DIY · Card 94 · Light · Motion
A ring of black card with twelve slits, twelve drawings of a ball, and a pencil to spin it on. Look through the slits and the ball starts bouncing: the same trick every film still plays on you.
01The brief
Twelve still pictures. One moving ball.
12 × 1
slits × turns a second = pictures a second
24
pictures a second in a cinema film
1834
when William George Horner described the first slotted drum
Nothing in a zoetrope moves except the drum. The pictures inside are still drawings. But spin it and peer through the slits, and the ball bounces, again and again, as long as you keep it turning.
It is the ancestor of cinema, cartoons and every screen you own. Its cousins on this site are the conveyor-belt cinema and the moiré patterns that seem to move when they slide.
02Kit
Card, paper, a pin and a pencil.
Tick off what you have
0 of 5 ready
03Method
Cut, slit, draw, spin.
Step 1 of 8 · Minute 0
You need black card, white paper, a ruler, a pencil, scissors, sticky tape, a drawing pin and a pencil with an eraser on the end.
The pictures are a blur.
Why
The slits are too wide.
Fix
Make them narrower, about 3 to 4 mm. Narrow slits give sharper pictures.
The ball jumps instead of bouncing.
Why
The change between pictures is too big, or the drum spins too slowly.
Fix
Spin a bit faster, or redraw with smaller steps between pictures.
The drum wobbles.
Why
The pin is not in the exact centre.
Fix
Measure again and move the pin to the true centre of the base.
04How it moves
As the drum turns, a slit sweeps past your eye and you catch one picture on the far wall. Then the dark card blocks your view, and by the next slit the next picture has moved into place. Change the slits and the speed:
Pictures a second
12
One bounce lasts
1.00 s
Each picture shows for
83 ms
Smooth motion, like an old cartoon. Each slit that sweeps past your eye shows you one picture for a split second, then the dark wall between slits hides the jump to the next. Pictures per second = slits × turns per second.
Flashes can also fool you. Here is a wheel lit by a strobe light:
Real turn per flash
58°
Turn you see
-2°
The wheel seems to
turns backwards at 0.10 rev/s
Six spokes look identical every 60°. If the wheel turns 58° between flashes, your eye cannot tell that from turning 2° backwards, so the wheel seems to creep the wrong way. Try 2.5 rev/s at 15 flashes: the wheel freezes. The orange spoke gives the game away.
05The big idea
Motion is
made up.
Your eye doesn’t send a film to your brain. It sends snapshots, and your brain fills in what must have happened between them. Give it still pictures at the right pace and it happily invents the motion.
That same guesswork can go wrong. Feed it a wheel that turns almost a whole spoke between glimpses and it guesses ‘a little backwards’. Sampling, filling the gaps and getting fooled are the whole story of the zoetrope.
06The math
1 · The wall
spacing = π d ⁄ n
d is the diameter of the base, n the number of slits.
2 · Frame rate
pictures ⁄ s = n × f
n slits, spun f times a second.
3 · Wagon wheel
seen = (360 f ⁄ s) mod (360 ⁄ k)
k spokes, wheel speed f, s flashes a second.
A 19 cm base has a circumference of π × 19 ≈ 59.7 cm. Split that into 12 and the slits sit 5 cm apart, which is why each picture on the strip is 5 cm wide. Spin it once a second and you see 12 × 1 = 12 pictures a second; give it a brisk twirl at two turns a second and it is 24, the rate of a cinema film.
The wagon-wheel formula works with remainders. A six-spoke wheel looks the same every 60°. At 2.4 turns a second and 15 flashes, it turns 57.6° between flashes. The remainder after dividing by 60 is 57.6°, the same as −2.4°, so it seems to creep backwards, 2.4° × 15 = 36° a second.
07Real life
From cinema to the phone in your pocket.
Cinema
A film is 24 still pictures a second. The projector’s shutter blacks out the screen while each frame is pulled into place, exactly like the wall between a zoetrope’s slits.
Cartoons ‘on twos’
Hand-drawn animators often draw only 12 pictures for each second of film and show each one twice. The zoetrope you just built runs at about the same rate.
Flip books
Each page is a frame, and your thumb is the shutter: each page snaps out of the way to reveal the next.
3D zoetropes
Some museums spin rings of small sculptures under a flashing light instead of slits. Each flash freezes the figures in a new pose and they seem to come alive.
Timing lights
Mechanics once set car engines with a strobe lamp. Flashing in time with the engine, it makes a spinning mark look still, so they could read it.
Screens
A phone screen redraws its picture 60 or more times a second. Every scrolling list and video is a fast row of still pictures, like the strip in your drum.
1834
William George Horner was an English mathematician, remembered today for a method of solving equations. In January 1834 he described a spinning drum with slits that made drawings move, and named it the Daedaleum, after the legendary inventor Daedalus.
It was nearly forgotten until the 1860s, when the American William E. Lincoln moved the slits above the pictures, which made the picture strips easy to swap. He called it the zoetrope, from the Greek for ‘wheel of life’. Milton Bradley sold it with sets of picture strips, and it became one of the great toys of the Victorian age.
08Is and isn’t
It works because of ‘persistence of vision’.
That old explanation, that each picture lingers on the eye, is too simple. Today scientists say the brain actively reads a series of shifting still images as motion.
Faster is always better.
Spin too fast and each picture flashes so briefly that the scene grows dim and blurry. There is a sweet spot.
You could take the slits away.
Without slits you would see the pictures smear past in a blur. The slits work as a shutter, showing each picture for an instant.
Films show continuous movement.
A film is a sequence of still photographs, usually 24 a second. Nothing on the screen ever really moves.
09Go further
A
Your own film
Draw a running dog, a growing flower or a blinking face. Keep the background the same in every picture.
B
Count the frames
Draw 11 pictures for 12 slits. What happens? Now try 13. Does the ball drift forward or backward?
C
Spokes on a bike
Watch a bicycle wheel spin in a video. Can you spot the wagon-wheel effect?
D
Phone strobe
Spin your drum with no slits under a flickering light, or film it with a phone. Do the pictures freeze?
E
Photograph a run
Take 12 photos of a friend jumping and print them small for your strip. That is what Eadweard Muybridge did with horses in 1878.
10Questions
Tape a strip of black card with evenly spaced slits round a card circle to make an open drum. Put a strip of pictures inside, below the slits, each one slightly different from the last. Spin the drum on a pin and look through the slits: the pictures move.
Each slit lets you glimpse one picture for an instant, and the dark wall between slits hides the jump to the next one. When enough pictures arrive each second, your brain joins them into smooth motion.
One per picture. Twelve is a good number for a homemade drum: spun about once a second, it shows twelve pictures a second, which is enough to look like motion.
The British mathematician William George Horner described a slotted drum he called the Daedaleum in 1834. In the 1860s the American William E. Lincoln named it the zoetrope, ‘wheel of life’, and Milton Bradley sold it as a toy.
A camera takes separate pictures, just like a zoetrope shows them. If a spoked wheel turns almost a full spoke-gap between pictures, it looks as if it moved a little backwards. This is called the wagon-wheel effect.
Safety · Take care with scissors and the drawing pin. People who are sensitive to flashing lights should not use a strobe light for the extra experiments.
All DIY cards →Keep wandering
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