The Vacuum Tube

Part 1 ended with a switch that electricity can flip, the relay, and with the two things it could not do: flip fast, and make a weak signal strong. Both of those are the vacuum tube's story. Go back to the microwave oven from the opening of Part 1. The magnetron behind its back wall is the great-grandchild of a device found by accident in 1883, inside a light bulb, by a man who patented it and then put it in a drawer. This part builds that device up in three steps, follows it through the first computers, and ends where the series really begins, with the transistor.

The vacuum tube: from light bulb to amplifier

The device that made electronics possible was found by accident, inside a light bulb, and the quickest way to understand it is to build it up in three steps.

Start with the bulb itself. Thomas Edison's lamp of 1879 is a thin filament inside a glass envelope with the air pumped out. The filament is heated white-hot by the current through it, and the vacuum is there because a filament that hot would burn away in seconds if there were any oxygen to burn in. Around 1883 Edison noticed something odd. If he put a second, separate piece of metal inside the bulb, a current would flow across the empty space from the hot filament to that plate, but only when the plate was connected to the positive side of the battery, and never the other way round. He patented the effect and did nothing with it.

Here is what was happening. Heat the filament enough and it boils off electrons into the vacuum, a cloud of them hovering around the wire. Make the plate positive and it attracts the cloud: electrons stream across the gap and a current flows. Make the plate negative and it repels them, and since the cold plate emits no electrons of its own, nothing can flow back. Current goes one way only. In 1904 John Ambrose Fleming, who had worked for Edison, turned this into a component, the diode (two electrodes), a one-way valve for electricity, which is why British engineers call every tube a valve to this day.

Why a one-way valve matters

It is fair to ask what a component that only lets current through one way is for, and the answer is two things without which there would be no radio, no television, and no plug-in electronics of any kind.

The first is turning mains electricity into something a circuit can use. The power in your walls is alternating current: it reverses direction fifty or sixty times a second, because that is the easiest kind to generate and to send over long distances. A heater or a light bulb does not care which way the current goes. But an amplifier, and every circuit in this series, needs a steady push in one direction, direct current, the way a battery gives. Put a diode in the path and it passes the forward half of every cycle and blocks the reverse half, leaving a series of one-way pulses. Add a capacitor, the small tank from Part 1's side note, to fill in the gaps between the pulses, and the result is a nearly steady supply. This is rectification, and every radio, television, and amplifier that ever plugged into a wall had a rectifier tube at the back. The rectifier is still there in everything you plug in today, as a semiconductor diode the size of a grain of rice, and it is the first thing the current meets inside a phone charger.

The second job is picking a signal out of a radio wave. A radio station's wave arrives at your aerial as a very fast alternation, hundreds of thousands of times a second, whose strength rises and falls with the music. The alternation itself is far too fast to hear and averages to nothing. Pass it through a one-way valve and only the forward halves survive, and now the average of what is left rises and falls with the music. That is detection, the way the first radios recovered speech from the air, and the crystal set that a child could build in the 1920s used a semiconductor diode for it, a "cat's whisker" wire touching a crystal of galena, fifty years before anyone understood why it worked.

The grid, and the first amplifier

The diode is a valve with no handle. In 1906 Lee de Forest gave it one. He put a third element between the filament and the plate, a mesh of fine wire he called the grid, and found that a small voltage on the grid controlled the large current from filament to plate. Make the grid slightly negative and it repels the electrons, choking the stream. Make it less negative and the stream flows freely. The grid itself draws almost no current, so a tiny signal on it, a faint radio wave or a whisper from a microphone, is reproduced as a large swing in the plate current. That is amplification, and it is also switching: drive the grid hard negative and the tube is off, let it go to zero and the tube is on. The device is the triode (three electrodes), it is the third drawing in the figure above, and it is the ancestor of every transistor in your graphics card.

The garden tap is the analogy that carries this whole series. The handle is the grid, the water is the electron stream, and turning the handle moves no water itself, it just opens or closes the path. Hold onto that picture, because Part 3 uses exactly the same tap to explain the transistor, and the point of the whole story is that the tap stayed the same while everything around it changed.

