HYDRAULIC
AND PNEUMATIC ENGINEERING 101
THE
WATER BAROMETER
Any liquid
can be used in a barometer but
liquids lighter than mercury
require longer tubes. This is true of
the water barometer. Mercury is
13.6 times as heavy as water and
since the atmosphere supports a
column of mercury 30 in. high it will
support a column of water 13.6 x 30 =
408 in. high, that is, a column
408/12 = 34 feet high.
Otto
von Guericke, the inventor of the
Magdeburg hemispheres, made a water
barometer in 1650, and had it so
arranged that the top of the tube
stuck up through the roof of his
house. He had a small wooden figure
floating on the water in the tube and
in fine weather, when the water
column rose, the figure rose above
the roof, but in bad weather the
figure retired from sight. This
frightened and mystified his
neighbors very much and they accused
him of being in league with the evil
one.
102
HYDRAULIC AND PNEUMATIC ENGINEERING
EXPERIMENT
No. 48
To
show that the
vertical height to which the atmosphere will lift water
in
a tube
is
independent of the length or slant of the tube.
Make
the experiments (1), (2) and (3), Fig.
134. Suck air out through the upper coupling
on the tee and close the clip.
Is
the vertical height of the water in one
tube above the water in the tumbler
always the same as that in the other?
Make experiments of your
own.
EXPERIMENT
No. 49
To
show that the
height to which the atmosphere
will
lift water in a tube is independent
of the size or shape of the tube and
of the water surface outside
the
tube.
Make the
experiments (1),
(2), (3) and (4) Fig. 135. Is the
height of the water always the same in
the two tubes?
Make
experiments of your own.
HYDRAULIC
AND PNEUMATIC ENGINEERING 103
EXPERIMENT
No. 50
To show that
the atmosphere lifts
heavy salt water to a less
height,
and
light gasoline to
a
greater height, than it lifts
fresh
water.
Make the
experiments illustrated
in Fig. 136.
EXPERIMENT
No. 51
To
show that the
atmosphere will lift
weights.
Make
the experiments illustrated in Fig.
137.
104
HYDRAULIC AND PNEUMATIC ENGINEERING
EXPERIMENT
No. 52
To show that
the
atmosphere will lift 15 lbs. per
sq.
in. but no more.
The
plungers have an area of 3/10 sq. in.
If then, the atmosphere will lift 15
lbs. on 1 sq. in., it will lift 3/10 x
15 = 4 1/ 2 lbs. on 3/10 sq. in.
Soap
the plungers, have water between
them but no air, pour an inch of
water above the upper plunger to make
it air-tight, attach a pail weighing
less than 4 1/2 lbs. to the lower plunger,
Fig. 138 and raise the upper plunger.
Does the atmosphere lift the lower
plunger and weight?
Add
water to the pail until the total
weight is 4 1/2 lbs. and raise the upper
plunger. Do you find that the atmosphere
does not lift the lower plunger? It
does not do so because the
atmospheric pressure on 3/10 sq. in.
cannot lift 4 1/2 lbs. and also
overcome
the friction.
Hold
the
upper plunger and lift the tube. Does
the atmosphere now lift 4 1/2 lbs.
weight? It does so because the
friction helps it in this case.
HYDRAULIC
AND PNEUMATIC ENGINEERING 105
Repeat
with the water and pail weighing 6 lbs. Do you find that the
atmospheric pressure on 3/10 sq. in. will not lift 6 lbs. even
with
the
help of the friction.
You
have shown here roughly that the atmospheric pressure on 3/10
sq.
in. will lift 4 1/2 lbs. but no more. This shows that the
atmospheric
pressure
on 1 sq. in. will lift 4.5 x 10/3 = 15 lbs. but no more.
Make
your own experiments.
AIR-LIFT
PUMPS
The
air-lift pump,
Fig. 139, is operated by compressed
air. It consists of two pipes one inside
the other, both open at the bottom
and without valves. The pump is at
least half-submerged,
that is, the bottom is at
least as far below the surface of the water in
the well as the top is above it.
