Storm in a Teacup: The Physics of Everyday Life(77)

It was another thirty years before J. J. Thomson discovered that what was flowing wasn’t really rays at all, but a stream of individual negatively charged particles—the particles we now call electrons. By then it was too late to change the name of the apparatus, though, and it was still always called a cathode ray tube. We know today that applying a voltage across it generates an electric field stretching from one end to the other, and so electrons will hop off the negative end and rush toward the positive end. Any particle with an electric charge will be accelerated by the electric field, which means it will be constantly pushed along. So the electrons don’t just move toward the positive end because they’re attracted by it, they’re speeding up as they go. The higher the voltage difference between the two ends, the faster they’re going when they get to the other side. In a CRT TV, they can be going at more than a mile per second by the time they hit the screen. That’s a significant fraction of the speed of light, the fastest that anything in the universe can travel.

So the same basic process that led to the discovery of the electron in the first place was in use inside every TV in the world until a couple of decades ago. Each CRT TV has a device at the back that produces electrons. The middle of the TV is a completely empty chamber—a vacuum with no air in it—so there are no obstacles at all, and so the electrons “fired” from the “electron gun” stream across that empty space until they hit the screen. It’s the purest form of electrical current—charged particles moving in a straight line.


MY AUNT OPENS a box full of bits and pieces she saved from Jack’s workshop when he died. There are glass tubes that look like cylindrical lightbulbs, with a weird metallic insect-like structure inside each one. These are valves, used to control the flow of electrons in circuits. Early on, most of Jack’s job seemed to involve working out which one of these had malfunctioned, and replacing it. My mother, my aunt, and my grandmother clearly have a lot of affection for these, because there were so many of them around back then, and so many different types. And then in the corner of the box there’s a large circular magnet, now broken in half.

This is the great connection, and it represents the moment the penny dropped for the physicists of the late 1800s. If you want to control electricity, you need magnets. If you want to control magnets, you need electricity. Electricity and magnetism are part of the same phenomenon. Both an electric field and a magnetic field can push on a moving electron. But the result of the push is different. An electric field will push an electron in the direction of the field. A magnetic field will push a moving electron sideways.

Creating a beam of electrons is all well and good, but the real cleverness of these old television sets was to control what that beam was pointed at. And the deep connection between electricity and magnetism is at the heart of it. As an electron zooms through a magnetic field, it gets pushed to one side. The stronger the magnetic field, the more it gets pushed. So by changing the magnetic fields inside an old TV, the electron beam could be pushed and pulled to a point wherever it was needed. The large, permanent magnet that my aunt showed me was used very close to the electron gun, to do the basic focusing. But the steering electromagnets positioned a bit closer to the screen were being controlled directly by the signal from the aerial. They pushed the electron beam so that it scanned horizontally across the screen, one line at a time. The beam itself was being switched on and off during each line, and where it hit the screen, a bright spot was created. The “line output transformer” that Nana mentioned was the bit of kit that controlled the scanning. To make a smooth picture, 405 lines were scanned, 50 times each second, with the electron beam flicking on and off at exactly the right time for each pixel.

This is an incredibly intricate electronic dance. To see any picture at all as a result of it requires a lot of fiddly components all doing the right thing at the right time. So early televisions had lots of knobs and dials to make adjustments, and it sounds as though the temptation to mess with them was too much for many TV owners. Jack had the knack of knowing how to readjust them. It must have seemed like magic at the time. For centuries, craftsmen had been respected for what they could produce, and everyone could appreciate what they’d done even if they couldn’t do it for themselves. Now the world had changed. Electronics engineers could make a device function, but it was impossible to see what exactly they’d done or why it had worked.

It’s odd that silent, invisible electrons locked away in a vacuum were the key to the huge richness of visual broadcasting with all its sound and spectacle. And for fifty years televisions were based on the same simple principle. Put an electron in an electric field, and you’ll speed it up or slow it down. Put that moving electron in a magnetic field, and its path will curve. Leave it there long enough, and it will go around in circles.

The massive physics experiment at CERN in Geneva, famous for the discovery of the Higgs boson in 2012,?? works on exactly the same principles as a cathode ray tube, although the particles it can shift around aren’t just electrons. Any charged particle can be accelerated by an electric field and have its path curved by a magnetic field. The Large Hadron Collider, the experiment that finally confirmed the existence of the Higgs boson, had protons zooming around in its guts. In this case, the speeds reached were incredibly close to the speed of light, so fast that even with extremely powerful magnets to steer the zooming particles, the circle had to be 17 miles in circumference.