Before we can tell you how the telephone
works, we must say a little about sound. Sound waves consist of small
and rapid changes in air pressure, a number of changes per second is
called the frequency of the sound to reproduce sound. We need a diaphragmhere. The boy is making one with a piece
of tissue paper and a comb. The paper is the diaphragm. And like all diaphragms,
it responds to the changes of pressure to transmit sound. As in the telephone, we must
connect to diaphragms together. Here we are using a taut string. When sound waves hit one
of the diaphragms, it vibrates. The vibrations travel along the string to the other diaphragm,
which reproduces the sound. This method has many obvious snags, but the basic principles
apply to all telephones. In 1874, the inventor of the telephone, Alexander Graham Bell, wrote,
If I could make a current of electricity vary in intensity,
precisely as during the reproduction of sound, I should be able
to transmit speech telegraph. This is a model of part of the apparatus
with which Bell made his words come true. It is connected to another identical
instrument by a pair of wires. Let's see how it is made and how it works. Firstly, there is a diaphragm
made of parchment. It is attached to a soft iron strip mounted close to an electromagnet
which has a permanent magnet core. The instrument could be used as
either a microphone or as a receiver. It works by generating current from the movement of the strip
in the field of the permanent magnet. No battery was needed,
but the current was weak and the system could not be used to speak
over long distances. In the modern television,
we have overcome this difficulty. So let's see what changes have
been made over the years. This is a diagram of a modern microphone. The first point to note is
that it is powered by a battery. These two wires carry direct current
from the battery and also connect the microphone to the telephone
wires or lines circuit. The second point to note is that the circuit is completed through a
container filled with granules of carbon. We call the instrument
a carbon granule microphone. This is the diaphragm. It is made of metal and is attached
to one end of the container. Now let's see what happens when
a sound wave hits the diaphragm. The wave pushes the diaphragm in and compresses the granules
into closer contact. This decreases the resistance
and allows more current to flow. The diaphragm then falls back
and the granules move apart again, increasing the resistance
and reducing the current. Let's see this again. When you are speaking,
these changes occur very rapidly, the result is that the resistance
of the granules and consequently the strength
of the current there is in accordance with the frequency
of the sound received by the microphone. In this way, the sound waves are turned into a rapidly fluctuating current,
which you see here on the meter. It may help you to think of this current as made up of two parts, the steady
current from the battery like this and a fluctuating current
due to the sound waves. It is only the fluctuating part, which we call the speech current, that carries your words along the line
to the receiver in the receiver. The current is turned back into sound waves again so that your
speech can be heard. The diaphragm is attached to an iron bar which is pivoted and mounted close
to the poles of an electromagnet. The bar is called a rocking armature and we use permanent magnetism
in the electromagnet. In principle, the modern receiver is not unlike Bell's original,
but it is very much more sensitive. Now let's see what happens when the speech
current arrives at the receiver. Each fluctuation of the current there is the attraction of the armature
making it and the diaphragm vibrate at the same frequency,
so reproducing the original sound. That is what happens when you speak
to your friend on the telephone. So far, however, we have said nothing about how one
telephone is connected to another. Let's look into this now. Here is a microphone. Remember, it needs direct current
from a battery to make it work. Now, the receiver, one might expect to connect the microphone, battery
and receiver in series so that they can speak to be
the system works, but it is wasteful because the battery has
to do more work than is really necessary. The speed current is weakened and we
cannot speak over very long distances. For this reason, we connect our telephone system in a different way as we can work
the receiver from the speed current loan. We removed the steady current by connecting the microphone
through a transformer. Transformer's help us connect telephones because they will pass a fluctuating
current, but stop any steady current by placing the transformer close
to the microphone and using it to step up the low input voltage,
we can send messages along the line at a higher voltage and therefore
over greater distances. Here to show how they are connected,
we have put the microphone and receiver close together, but in reality,
of course, there may be many miles apart with this circuit.
We can only speak in one direction to speak in both directions. We could use another
identical circuit like this. This method is rather wasteful, however,
because there is no reason why we cannot use only one pair of wires by rearranging
the connections like this. The microphone and receiver are now coupled together through
the transformers at each end. We also need switches
or telephone hooks to break the circuit and save the batteries
when they are not in use. Lastly, we need bells and they are connected so that they can be
rung even though the switches are open. Current to ring the bells is supplied from the telephone exchange,
which also contains all the equipment needed to connect one
telephone with another. Here we have shown all the parts spread out,
but in reality, apart from the battery, they are box together in the modern
telephones with which we are familiar. We can now see what happens
when a call is made. Suppose a wish is to telephone be first of all a must lift the handset. When he does this, the switch closes
and a signal is sent to the exchange. Meanwhile, direct current flows in the
microphone circuit ready for a to speak. We are assuming that A is
connected to a manual exchange. The signal stays on until the operator
answers A by plugging into his number. She now plugs into line B and makes B's bell ring by moving a P
when B answers by lifting her handset, the switch closes,
the bell stops ringing and the operator connects a direct to B,
you can now speak to your friend. One Where are you at both telephones direct
current is energizing the microphones. The speech current is transmitted through the transformers and along
the line like this. This then is a simple form of circuit
by which our telephones can be connected in busy places. Only one battery is used
and that is kept at the exchange. Of course, there are not just two
telephones, but many hundreds and indeed thousands, all connected
to the one exchange. The operations necessary to connect one
subscriber to another are performed at the telephone exchange, either manually
as we have seen, or automatically. In every case, a direct link is established
between the caller and the called. Today, we can speak to people on the other side of the world almost as easily as we
can to our friends around the corner. We can do so because of the fact that one diaphragm can be made to reproduce
the fluctuations of another.