The
air which is compressed in the storage tank passes into
the outer pipe of the pump, forces the water down to
the bottom of the inner pipe, and forces the water in the
inner pipe up into the tank. After the first lot of water
has been forced out of the inner pipe the pump settles
down to its regular operation which is as follows. Compressed
air from the outer pipe enters the inner pipe, the
pressure in the outer pipe is thereby lowered and the water
rises in the outer pipe above the bottom of the inner
pipe, more compressed air comes from the tank and forces
the water down in the outer pipe but up in the inner
pipe. This operation takes place over and over again rapidly,
and
alternate layers of air and water are forced up the inner pipe
as shown
in Fig. 139. The water thus flows from the inner pipe into the
tank
in spurts as you will show in your next experiment.
Another
form of air-lift pump is illustrated in Fig. 140. Here the air
enters through the inner pipe and the mixture of water and air
is
106
HYDRAULIC AND PNEUMATIC ENGINEERING
forced
out through the outer pipe. The water comes out as a
continuous heavy
spray because the air is mixed with the water in bubbles
rather than
in layers.
These
are called
air-lift pumps but the water is not raised by the air
pressure. It is raised by the weight of the water in the well
outside
the pump, because the water rising in the pump is really a
mixture of
air and water and is lighter than a water column of the same
height.
You
can illustrate this by means of experiments
shown in Fig. 141. In (1) both sides
of the U tube are filled with water
and you know from your experiments
that the water will be at the same
level in both sides. In (II) one side
is filled with kerosene oil which is only
8/10 as heavy as water, and you know
that a column of water 8 in. high will
support a column of oil 10 in high. Similarly
in (III) a depth of water of 8 inches
will support a column of oil 10 inches
high. If now the oil in (III) were replaced
by a mixture of air ond water which
was only 1/2 as heavy as water, you
can see that the 8 inch depth of water
would support a column of the mixture
16 inches high, and so on.
The
bottom of the air-lift pump is always
placed at least as far below the sur-
HYDRAULIC
AND PNEUMATIC ENGINEERING 107
face
of the water as the top is above, and the water outside the
pump lifts
the lighter mixture of air and water to the top. You will
illustrate
this
in the next experiment.
EXPERIMENT
No. 53
To
make and operate
two air-lift pumps.
Make an
air-lift pump. (1) Fig. 142. Use a quart
sealer to represent the well, fill it to the
top with water, and insert the air-lift pump
until it is half submerged, that is, until the
water in the sealer is at a point half way between
the bottom of the wide tube and the top
of the elbow of the discharge pipe.
Force
air in through the hose and
observe what takes place near the
bottom of the pump.
Do
you observe that the water level
in the pump moves alternately down
below the end of the discharge
pipe and then up above it, and
that alternately water and air
are forced up the discharge pipe?
Do you observe further
that when you force air in at
just the right rate the pump works
steadily and the water comes
up the discharge pipe in
108
HYDRAULIC AND PNEUMATIC ENGINEERING
spurts
at regular intervals.
In
the other type of air-lift pump the compressed air passes down
the
inside pipe and the mixed air and water move up the other
pipe.
Make
a pump of this kind, (2) Fig. 142 and blow air in through the
inside
pipe.
Do you find that air
and water are forced up over the top of the outside
pipe?
Repeat the experiment
with the pump deeper in the water.
Do
you find that it works better the deeper it is in the water?
LAWS
WHICH APPLY TO GASES
PASCAL'S LAW
In
the remaining pages of this book you will study three laws
which apply
to gases and you will illustrate many practical applications
of these
laws. They are Pascal's law, Archimedes' law, and Boyle's law.
You
will begin with Pascal's law.
You
learned on pages 49, 50 and 51, Pascal's law which states one
property
of liquids; namely, pressure exerted on a liquid is
transmitted by
the liquid equally and undiminished in all directions. This
law also
states
a property of gases as follows: pressure
applied to a gas is transmitted
by the gas equally and undiminished in all directions.
You
are very familiar with one application of this law, namely in
the
pneumatic
tire. The air in a bicycle or automobile tire exerts pressure
outward
equally at every part of the tire.
