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Telephone


Grandad told Junior that he could look more closely at the old telephone.
"It looks very old," said Junior.
"No, it's quite a new one," replied Grandad. "It was made in 1954; almost new!"

Grandad's telephone

"1954 was a long time before I was born", commented Junior. "It was more than 50 years ago!"
"Well, I suppose it was, "said Grandad,  "But it still works just as well as when it was new. Our first telephone was a sort of wooden box screwed to the wall. It had an arm sticking out of the side where the earpiece went, and a carbon microphone at the front. When you lifted the earpiece, it connected the 'phone to the exchange, then you could talk to the operator."
"What is the 'operator'?" asked Junior.
"That was the person at the telephone exchange who you asked for the number to which you wanted to connect. She would then put the plugs in the right places t o connect things up."

Junior had been fiddling with the old telephone.
"I can turn this round bit on the front!" he said.


Telephone dial

"That's the dial", said Grandad. "If you put your finger in the hole where the first digit of your number is, and turn the dial clockwise until your finger hits the stop, you can dial that digit.  When you let go, the dial turns back again and makes and breaks the line circuit as many times as the digit you chose."
"It goes back quite slowly, and makes a sort of ticking noise", observed Junior.
"Do you remember that clockwork gramophone, with the speed control governor?" asked Grandad.
"Yes, it stopped the spring unwinding all at once and made the record play at the right speed," replied Junior.

"Well, behind the telephone dial is a governor. It works like the gramophone one. When you turn the dial with your finger, it winds up a spring. When you let go, the dial turns back slowly, because of the governor. As it goes back it operates a switch once for every hole that it was wound up. That signals to the exchange to tell it what digit you dialled. If you do it for each digit of the number that you want, the exchange operates lots of circuits that connect the phone to the other end," said Grandad.

Inside the Telephone Dial
Inside the Telephone Dial
The governor is at the bottom left.

"Gosh, that's clever," said Junior. "What happened to the lady at the exchange?"
"She stayed there for a while, and you could talk to her by just dialling 100," said Grandad. "She would help if there was a problem connecting the call."
"The exchanges were called 'automatic exchanges' if they worked with a dial telephone like the one you've been playing with. Instead of lots of people connecting the calls, they were full of rows and rows of equipment that automatically connected the calls. These exchanges were sometimes calld 'Strowger' exchanges, named after the man who work it all out, Almon Strowger. By 1912 an automatic exchange was operating in Epsom, England, and others were soon built. They were very big and very noisy buildings, with the selector mechanisms chattering away as they connected the calls."

Inside the Old Phone
The inside of the 'phone from below

Junior had been half listening as he played with Grandad's telephone.
"When I dial '0' it clicks ten times," he observed.
"It would be rather difficult for the exchange to work out if it clicked zero times!" commented Grandad, "So they used ten times."

"There are letters in the holes as well as numbers," said Junior. "Could you write text messages with these old phones?"
Grandad laughed.  "No," he chuckled, "The letters were just used in place of numbers to help people remember the numbers. Usually the first part of the number was an area code, for a part of London, say. By having carefully chosen places grouped in an area, they could call it by three letters instead of a number. This way, for example, you could dial 'BAR' for BARnet in London instead of the digits 227.  There were some famous numbers, like WHItehall 1212 for Scotland Yard, the Police headquarters.  I suppose people found it easier to remember."
He chuckled again. "But text messages were not possible until only a few years ago, with modern mobile telephones. They work in a totally different way.

Junior thought again.
"Our telephone at home doesn't have a round dial, just a load of push-buttons," he said. Does that work the same way?"

Modern Telephone
Daddy's telephone has buttons instead of a dial.

"Not quite," replied Grandad. "Inside the telephone is a modern device called an 'integrated circuit'. When a button is pressed, this integrated circuit sends the necessary information to the exchange. Some will do the same things as the dial, making and breaking the line a number of times for each digit. This is called 'Loop Disconnect'. These days, though, most telephones use a different method. The integrated circuit generates two notes, called 'tones', different ones for each button.  You can hear these in the telephone earpiece when you press the buttons to call a number. The exchange also has a lot of integrated circuits, and they listen for the combinations of tones and know which button you pressed. This is called 'Dual Tone Multi Frequency' or just DTMF for short. The information takes the same time regardless of which digit you press, unlike the Loop Disconnect, which takes longer for higher digits and longest for '0'. Also, there are several combinations spare, so that extra buttons, usually '#' and '*' called 'Hash' and 'Asterisk' can be used, as well as others."

Junior wanted to take Daddy's telephone to pieces so that he could see what stuff was inside it, but Daddy thought it would not be a good idea.

"Do modern exchanges still have all the mechanical bits clattering, or can the integrated circuits deal with both Loop Disconnect and DTMF?" asked Junior.
 "Modern exchanges are based on computers. They have big racks of equipment that connect the computer to people's telephone lines. These connection units are on big printed circuit boards, like this one.

An old Telephone Linecard

"These are known as 'Line Cards' because they connect to people's telephone lines. They do a lot of the work themselves, and stop problems on the telephone line from damaging the computer," said Grandad.
"In my days of working with these line cards, each  board could connect to about eight or sixteen telephones. The computers that did the main work could understand Loop Disconnect and DTMF signalling, so there was no need for all the clattering equipment. Exchanges became much smaller as well, which is a good job, or we would need gigantic telephone exchanges in every town and village."

While Grandad was talking, Junior had found a little drawer at the bottom of the old telephone.
"That is to put a card or booklet in." said Grandad. "You could keep your list of people's numbers in there."


Drawer at base of phone

"That's useful," replied Junior. "I wish our telephone had one of those. We just use bits of paper."

"So do we. We always forget about the drawer." said Grandad.