Part I-How a Crystal Set Works
| ON A CALM day you can hear a man shouting a quarter of a mile away.
But radio can carry his voice for hundreds or even thousands of miles.
It does this by carrying the sound waves on the back of the radio waves.
You can see something like a sound wave if you drop a stone into a still pond. But sQund waves travel at a speed of about 760 miles an hour, and the wave crests go by at the rate of about 4,000 every second. (In scientific language we say "they have a frequency of about 4,000 cycles per second.")
They travel at the speed of light, about 186,000 miles a second. The wave crests go by at the rate of about thirty million every second (a frequency of about thirty million cycles per second). The radio transmitter gives the sound waves a piggy-back
ride on the radio waves by making the outside shape of the radio wave follow
the shape of the sound wave:
The result is called a "modulated wave."
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The modulated wave is picked up by the AERIAL of
your crystal set. The aerial is usually a long wire held up on posts but
separated from the posts by insulators that prevent electricity from passing.
Now your crystal set has to do two things. First it must pick the right wave out of all the radio waves that are coming in to the aerial. Then it must turn the modulated wave into a sound wave that you can hear. Each radio programme is sent out on a different frequency, so in order to find the programme you want you must tune in on a particular frequency. Your crystal set has two parts which allow you to do this. They are called the INDUCTOR and the VARIABLE CAPACITOR. 
The inductor is simply
a length of wire wound SUPPORTS on a tube. The variable capacitor looks like this: As you can see, it has a set of fixed metal plates and a set of movable plates sandwiched between them. By twisting a knob you can alter the amount of overlap between the two sets of plates and "vary the capacity." That means you alter the amount of electricity it will hold, and so change the frequency that the inductor and capacitor will accept. You can do an easy experiment to show this. If you partly fill a narrow-necked bottle with water and then blow across the neck of the bottle, you will hear a note. No matter how hard you blow you cannot change the note-but if you put in some more water you will find that the note becomes higher in pitch. Find out what happens to the pitch of the note when you empty all the water out. |
| As soon as you have your signal, you can start to
get the sound from it -- but what about all the other signals the aerial
picked up? You can get rid of them by sending them to earth. A crystal
set needs a good EARTH, or connection to the ground.
Your crystal set has two parts for turning radio signals into sounds. They are the CRYSTAL DETECTOR and the EARPHONES.
When the modulated wave was picked up by the aerial it came down
the wire to your crystal set as an "alternating current" -- that is to
say, a current which flows first in one direction and then in the other.
Alternating current in a piece of wire is represented by |
When the alternating current reaches the crystal
detector it gets sliced in half, for the crystal detector will Only allow
current to flow in one direction (let us say from left to right). So after
it has been through the crystal the signal looks like this:
This "half signal" is passed to the earphones. Each
earphone contains a magnet with a coil of wire round it. As the pulsating
current passes through this coil it increases and decreases the strength
of the magnet.
But remember that the current is pulsating thirty
million times a second. The diaphragm cannot possibly move at this speed,
so it takes the average. We can draw the average half-way tip each pulse:
As you can see, it is the same shape as a sound wave.
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Part Il-Building an Inexpensive Crystal Set
| FIRST OF ALL, here is a list of the parts you will need. Any good wireless
shop will supply them quite cheaply, though you may save money if you buy
the earphones second-hand from a surplus store.
.ooo3 uF (microfarad) variable capacitor
|
Any radio set looks better in a neat container. A cabinet for
your crystal set can easily be made from a square foot of either hardboard
or three-ply wood. Cut the parts required -- that is, two sides,
back, front, top and base-using the measurements shown on the next page.
Then drill the five holes in their right positions.
Glue the sides together, and slip two or three strong elastic bands round this frame to hold it firmly in place. Glue the top in position and weight it down with something fairly heavy until the glue is dry. When the glue is thoroughly dry, add the small corner supports which will later take the screws for securing the base. |
Cut out a semi-circular piece of paper, about three and a half inches
in diameter, and paste it on the top of your cabinet just above the centre
hole. This is the tuning dial on which you will mark the positions of the
stations you receive.
The final appearance of the cabinet will be improved
by a coat of varnish or wax polish.
You are now ready to make the crystal set itself.
STEP 1 -- MAKING AND FITTING THE INDUCTOR
All you need for the inductor is a core and some
wire (see list of parts). The core must be fairly firm so that the wire
can be wrapped tightly round it, and the cardboard centre of a toilet roll
is ideal for this purpose. Trim it to the right length for the cabinet
and cut a small notch at each end to allow your screwdriver to get inside
when you fasten the inductor in position.
Make two small holes near one end of the core, and
secure the end of the wire by passing it into one 1 hole and out of the
other. Draw out enough wire to make a connection later.
Wind the wire neatly on to the core, taking about
sixty turns and making sure they do not overlap. (By experimenting with
the number of turns on the coil you may find you can vary the stations
you receive.) Make two holes at the other end of the core, and secure the
wire by threading it through them as before. Wrap a layer of transparent
tape round the wire coil to hold it in position.
Now fasten your inductor in the cabinet with two
small screws as shown in the diagram.
STEP 2 -- FITTING THE CAPACITOR
Pass the spindle of the variable capacitor through
its hole in the top of the cabinet and bolt it from the outside. Fit the
tuning knob on top of the spindle.
STEP 3 -- FITTING FOUR SOCKETS
Pass each socket through the hole provided for it,
and bolt firmly from inside.
STEP 4 -- SOLDERING THE CONNECTIONS
Join up the various connections, soldering each
joint as shown in the diagram. You can now screw the base on to the cabinet.
STEP 5 -- Solder plugs on to earphones.
STEP 6 -- Solder plugs on to aerial and earth leads (see Notes below).
STEP 7 -- Insert aerial, earth and earphone plugs in their sockets,
and adjust the tuning knob until a signal is heard. Mark the positions
of the stations on your paper dial.
Good luck and good listening!
NOTES ON AERIAL AND EARTH
The old-fashioned sort of crystal detector was a piece of galena which
you touched with a wire called a "cat's whisker." After much scratching
you found a "working point" on the crystal and heard a signal of some kind.
Nowadays we use a "germanium diode" set permanently in plastic so that
it is always on a working point.
when it is flowing from left to right, and by 
when
it is flowing from right to left in the wire. Thus a modulated signal,
lasting for some time, can be drawn as before:
The magnet is arranged so that it attracts a thin metal plate called a
"diaphragm." The diaphragm will try to move backwards and forwards as the
magnet gets Stronger and weaker.