EXPERIMENT
No. 54
To illustrate
Pascal's law as it applies to gases.
Shove
the plunger in (1) Fig. 143, down, and feel the air at the
nozzles. Are
the pressures equal?
Blow a
soap bubble (2). Is it a perfect sphere? This shows that the
air exerts pressure equally in all directions against the
inside of the
bubble.
Make a three legged
siphon filled with air (3), place two legs in tumblers
of water, place the third leg in the wide tube partly filled
with
water, and raise and lower the wide tube.
HYDRAULIC
AND PNEUMATIC ENGINEERING 109
The
water in the wide tube exerts pressure on the air in the third
leg.
Is this pressure exerted equally and undiminUhed by the air,
that is,
is the water level in the three legs always at the same
distance below
the water outside?
Repeat
this with the apparatus (4). Is the result the same?
You
have here proved Pascal's law, namely that a gas transmits
pressure
equally and undiminished in all directions.
110
HYDRAULIC AND PNEUMATIC ENGINEERING
BALLOONS
AND THE BUOYANT FORCE OF AIR
The Law
of Archimedes applied to Gases
Balloons
float in air and this fact is due to a property of air which
is
expressed by the law of Archimedes.
You
have already made experiments on this law with liquids and you
have
shown that t
he buoyant
force of a liquid on a body is equal to the weight
of the liquid displaced by the body. This is the
law of Archimedes
as it applies to liquids.
The
law of Archimedes
in regard to gases is: t
he
buoyant force of a gas
on
a body is equal to the weight of the gas displaced by the
body.
How
the Total Lift of a Balloon is Calculated
The
weight of air is about 1 1/4 ounces per cubic foot at ordinary
temperatures
and at the surface of the earth. If then a balloon displaces
1,000,000
cubic feet of air, its total lift or buoyancy is 5/4 x
1,000,000 =
1,250,000
ounces = 1,250,000/16 lbs. = 78,125 ibs. and so on. The useful
load
a balloon can lift is its total lift minus the weight of the
envelope,
of
the gas in the envelope, of the cars, of the engines, and of
the fuel.
In
Fig. 145 we show the relative strengths in dirigible balloons
of
Germany,
France and Great Britain at the beginning of the war. Britain
HYDRAULIC
AND PNEUMATIC ENGINEERING 111
and
France built many dirigibles during the war and one of the
latest built
by Britain displaces 1,600,000 cubic feet of air. Its total
lift
therefore
is 1,600,000/16 x 5/4 = 125,000
lbs.
The balloon is filled
with hydrogen which weighs about 1/14 as much as
air, and therefore 1/14 of the total lift is used up in
lifting the
hydrogen
gas. The weight of the hydrogen is 125,000/14 = 8928 lbs.
Hydrogen
gas has been used in balloons because it is the lightest gas
known. It has one great drawback, however, in that it burns
very
readily.
There is another gas called helium which is twice as heavy as
hydrogen
but which has the great advantage that it does not burn.
Before
the war helium was very expensive but during the war it was
found
that the helium which occurs in some of the natural gases of
the United
States could be separated at a reasonable cost. It is expected
that
the dirigibles of the future will be filled with helium, and
since it
does
not burn, it will be possible to put the engines in a room
inside the
balloon as shown in Fig. 146.
112
HYDRAULIC AND PNEUMATIC ENGINEERING
Although
helium is twice as heavy as hydrogen its lifting power is only
1/13 less because the lift of a balloon depends primarily on
the weight
of air displaced. You can show this as follows :
If
a balloon displaces 140,000 lbs. of air and it is filled with
hydrogen,
it holds 140,000/14 = 10,000 lbs. of
hydrogen, since hydrogen weighs 1/14 as
much as air.
If the balloon
is filled with helium it holds 140,000/7 = 20,000 lbs. of
helium,
since helium weighs 1/7 as much as air.
The
lift minus the weight of hydrogen = 140,000 - 10,000 = 130,000
lbs.
The
lift minus the weight of helium = 140,000 - 20,000 = 120,000
lbs.
