The Classroom Demo That Failed in Front of Everyone

The monkey’s name is Gerald, and on an October morning two years ago he hung from an electromagnet on the stage of the Ansel High School auditorium, in front of 640 students and the superintendent, and declined to be shot.

The demo is called monkey and hunter. You aim a dart straight at a target hanging in the air. The instant the dart leaves the tube, the target drops. Everybody’s gut says the dart will sail over it, since you aimed at where the monkey used to be. It hits him anyway, because dart and monkey fall the same distance in the same time, so the aim stays true however fast you shoot. It’s the most beautiful thing I know how to do with a length of PVC.

A strip of foil across the muzzle of a blowpipe completes a circuit. The dart tears the foil, the circuit opens, the magnet shuts off, and Gerald falls. Gerald is a six-inch beanbag monkey with a steel washer sewn into his head. The department’s old magnet had vanished in a renovation, so I built a new one in my garage from a hardware-store bolt and a couple hundred feet of magnet wire.

I rehearsed for a week across a lab bench, about seven feet, aiming at his belly. I hit him every time, usually in the feet. I should have thought harder about that, but I was busy being pleased with myself.

Science Week gave me twelve minutes between the jazz band and a talk about vaping. I set up at twenty feet so the back rows could see. I took a voice vote on whether the dart would go high, and “high” won by a landslide. Then I blew.

Gerald dropped. The dart passed well under him and stuck in the curtain.

There was a good-natured laugh. I reset the foil and tried again, and the dart went under him again, and the laugh got less good-natured. On the third try it went under him a third time, and a freshman somewhere in the dark yelled, “Free Gerald!” and the room came apart. The superintendent was writing something down, and I have decided it was a grocery list. I said something about this being what science is really like, which nobody believed, and the jazz band came back out.

At lunch a junior named Marisol Teague walked into my room and said, “It went under all three times.”

I said I knew that, I’d been there.

“No,” she said. “It went under by the same amount.”

I had spent the whole morning assuming my aim was bad. But bad aim scatters. It goes left, it goes high. Three misses at the same height means the rig is doing something on purpose.

So I gave my physics section the rig for the week and got out of the way, and that is all the credit I get for what follows. They listed suspects on the board. Mine were foil stiffness and air drag. Theirs was “Mr. A’s lungs,” which got the most votes. They timed the dart with a pair of foil gates and got about 12 meters per second, consistent from shot to shot, which cleared my lungs and hurt my feelings a little.

Then a kid named Jalen filmed the muzzle and the monkey together on his phone, in slow motion, at 240 frames per second. They counted frames from the foil tearing to Gerald’s first twitch. Eleven frames came to about 46 milliseconds.

The magnet wasn’t letting go when it was told to. The field in a coil takes a moment to collapse, and a bolt from the hardware store is ordinary steel, which stays a little magnetized after the current stops. My bolt held on, so the dart got a head start on falling, and Gerald was always late. Marisol put the arithmetic on my board. For a small delay, the dart ends up lower than the monkey by gravity times flight time times delay.

20 feet = 6.1 m, at 12 m/s: flight time 0.51 s
miss = 9.8 x 0.51 x 0.046 = 0.23 m = about 9 inches

7 feet = 2.1 m, at 12 m/s: flight time 0.18 s
miss = 9.8 x 0.18 x 0.046 = 0.08 m = about 3 inches

Somebody had filmed the assembly, of course, and they measured the miss against the curtain. Nine inches, near enough. Then they got to the second line, and the room got loud at my expense, because three inches below Gerald’s belly is Gerald’s feet. My week of successful rehearsals had been failing the whole time, by three inches, into a target just big enough to hide it.

The fix was one layer of masking tape over the head of the bolt. A tiny gap between the steel and the washer weakens that leftover grip enormously, and eleven frames dropped to two. That Friday, in the empty auditorium, with fourteen kids in the front row, Gerald took one in the stomach, and they cheered louder than anyone had cheered for the jazz band.

I had been running some version of this demo for nineteen years. In all that time, nobody had asked me anything about it except whether they could have a turn. A demo that works is a magic trick. They watch, they enjoy it, and they could not tell you what would change the result.

That spring a new sentence started appearing in lab reports. “Is it off by the same amount?” I never assigned it. It just spread around the room like a cold.

Every fall now I peel the tape off, stand at the back of the room, and miss Gerald by a little under nine inches. Then I hand over my phone and sit down. If you are a future junior at Ansel High and you’ve found this, keep your mouth shut and act surprised.