The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

The spark took Tara to a case near the back of the small museum that held an old school slate in a wooden frame. Someone had drawn on it in chalk, and then someone else had sealed the chalk under glass so it would not rub off. It was a diagram: a tube with a flat end, a few lines like rays, a lens, some labels in small capitals.

The card said: Copied from memory, twelve years later, by a teacher. Drawn first by a fifteen-year-old.

This is a true story. Where nobody knows something, the story says so.

Philo Taylor Farnsworth was born on 19 August 1906, in a log cabin near Beaver, Utah. His family were farmers. When he was eleven, they moved to a ranch near Rigby, Idaho.

The ranch had something most farms in that part of the country did not have in 1918: electricity, from a small generator. Philo had never lived with electric light before. He was fascinated. He learned how the generator worked, and when it broke he mended it. He wired up electric motors to do jobs around the farm, including one for his mother's washing machine.

In the attic of the house the family found a pile of old magazines about science and radio, left behind by the people who had lived there before. Philo read all of them. He read about the electron, which scientists had only identified about twenty years earlier. He read about radio, which could send sound through the air. And he read about an idea that many inventors were working on and nobody had solved: sending pictures through the air the way radio sent sound.

Most of the people working on it used spinning discs with holes in them, a mechanical system. A disc turned in front of the scene, and each hole let through a small piece of light in turn. It worked, a little. The pictures were tiny and blurred and flickering, because a disc can only spin so fast.

Philo thought the answer was not a machine with moving parts. He thought it was the electron itself.

Here is the story Philo Farnsworth told, many times, about what happened next.

In the summer of 1921, when he was fourteen, he was working in a potato field with a horse-drawn harrow, going up and down, row after row. At the end of one row he turned the horse and looked back at the field, at the parallel lines he had made.

And he thought: that is how you could do it. You could break a picture into lines, the way a field is broken into rows. A beam of electrons could sweep across the picture one line at a time, very fast, reading how bright each spot was, then jump back and do the next line, and the next. At the other end, another beam could paint the lines back onto a screen in the same order. If it was fast enough, the eye would see a whole picture.

It is a good story. It may be true. The only person who was there was Philo, and the horse.

What we know more surely happened in the spring of 1922, at Rigby High School.

Philo was fifteen. He was taking chemistry with a teacher called Justin Tolman, and he was so far ahead of the class that he asked Tolman if he could take an advanced course meant for older students. Tolman said yes, and then found himself staying after school to listen to his student explain an idea.

One day Philo went up to the blackboard and drew it for him. A special tube that would turn light into a stream of electrons. A way to steer the stream line by line with magnets. A receiver that would turn the electrons back into light. He filled the board with diagrams and equations.

Tolman did not fully understand it. But he understood enough to know it was serious, and he did something that turned out to matter enormously: he remembered it. Philo tore a page from his notebook and drew a sketch of the tube for his teacher, and Tolman kept the memory of the drawing for years.

Philo did not have money, and inventing a new kind of television needed a laboratory. When he was nineteen he found two businessmen in Salt Lake City who were willing to back him, and he moved to San Francisco to build a laboratory there. He had married Elma Gardner, whom everyone called Pem, and she worked in the laboratory beside him, drawing diagrams and keeping notes. Her brother Cliff taught himself to blow glass so he could make the delicate tubes Philo designed. For more than a year almost nothing worked. Tubes cracked. Signals vanished into noise. The backers kept asking when they would see something for their money.

On 7 September 1927, when Philo was twenty-one, his team pointed their camera tube at a glass slide with a single straight line painted on it. In the next room, on a small bluish screen, a line appeared. When they turned the slide, the line on the screen turned too.

It was the first picture ever sent by an all-electronic television system. No spinning discs. Only electrons.

The story goes that one of the backers, who had been asking for months when he would see some dollars, came to a demonstration not long afterwards, and the picture on the screen was a dollar sign. It is the kind of story people enjoy telling about inventors and their investors, and it may even be true.

There was a problem. A large company, the Radio Corporation of America, had a scientist named Vladimir Zworykin who had filed a patent application for an electronic camera tube in 1923, earlier than Philo's own application in 1927. If Zworykin's idea came first, RCA would own electronic television, and Philo's patents would be worth very little.

In 1931 the head of RCA, David Sarnoff, visited Philo's laboratory himself. He is reported to have offered a large sum for the whole operation, and Philo, who wanted to keep his independence and be paid fairly for his patents, refused. The patent office then held a hearing to decide who had invented the idea first. It was called an interference case. RCA had far more money and lawyers. Philo's side needed to prove that he had had the idea before 1923.

So his lawyer went looking for Justin Tolman.

In 1934, twelve years after the afternoon at the blackboard, Philo's old chemistry teacher gave evidence. He described the lessons after school. He described the drawing. And he took a pen and drew, from memory, the sketch his fifteen-year-old student had made for him in 1922. It matched Philo's design.

In 1935 the patent office decided that Philo Farnsworth had priority for the invention. RCA appealed, and lost. Years later RCA agreed to pay Philo's company for the use of his patents, the first time it had ever paid royalties instead of collecting them.

Philo Farnsworth's company struggled anyway. RCA had the factories and the money, and when television finally reached people's homes, after the Second World War, it was RCA's name on many of the sets. Philo spent the rest of his life on other inventions, including a small device for nuclear fusion that scientists still build in laboratories, and some students build in garages. He was ill and in debt for much of it, and he did not much like what television was usually used for. Years later, his wife Pem said that in July 1969 they watched the first people walk on the Moon together, live, on a television screen, and that he turned to her and said this had made it all worthwhile. She is the only source for that. He died in 1971.

Every television picture for the next half-century was made the way he described on the blackboard: one line after another, very fast.

Tara looked at the slate for a long time.

She thought about the field, which might have happened exactly as Philo said, or might be a story a man told so often that it became the shape of his memory. She thought about the teacher who stayed after school, not understanding everything, and kept the drawing in his head for twelve years, and turned out to be the most important witness in the history of television. And she thought about which part of the story she would have trusted most: the boy's memory of the potato field, or the teacher's memory of the drawing.

Both were memories. One was given under oath.

She thought, too, about how the story is usually told: a farm boy, a flash of genius in a potato field, and then television. The version on the slate was messier and better. It had a teacher in it, and a wife who kept the notes, and a brother-in-law who learned to blow glass, and backers who wanted their money, and a company that nearly won. Nobody in it did anything alone.

If the most important evidence for something you did at fifteen was in someone else's memory, who would you want that person to be?

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

Back on the Green

At the Whetstone, Tara wrote one line and dated it.

The only person who was there was Philo, and the horse.

Danya laughed. "That's a source problem," he said. "The horse isn't talking."

Which line from this story would you keep?