Copper Box Wireless
Grandad must have expected Junior to want to see this one. He had put it out all ready .
"Gosh!" said Junior, "That's a funny looking radio!"
"Wireless," corrected Grandad, "We called them wireless sets, because they were not connected up with wires like telephones."
"My 'phone doesn't have wires to connect it," said Junior.
Grandad pretended not to hear him.
"Later on they were called 'Radios', because they received radio waves," he said.
"Tell me about this set," demanded Junior.
"OK. This is a one-valve reaction set, using a screen-grid valve," replied Grandad.
"Reaction was a way of making the set much more responsive to the signals. It works by taking some of the amplified signal from the valve anode output, and feeding it back to the grid circuit in a way that would reinforce the original signal. That way you could get a louder signal and also more selectivity, that's the ability to select the signal that you want even when others are close.
"The main problem with reaction was that it had to be adjusted just right, different for each station. If it was too much, the set made it's own signal (we call that 'oscillation') and would pass it up through the aerial and annoy all the neighbours. They would hear a loud whistle on the station that they couldn't remove. Not at all popular!
"What is a 'screen-grid' valve?" asked Junior.
"Oh, that's like a triode but with an extra grid near the anode," said Grandad.
"The extra grid stopped some of the amplified signal on the anode getting coupled back to the main grid, called the control grid, by capacitance. This improved things quite a lot, but was most useful for sets that amplified the aerial signal before using a detector valve, so that the amplified signal didn't get back to the grid and cancel some of the amplification."
"Why didn't that happen inside the detector? Why didn't it just behave like reaction, and make it better?" asked Junior.
"It was in the wrong phase," replied Grandad. "It fed back in a way that tried to cancel itself. With reaction, the signal is fed back the other way, so that it reinforces itself. It's hard to explain it.
"Think of a moving staircase," said Grandad.
"You mean an escalator," said Junior.
"Yes," replied Grandad. "If you try to run up the downward one, it's jolly hard work, because it is trying to take you back down and cancel what you do. That's like negative feedback, like the valve tries to do.
"But if you run down the downward one, you get down even faster because it helps you. That's like positive feedback or reaction."
"I see what you mean," said Junior. "Doyou run up the down escalator, Grandad?"
"Not so often as I used to," he replied.
"I built this one in a box made out of copper in order to screen it from unwanted effects, but someone has introduced an unwanted effect by putting something heavy on it and bending the back!" he told Junior.
"What are the other unwanted effects?" Junior asked.
"Well, there are two main thingsl. The most annoying was 'hand capacity' that would be called 'hand capacitance' these days. This was an annoying thing that happened as you tuned the set, or altered the reaction, causing it to alter just because you had put your hand on the knob. When you moved away again, it all altered again, so getting things right could be quite difficult!
"The other thing that happened was that interference could be picked up directly on the set, rather than the proper signal from the aerial. The box was meant to help this as well. It used to have a copper lid, but that seems to have gone astray."
"Did the copper box work?" asked Junior.
"It helped a bit," replied Grandad. "There was less problem when tuning the set or adjusting the reaction. It was still a bit awful when you moved around though. Later on I realised that this was because the headphones went directly to the reaction coil. I should have fitted a condenser to earth to stop most of that. You learn more as you do more."
What's a condenser?" asked Junior, trying to learn more.
"It's a thing that lets alternating signal pass through, but blocks steady signals," replied Grandad.
"Because it can store up electricity, it seemed to 'condense' it like steam being condensed to water and then boiled off as steam again. These days it's called a 'capacitor', though, because it has capacitance. We used to call that 'capacity', because it seemed to be the amount of electricity that could be stored in the condenser. I don't know why it was changed; the old words seem a lot better to me."
"What is it capacitance measured in?" asked Junior.
"The unit of capacitance is the Farad," replied Grandad, "But one Farad is a jolly great lot! Most wireless condensers are only a small part of that, usually much less than a thousandth of a millionth!"
"Wow! That's small!" said Junior.
"Like so many other electronic units, the very big and very small are quite common," said Grandad. "See this condenser in the set, here? I made this one myself out of copper foil and mica sheets. It's about as long as my finger, and it has a capacity of 0.0039 microfarads. A microfarad is a millionth of a farad.
"In the old days this would be written as .0039mF, but these days the 'm' means 'milli' for thousandths and these days a Greek symbol 'µ', called mu, like the word for a cat mewing, is used."
"There are lots of noughts in that value," observed Junior.
"Yes, it was sometimes easier to multiply it by a million and change the units to milli-microfarads. That condenser would have been known as 3900 mmF or pF as it is now know, for picofarads. Either way will do; they are both the same."
"Why is there no screen-grid valve in the set?" asked Junior.
"It got broken when some great big lump trod on the back," replied Grandad.
"Who did that?" asked Junior.
Grandad just mumbled.