Grandad said "Have you heard about valves yet? The Americans call them tubes, short for 'vacuum tubes', but we call them valves."
"Why are they called different things?" asked Junior.
"Because they look at them in different ways," replied Grandad. The Americans see a tube full of vacuum, if anything can be full of nothing, and call it a vacuum tube. The British see a thing that controls the flow of something, in this case the flow of electrons, and so they call it a valve. The proper name for a valve is a 'thermionic valve' because the electrons are made by a thermal process. Do you know what that means?"
"I might," replied Junior. "I know that a thermometer measures temperature, and a thermostat keeps the temperature steady. They are both 'therm'-somethings, so I think it means temperature."
"Jolly good," said Grandad, "It means 'to do with heat'. The thermionic valve uses heat to make the electrons that it controls."
"What bit is heated?" asked Junior.
"There is a part right at the centre of a valve called the 'filament' in old valves. This is just a bit of wire that has electricity passed through it, making it glow hot like a light bulb. In fact, the workings of a valve were discovered when someone was trying to improve a light bulb."
"Who was that?" asked Junior.
"So many questions," said Grandad. "Why don't you find out when you get home and let me know the answer next time you come?"
"O.K." Junior agreed. "I'll do that, Grandad."
"What else is in a valve?", asked Junior.
"The type of valve that was used in an old wireless was called a 'triode'," said Grandad. "That means it had three of something. The '-ode' is probably taken from the end of 'electrode'. It has three electrodes. In this case it had a filament, a grid, and an anode. The anode is a metal tube that goes around the filament wire, with a gap. The grid is a coil of wire between the anode and the filament, in the gap. All these things are connected with wires that go through the glass and are connected to pins at the end of the valve. Now, tell me, how many pins does a triode have?"
"That's easy," said Junior. "It has three bits inside, the filament, grid and anode. So it needs three pins."
"Are you sure about that?" asked Grandad.
"Yes. No." replied Junior. "Is it a trick question, Grandad?"
"Not really," replied Grandad, "But you must remember what I told you about the filament."
"I know," said Junior, "The filament needs to connect to something at both ends, like a light bulb. So it needs two pins, not one. The triode needs four pins altogether, Grandad."
"That's better," said Grandad, "Your brain is working today. You're right - a triode needs four pins. They are normally labelled Anode, Grid, Filament+ and Filament-.
"In most modern valves, the filament is called a heater. It sits in a tube of metal, that's covered with a chemical that helps send out lots of electrons. The tube is called a cathode, and it needs an extra pin."
"Why have a cathode?" asked Junior. "Surely the special chemical could just be put on the filament wire?"
"Yes, they did do that," said Grandad. "Valves with chemical on the filaments could work well with only a dull red glow from the filament, so it used less electricity and lasted longer. They were called 'dull emitters' instead of 'bright emitters'. The old ones, without the chemical coating, had to glow really bright, just like a light bulb, and they lasted about as long as a light bulb - not very long! Valves used to cost a great deal of money, so making them run for longer was a really good thing."
"How much did a valve cost?" asked Junior.
"In terms of the amount of money that a man could earn, it was a lot. Often it was several week's work."
"Goodness!" said Junior. "Could they mend the valves if the filament stopped working?"
"Yes, there were businesses that repaired valves," said Grandad, surprisingly. "They opened up the valve and replaced the filament, then sucked out all the air and sealed it up again."
"That must have cost a lot!" said Junior.
"Yes", replied Grandad, "But still cheaper than a new valve.
"Look at that valve over there; the one with a glass 'pip' on the top, but hold it carefully."
"It's all sort of pinky orange inside," said Junior, "And look - there's writing on it. Let me read it."
"It says 'REPAIRED BY RADIONS LTD' ," he said. "It must be a repaired valve!"
"That's right," said Grandad. It is an old triode, from around 1920. It doesn't have a part number on it; probably when it was repaired, they rubbed it off. In spite of that, the insides gives it away. It is known as a 'R5v' because it is a 'R' type valve and has a 5 volt filament. The colour is from what is called a 'phosphorus getter'. They put a bit of red phosphorus in the valve, and after the air is all pumped out, the whole thing is heated up. Any gases left in the metal parts inside the valve are driven out, and get caught by the phosphorus, which boils up and sticks to the inside of the glass. That's why it has that colour. Later on they used magnesium, which works better and makes the glass look like a silver mirror."
"I've been looking at your valve," said Junior, "And I can't see a hole or anything where they got inside to repair it."
"Nor can I," replied Grandad. "I wonder how they did it?"
"Can anyone make a triode valve now?" asked Junior.
"Of course," replied Grandad. "Modern triodes and other valves with even more grids in between the filament (or heater) and the anode are still being made."
"I meant, can you make them like they used to?" clarified Junior.
"You bet!" replied Grandad. "A young friend of mine has recently made a triode valve himself, using a vacuum pump and a special heater called an induction heater, that heats the metal in the valve but not the glass. I've got a photograph of it over there."
Grandad pointed to the photo of the home-made valve.
"Does it work?" asked Junior.
"Yes, it does." replied Grandad.
"Thank you for telling me about thermionic valves or vacuum tubes," said Junior.
"Now can I see your old wireless, please?"
"Which one would you like to see?" asked Grandad, "The one in a wooden box, or the one in a copper box?"
So Junior made his choice
"Thank you very much for showing me your old wireless sets," said Junior, "I really enjoyed seeing how they were built and how easy it seems to build one."
"Hmmmmm," said Grandad, "It seems easy these days, but ninety years ago it was just being invented, and it didn't seem so easy then. People have learned a great deal about these things since then."
"Can I make a wireless set one day?" asked Junior.
"We'll make one together," replied Grandad, "But not for a while. I need to get all the bits together. It will definitely need a good aerial and earth. We'll make a crystal set, shall we?"
"Yes please," said Junior. "I can hardly wait!"