II. Learn the following words

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Modern telegraph systems are based upon the fact that electric current will flow through a wire which forms a circuit. The circuit can be broken to interrupt the flow of electricity and closed again for a short time by a key to send an electrical impulse through the circuit. Here the wire is not joined all the way round, but the circuit is complete because electricity will flow through the wire and back through the earth. This is called an " earth return". When the key is pressed the circuit is closed. Electricity from the flows through the wire and earth circuit and causes the buzzer to make a noise. Using the Morse code, or some other code, messages can be sent in this way.

Men wanted to increase the speed of sending messages. They knew that current flows very quickly along a wire, much more quickly than even the fastest operator can use a machine to send messages by hand. One way of getting higher speeds was to use the fact that current could flow one way round the circuit, or the other.

A flow in one direction could be the same as a dash in Morse, and a flow in the other could be the same as a dot. The system is called " double current working". It is faster than normal, or " single", current working. A dash or a dot is signaled for the same length of time, but in the ordinary Morse code a dash in signaled by an impulse lasting three times as long as a dot.

A machine was invented to receive messages by making a mark on a paper tape with dots on one side of a line and dashes on the other. Through the years other machines were invented, both to receive and transmit messages. These machines are called teleprinters. They use a code called the International 5-unit teleprinter code instead of the Morse code. It is called a " 5-unit" code because a letter or a figure is made up of five impulses of electrical current, either positive or negative — three positive, two negative; one positive, four negative; and so on. All the letters are the same length in time. This is quite different from the Morse code, where E is the shortest and " nought" the longest.

A teleprinter is really a machine which sends a typewriter message over a telegraph circuit. In fact, it looks like a typewriter and has the same keyboard. Every time one of the keys on the keyboard is pressed it sends the five impulses that make one letter or figure. It also sends a " start" impulse. This sets the teleprinter (or any number of them that are connected) al a receiving station ready to print the letter that follows immediately. A " stop" impulse brings all the teleprinters to rest after each letter or figure.

In the early days of telegraphy direct current was used. This is current which flows along a line in one direction at a time. Most modern telegraph systems use alternating current. This is current which flows backwards and forwards along the wire. Alternating current is usually supplied to people's houses at 50 cycles per second. This means that every second the current reverses its direction 100 times. Alternating current used to send telegraph messages reverses its direction much more frequently than this, it works on much higher frequencies. These frequencies range from 420 cycles per second to 3, 180 cycles per second.

As the number of people who wanted to send telegraph messages increased, more and more messages had to travel along the same telegraph line. Using alternating current, one line can be used to send many messages at the same time if each message is sent out at a different frequency. A transmitting machine sends a message at one of the high frequencies mentioned, say 420 cycles. A receiving machine also working on a current alternating at 420 cycles per second will get the message. But it will not get a message being transmitted at the next highest frequency (which is 540 cycles per second) by another machine, even though this message is traveling along the same line at the same time. To get this message another receiving machine, working on current alternating at 540 cycles, must be used.