What the tube made possible

The triode changed the world faster than any component before or since. In 1915 it let a telephone call cross the American continent for the first time, with three tube repeaters along the way, at Pittsburgh, Omaha, and Salt Lake City, restoring a voice that thousands of miles of copper had worn down to almost nothing. From 1920 it made radio broadcasting possible, both the transmitters and the sets in living rooms. It made television, radar, the electric guitar, sound in cinemas, and long-distance telephony. And because it could switch in a microsecond where a relay took a millisecond, it made the electronic computer: the AND and OR of Part 1, with a tube's grid in place of a relay's coil and its plate current in place of the contacts, a thousand times faster.

Colossus, the British code-breaking machine of 1943, used 1,600 tubes in its first version and 2,400 in its second. ENIAC, completed in 1945 in Pennsylvania, used 17,468 of them, could do 5,000 additions a second, filled a room of about 170 square metres, and drew 150 kilowatts. Nearly every computer of the 1950s was a tube computer: UNIVAC, the IBM 700 series, the early Manchester machines. The largest of them all was the SAGE air defence system, which went into service in 1958 with around 50,000 tubes per computer, two computers per site, and a power bill of three megawatts for the site. It was the biggest computer ever built, by most measures it still is, and it was obsolete before it was finished, for reasons the next section explains.

The tube also never went away, and this is the part people miss. It survived wherever the job was too hot, too powerful, or too high in frequency for a semiconductor to do cheaply. The magnetron in your microwave is a tube. The X-ray tube in a hospital CT scanner, a dentist's surgery, and an airport baggage scanner is a tube, pumping electrons into a metal target at a hundred kilovolts. Communications satellites amplify their downlink with travelling-wave tubes, because for decades nothing else could produce that power at those frequencies with that efficiency in orbit, and many still do. Broadcast radio and television transmitters ran on tubes the size of dustbins into the 2000s. Particle accelerators drive their beams with klystrons, tubes the size of a person. The fluorescent display on an older microwave clock or car dashboard is a small vacuum tube. And a large share of guitar amplifiers, and a smaller share of expensive audio equipment, still use tubes on purpose, because the way a tube distorts when overdriven is the sound of rock music, and no transistor circuit quite copies it. If you own a Fender or a Marshall, you own a handful of triodes that would be recognisable to de Forest.

What was wrong with it

Everything that was wrong with the tube followed from one fact: to boil electrons off the cathode, you have to heat it. A typical small tube burns a couple of watts in its heater alone before it does any work, and the largest single item in ENIAC's 150 kilowatts was the heaters, with a ventilation system to match. A tube takes half a minute to warm up before it works at all, which is why old radios and televisions came on slowly. And the heater, like the filament in a light bulb, eventually burns out. A good tube lasts a few thousand hours. That is fine for a radio with five tubes. It is a catastrophe for a computer with seventeen thousand, because with that many, one of them is always about to fail. ENIAC's engineers ran the heaters below their rated voltage and never switched the machine off, and still lost a tube every couple of days, each failure meaning a hunt through the racks. SAGE used tubes specially built for reliability and still needed a full-time crew replacing them.

Then there is size. A tube is a glass bottle with a vacuum in it, and a vacuum needs a certain volume to hold electrodes apart. The smallest practical tubes were the size of a thumb, and no amount of ingenuity made them the size of a grain of rice. A computer's power is set by how many switches it has, and with tubes, every switch was a thumb-sized, two-watt, glass object with a lifetime of a few years and a warm-up time of thirty seconds. You could build ENIAC. You could not build anything a hundred times bigger, and by the late 1950s engineers had a name for the wall they had hit. Jack Morton of Bell Labs called it the tyranny of numbers in 1958. Every extra switch added cost, heat, failure rate, and thousands of hand-soldered joints, and the joints failed too.

Try it below. Pick a technology and a number of switches, and see what the machine would cost you in power, space, and reliability. Then slide up to the 76 billion switches of the graphics card that this series is about.

The figures are rough, the trend is not. Tubes and relays are fine up to a few thousand switches, marginal at tens of thousands, and impossible beyond. Yet everything interesting a computer can do, including everything in this series, needs millions of switches at the very least. The tube got computing started and then stood in its way.