That
is, the lift with helium is only 1/13 less.
EXPERIMENT
No. 55
To
illustrate the
buoyant force of air.
Blow a
soap
bubble with illuminating gas (1) Fig. 147. Does
the bubble rise?
HYDRAULIC
AND PNEUMATIC ENGINEERING 113
Blow
up a balloon with illuminating gas by means of the force pump
(2)
Fig. 147. Does the balloon rise?
The
bubble and balloon rise because they displace a greater weight
of
air than their own weight plus the weight of the gas in them.
EXPERIMENT
No. 56
To illustrate
the
buoyant force of air by means of a balloon filled with
hydrogen.
114
HYDRAULIC AND PNEUMATIC ENGINEERING
If
the metal zinc is placed in an acid, the metal is dissolved by
the acid
and hydrogen gas is produced. You can make hydrogen gas and
fill
the large balloon with it as follows.
Purchase
at a drug store 2 ounces of strong hydrochloric acid (also
called
muriatic acid) which should cost about 5 or 10 cents; also
purchase
at an electrical shop a zinc rod for a Leclanche battery,
which will
also cost about 5 or 10 cents, or purchase two zinc strips.
Pour
the acid into the bottle and add an
equal volume of water. This dilutes
the acid and slows up the production
of the hydrogen. If the hydrogen is
produced too fast it will bubble acid
up into the balloon.
Now
to
make hydrogen and to fill the
balloon, proceed as follows: Arrange
the bottle as shown in Fig. 148 and
attach the large balloon to the
elbow by means of a short piece
(about 1 1/2 in.) of a
stretched
rubber
band. When you have done
this
place the zinc rod or zinc strips
gently in the bottle, insert the
stopper at once, and allow the
hydrogen to fill the balloon. It will take about 5 minutes to
fill
the large balloon completely.
When
the balloon floats well in the air, slip it off the elbow with
its
stretched
rubber band. The band will contract and close the
balloon.
Now
release the balloon. Do you find that it floats up to the
ceiling?
Precautions.
Be very careful not to get any of the acid on your hands or
clothes. It will burn very bad holes if it does.
When
you are through empty out the liquid left in the bottle, as it
is
of no further use, and rinse the bottle and rod very
thoroughly with
water.
You must not use the zinc in
small pieces because it produces the hydrogen
too fast and makes the acid bubble up into the balloon. Use
the
zinc in the shape of a rod or strips.
HYDRAULIC
AND PNEUMATIC ENGINEERING 115
EXPERIMENT
No. 57
To shoot down
a
balloon.
We show in
Fig.
149 three views of a balloon shot down by means of
incendiary bullets. These bullets set the hydrogen on fire,
the
envelope
burns, and the car and machinery fall to the ground.
A
toy balloon filled with hydrogen as in the last experiment
floats up
to the ceiling. It will come down by itself in a few hours
because
the
hydrogen gradually passes out through the rubber.
If
you are in a hurry to get the balloon down, and if you have an
air
riflle, you can shoot a hole through the balloon : the
hydrogen will
then
escape and the balloon will fall at once. This method,
however, spoils
the balloon.
116
HYDRAULIC AND PNEUMATIC ENGINEERING
You
can shoot the balloon down with a syringe without
destroying it as shown in Fig. 150. The water
on the balloon will make its weight greater than
the buoyancy of the air displaced by the balloon
and this will bring it down.
If
you let the water evaporate, the balloon will rise
again because it again becomes lighter than the air
it displaces. You can then shoot it down again with
water.
EXPERIMENT No. 58
To
illustrate the buoyant force of a gas heavier than
air by means of a soap bubble filled with air.
Purchase
at a drug store one ounce of ether and pour
it into an empty 12 qt. pail, cover the pail with
a newspaper and allow it to stand for about 10
minutes.
The ether will
evaporate and produce ether gas. This
being heavier than air will remain in the bottom
of the pail and force the lighter air out at
the top.
Now
dip the end of the wide tube in the soap suds
and shake off the excess soapy water. Blow a
large bubble and detach it about 6 in. above the
bottom of the pail.
