perpendicular
to the direction in which one blows, but
to avoid this a pin should be stuck thru
the card's center and tlien into the hole in
the spool (care being taken not to stick
it into the wood of the spool). Fig. 16-B
shows diagrammatically what happens. The
air from the moutli passes down the hole
in the spool and out along the upper surface
of the card. It is a well-known fact
that the pressure is greatest where the
speed is least and vice versa. The air
underneath the card is practically still,
while that just alK>ve the card is in rapid
motion, and hence the pressure against the
card from beneath is greater than that
from above. Hence the card tends to get
as close to the spool as possible and does
not fall.
EXPERIMENT 21—
In the First Lesson we learned that at
any depth in a liquid there is a pressure
due to the weight of the liquid above that
depth. We also learned that air has
weight and consequently we conclude that
the air (at the surface of the earth) has
pressure due to the weight of the air above
it. The higher up we go, the less air
there is above us and hence the pressure
is less. If one sucks in at the stem of a
pipe (see Fig. 17) at the bowl of which
is stretched a piece of sheet elastic, the
pressure of the air above it pushes the
elastic down. Suction is not a mysterious
force: it is simply a removal of the air
from one side so that the pressure from
Removing the Air V^ithin a Pipe Bowl by
Sucl