[Connections], [Challenge], [Flash Fact], [Teacher's Notes]
WATCH THE LIGHT.
The first telephone was invented in Canada. In 1874, Alexander Graham Bell got thee idea for a "talking telegraph" system. Bell built the first working telephone in 1876. Five months later, he sent the first long distance call, from Branford to Paris, Ont ario.
Bell's telephone contained a tiny cup filled with bits of carbon. This cup was the first transmitter disc. By controlling electric current, it helped to make voices clear over the telephone.
The first Canadian to install a telephone was Prime Minister Alexander Mackenzie. Bell had a phone in his laboratory, too. Whenever he arrived, he used to cover it with a towel so that its ringing would not disturb him!
Canada's first telephone exchange opened in Hamilton in 1878, with seven lines and fifty customers. Now, Canadians use telephones more than anyone else in the world. Obviously, not all Canadians feel the way Bell did.
A modern telephone has a different way of changing the current in a telephone wire. When you speak into a telephone, your voice waves hit a metal plate called a diaphragm. The diaphragm is attached to a magnet. A coil of wire is wrapped around the magnet, but not touching it. When your voice moves the diaphragm, it moves the magnet inside the coil to create a small electric current. So in this way, the sound waves are changed into electric current.
Different sound waves cause different amounts of current. A loud voice moves the diaphragm more, which moves the magnet more, which makes a stronger current. As your voice changes, different amounts of current are sent along the telephone wire as electric signals.
When the signals arrive at another telephone, it all happens in reverse.The changing current travels through the coil and its magnetism moves the magnet inside. The magnet vibrates the diaphragm which makes sound waves. The electric signals have been chan ged back into sound waves and someone hears your voice.
Not only telephones, but all of modern communications depends on electric signals. Sound and pictures are changed into signals and sent along wires. At the other end of the wires, the signals are received and changed back into sound and pictures. Sending signals along wires works fine if you don't have to send them too far.
When wires cannot be used, the signals are sent through the air by electromagnetic waves. These waves travel in straight lines from a sending antenna to a receiving antenna. If the distance is far enough that the waves have to travel over the curve of the earth, a series of antenna towers pass the signal along. However, satellites are often used to send long distance signals. Canada was the first country to use satellites to pass along communication signals.
Sound travels through the air like waves travel through the water. You cannot see sound waves, but your ears can sense them.
You know that carbon resists electrons. When you loose group of carbon bits is squeezed, its resistance becomes greater. So fewer electrons can flow as long as the carbon is being squeezed. In this case, it was the sound waves of your voice that did the s queezing. When you spoke loudly into the transmitter disc, the carbon bits were tightly squeezed. So, the resistance was greater and the light was dim. When you spoke softly, the carbon bits were less squeezed and the resistance was less. So, the light w as bright. The disc was acting like a sliding resistor. It was changing the amount of electricity flowing through the circuit. The vibrations sent by your voice were the signals that operated the transmitter disc.
Demonstrate sound waves by stretching a balloon over the top of an empty jar. Sprinkle some grains of sugar or salt on the balloon. Then, while students observe the grains closely, clap your hands loudly. The grains will "dance" as you clap. Sound waves m ake eardrums vibrate the same way.