Do you
find that the soap bubble filled with air floats
on the heavy ether gas?
The
buoyant force of the ether gas is the weight
of this gas displaced by the bubble. This buoyant
force is sufficient to support the soap bubble
film and the air inside of it.
HYDRAULIC
AND PNEUMATIC ENGINEERING 117
COMPRESSED
AND EXPANDED GASES
BOYLE'S
LAW
You
will now illustrate Boyle's law and
then make a number of appliances which
make use of this law, namely, the air
brake, flame thrower, fire extinguisher,
air pump, bicycle pump, sand blast,
pneumatic paint brush, diving bell,
pneumatic caisson, and submarine air
supply.
Boyle's law is:
The volume of a gas
varies inversely
as the pressure on it.
This
is illustrated in Fig. 152. In (1)
the tube is full of air and the
pressure on the air is one atmosphere
because the tube is open to the
atmosphere. In (2) the pressure on
the air is 2 atmospheres and the
volume of the air is 1/2 what it was
in (1). In (3) the pressure on the air
is 3 atmospheres and 1/3 what is was
in (1) and so on.
In (4)
the air in the tube below the plunger is under 1 atmosphere
pressure
because the tube is open to the atmosphere. In (5) the tube is
closed, the plunger is raised until the pressure on the air is
1/2
atmosphere
and its volume is two times what it was in (4). In (6) the
plunger
is raised until the pressure on the air . in only 1/3 and its
volume
is three times what it was in (4).
These
illustrate Boyle's law.
118
HYDRAULIC AND PNEUMATIC ENGINEERING
Boyle's
law is usually illustrated by means of the apparatus
shown in Fig. 153. The glass tube A is closed at
the top and is partly filled with air, the second glass tube
B is open at the top, and the two tubes are connected
by a rubber tube filled with mercury.
The
mercury surfaces at the beginning are at the same
level, (1) Fig. 154, and since the pressure on the mercury
surface in B is 1 atmosphere, the pressure on
the air in A is also 1 atmosphere.
If
now B is raised until its mercury surface
is 30 in. above that in A, the air in
A is under 2 atmospheres pressure
and it is compressed to 1/2 its first
volume, (2).
If B is raised
until its mercury surface is 60
in. above that in A, the
air in A is under 3 atmospheres
pressure and it is compressed to 1/3
its first volume (3), and so on. If
on the other hand, B is lowered, (5),
until its mercury surface is 15 in.
HYDRAULIC
AND PNEUMATIC ENGINEERING 119
below
that in A, the air in A is under a
pressure of only 1/2 atmosphere and
it expands until its volume is 2
times its volume in (4).
If
B is lowered (6) until its mercury
surface is 20 in. below that in A,
the air in A is under a pressure of
only 1/3 atmosphere and it expands
until its volume is 3 times its volume
in (4), and so on.
Note: A
column of mercury 30 inches high
exerts a pressure equal to that of
one atmosphere. Similarly 15 in. =
1/2 atmosphere and 10 in. = 1/3 atmosphere.
EXPERIMENT
No. 59
To
illustrate
Boyle's law.
If you
have a
spring balance you can prove Boyle's law as follows: Use
the apparatus (1) Fig. 155 and compress the air to one half
its volume
as in (2). Is the average pull on the balance 4 1/2 lbs.?
Note:
Friction opposes the plunger when it is moving in, but it
helps
the plunger to remain in. You will find that it takes more
than 4
1/2 lbs. to compress the gas, but less than 4 1/2 lbs. to hold
it after
it is
compressed, the average is 4 l /2 lbs.
120
HYDRAULIC AND PNEUMATIC ENGINEERING
The
area of the plunger is 3/10 sq. in.,
therefore the pressure per square
inch is 4.5 x 10/3 = 15 lbs. or 1 atmosphere,
but the air on the outside
exerts a pressure of 1 atmosphere on
the plunger, therefore the total
pressure the plunger exerts on the
air in the tube is 1 + 1 = 2 atmospheres.
You have shown here that when
you double the pressure on a gas you
compress the gas to one half its
first volume.