How the transistor fixed it

In December 1947 at Bell Labs, John Bardeen and Walter Brattain pressed two gold contacts onto a sliver of germanium and found that a current into one of them controlled a larger current through the other. Their boss, William Shockley, worked out the theory and a more practical design within months. The three shared the Nobel Prize for it. The device was named the transistor, and what it did was nothing new: a small signal controlling a large one, the triode's job exactly. What was new was what it did not need.

It did not need a heater, because the electrons were already free inside the semiconductor. That removed the warm-up time, most of the power, and most of the heat at a stroke. Early transistors used a few milliwatts against a tube's few watts, a thousandfold difference before anyone had tried to optimise anything. It did not need a vacuum or a glass envelope, so it was rugged and could be made small: the first commercial transistors were the size of a pea, and, unlike the tube, there was no physical reason they could not be made smaller still. And it had no filament to burn out. A transistor that is kept within its ratings does not wear out at all, which turned the tyranny of numbers on its head. A machine with a million transistors is not a machine with a million things waiting to fail.

The transistor had its own problems, and for a decade the tube kept its jobs. Early germanium transistors were expensive, around eighteen dollars each in 1950 against seventy-five cents for a tube, and their behaviour drifted with temperature. They could not handle high power or high frequencies, which is why transmitters, radar, and anything with kilowatts stayed with tubes, and why the microwave oven still does. And they were noisy in the electrical sense. So the transistor won its first markets where small and low-power mattered more than anything else: hearing aids, which people had been carrying around with a tube amplifier and a battery pack the size of a book, took their first transistor in 1952 and had gone almost entirely transistor by 1954. That year also brought the Regency TR-1, the first commercially made transistor radio, with four transistors, which sold for about fifty dollars and made the word "transistor" mean "radio" for a generation. The first transistor computer ran at Manchester University in 1953, Bell Labs' TRADIC followed in 1954 with 684 transistors (and one tube, to generate its clock), and by 1959 IBM's 7090 mainframe was transistorised. No serious computer used tubes again.

Two further steps made the transistor into the switch this series is about. In 1958 Jack Kilby at Texas Instruments put several components on a single sliver of germanium, still wired together by hand, and in 1959 Robert Noyce at Fairchild did it on silicon with the connecting wires printed on in the same process as the transistors. That is the integrated circuit, it was the real answer to the tyranny of numbers, because the soldered joints disappeared along with the tubes, and it is the subject of Part 3. And in 1959, at Bell Labs, Mohamed Atalla and Dawon Kahng built a transistor that worked by a different mechanism from Bardeen and Brattain's, controlling the current with an electric field across a thin insulator rather than with a current. That is the MOSFET, it draws almost nothing to hold its state, it shrinks better than anything else ever invented, and there are 76 billion of them in the chip on the cover of this series.

Where you will find each one today

The pattern in the last row is the lesson of this part. Each technology kept the jobs where its particular strength mattered more than its weaknesses. The relay kept isolation and brute current. The tube kept extreme power and frequency. The transistor took everything where the count mattered, and it turned out that in computing, the count is the only thing that matters.

Where this leaves us

Electronics needed a component that lets a small signal control a large one. The relay of Part 1 did it with a magnet and a moving arm, and could switch a starter motor but not a radio signal. The tube did it with a heated cathode in a vacuum, could amplify anything, and built the first computers, but every one was a hot, thumb-sized, glass object with a lifetime of a few years, and that put a ceiling of tens of thousands on how many switches a machine could have. The transistor did the same job in a solid, with no heater, no vacuum, and no wearing out, and the ceiling disappeared. The tube and the relay are still around us, doing the jobs where they were never beaten. But the machine this series is about could only be built from the third.

So the next part starts where Stokes did, with the switch itself: what a MOSFET is, why it can only be on or off, how it got from one to 76 billion, and what it costs, in heat, to flip it a few billion times a second.

Next: The Switch: the transistor as a switch, why computers count in twos, what "4 nanometres" does and does not mean, and the end of the free lunch that made GPUs inevitable.