To
show that when you halve the pressure
on a gas its volume doubles, use the apparatus
(3) Fig. 155.
Start with a
distance of 2 inches between the
plungers, (3) then pull up the spring
balance until the distance is 4
inches, (4). Is the average pull on
the balance 2 1/4 lbs.?
A
pull
of 2 1/4 lbs. on 3/10 sq.
in. is 2.25 x
10/3 = 7.5 lbs. per sq. in. or 1/2
atmosphere. Since the pull of the
balance is only 1/2 atmosphere, the
air in the tube must be exerting the
other 1/2 atmosphere.
HYDRAULIC
AND PNEUMATIC ENGINEERING 121
THE
AIR BRAKE
One
of the commonest applications of compressed air is in the air
brakes
on trains. The air compressor A, on the side of the engine
boiler,
is operated by steam from the boiler. It compresses air in the
large
tank B, on the locomotive, and this compressed air is carried
through
the train pipe under the cars to the air brake under each car.
The
air brake on each car consists of a triple valve F, an air
tank E and
a cylinder C containing a piston P. The brake beam is attached
to
D.
The operation of the air
brakes is as follows: Air is pumped into the locomotive
tank B until its pressure is about 75 tbs. per sq. in. This
compressed
air moves through the train pipe, through the triple valve F,
and into the car tanks E.
When
the train is running, the pressure in each car tank E is equal
to
that in the locomotive tank B ; but there is no air in the
cylinder C
and
the brakes are "off", because the spring holds the piston P in
the
position
shown.
When the engineer
puts "on" the brakes, he turns a lever which closes the
valve between B and the train pipe, and which at the same
time, lets
the
air out of the train pipe. When the air pressure in the train
pipe
122
HYDRAULIC AND PNEUMATIC ENGINEERING
decreases,
the triple valve shifts in such a way that compressed air
passes
from the tank E into the cylinder C; this compressed air
drives the
piston out with a pressure of 75 lbs. per sq. in. and puts the
brakes
"on."
When the engineer
wishes to take the brakes "off" again, he turns the lever
back. This closes the train pipe and at the same time allows
air to
flow from tank B through the train pipe to the triple valve F.
When the
pressure in the trian pipe increases, the triple valve shifts
back in
such
a way that it lets air pass from B into E, also it closes the
passage
from
E to C, and lets the air out of C. The spring then forces the
plunger
in and takes the brakes "off".
It
will be noticed that if the train should break in two by the
breaking
of
a coupling, the rubber air hose connection on the train pipe
is broken
and
the air is let out of the train pipe. This automatically sets
the
brakes
on each car and both parts of the train are brought to a
standstill.
You
will now make and operate an air brake and illustrate the
working
of the cylinder, triple valve, and air tank.
EXPERIMENT
No. 60
To
make and operate
an air brake and to illustrate the working
of
the
triple valve, cylinder, air tank, and train pipe.
Use
the apparatus as shown in Fig. 157,
open clip 1, and blow air into the
rubber tube.
Your mouth
here represents the compressor and air
tank on the locomotive, and while
you are blowing air into the tank E
you are representing the
conditions when the train is running
and the brakes are "off". You will
notice here that when clip 1 is open and 2 is closed
the triple valve is admitting air to the tank E, the cylinder
C is
open,
and the brakes are "off". Clips 1 and 2 represent the triple
valve. Now
close clip 1 and open clip 2. Do you observe that the
compressed air
in E forces the piston out? This is exactly what happens when
the
HYDRAULIC
AND PNEUMATIC ENGINEERING 123
engineer
puts the brakes "on". You will notice here that when clip 1 is
closed
and 2 is opened, the triple valve has closed the passage
between the
cylinder and train pipe, and has opened the pipe between E and
the
cylinder.
This is the condition when the brakes are "on".
If
you have a bicycle pump, use it instead of your mouth and pump
more
air into the tank E. You will then find that the piston is
driven out
with greater force.
At the
next opportunity examine the air brakes under a box car or
flat
car on a railway siding. Identify the air tank, cylinder,
piston rod
end,
the triple valve, and the train pipe. Notice that the outward
movement
of the piston rod, moves a lever, and that this lever in turn
sets the
brakes.
THE FLAME THROWER
You
have read of the flame throwers,
which were used during the war. You
will illustrate their action in the next
experiment.
A flame thrower
is a strong metal tank with a pipe
and nozzle leading from the bottom.
It contains a mixture of inflamable
oils in the lower part
and above this, hydrogen gas under
great pressure.
The tank is
carried on the back of the soldier,
as shown in Fig. 158, and when the
nozzle is opened the compressed
hydrogen drives the oil out with
great force. The oil is set on fire
by a pilot light just beneath the nozzle
and the moving stream becomes a
stream of flame or liquid fire.
124
HYDRAULIC AND PNEUMATIC ENGINEERING
EXPERIMENT
No. 61
To illustrate
the
action of the flame thrower.
It
is dangerous to illustrate the action of a flame thrower with
oil and
you will use water instead.
Arrange
the apparatus as shown in Fig. 159. To load the flame thrower,
place a clip on the rubber tube, put a stopper and elbow on
the end,
insert the stopper into a water faucet, open the faucet
gently, open
the
clip, and allow water to enter the bottle until it is one half
full,
then
close the clip.
The flame
thrower is now loaded; the water represents the oil and the
compressed air represents the compressed hydrogen.
Now
to use the flame thrower; replace the elbow and stopper at the
end
of the rubber tube by a nozzle, turn the bottle upside down,
point the
nozzle at the thing you wish to hit, and open the clip.
THE
FIRE EXTINGUISHER
The
common household fire extinguisher,
Fig. 160, is a strong brass cylinder
with a short piece of hose attached
at the top; this hose and its nozzle
are open at all times. The
extinguisher is charged as follows:
In the bottom there is a solution of
1 1/2 lbs. of sodium carbonate (Na2CO3)
and 2 l/2 gal. of water, and
above this there is an 8 oz. bottle
containing 4 ozs. of strong sulphuric
acid (H2SO4). This
bottle is fitted
with a loose lead stopper which falls
out when the extinguisher is turned
upside down.
To
use the extinguisher, you carry it
right side up
to the fire, then turn
it upside down
and direct the stream of water and gas upon
the fire by means of the short hose. Use all of
the water, because
once you have turned the extinguisher upside down, the liquids
are
mixed, and the extinguisher is of no further use until you
have re-
HYDRAULIC
AND PNEUMATIC ENGINEERING 125
charged
it. You should do this at once in order to be prepared again
for
a fire. In recharging you should follow the directions printed
on the
case.
The action which takes place
in the extinguisher is as follows: when you
turn it upside down, the sulphuric acid and sodium carbonate
react
chemically
and produce a large quantity of carbon dioxide gas. The volume
of carbon dioxide gas produced is much greater than the volume
of
the cylinder and therefore the gas exerts pressure on the
water and
drives
it out with great force.
The
fire is extinguished, partly by the water, and partly by the
gas. It
seems strange to speak of putting out a fire by means of gas,
but
carbon
dioxide gas has three properties which make it very valuable
for
this purpose: first, it does not burn; second, it does not
support
combustion,
that is, it does not help other things to burn; third, it is
heavier
than air. The carbon dioxide gas surrounds the
fire and smothers it, because it does not
support combustion
and it takes the place of the air which does
support
combustion.
EXPERIMENT
No. 62
To make and
operate
a fire extinguisher.
You will not use
strong sulphuric acid because it burns
practically everything it touches, but instead you
will use a dilute acid, vinegar; also you will use
baking soda which is sodium carbonate. Arrange
the apparatus as shown in Fig. 161. Pour six
tablespoonsful of vinegar into the
bottle, fill the bottle four fifths full
of water and shake, measure out one
level tablespoonful of baking soda
and place it on a piece of paper
ready for use.
Now to use
the fire extinguisher, go outside and
let one experimenter hold the
bottle and stopper while the other
holds the baking soda and the nozzle.
Dump the soda into the bottle, put in
the stopper quickly and hold it very
The
Science Notebook
