How is a slinky created?

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For the grown-up

The short answer. A slinky is one long steel wire, about 80 feet of it. At the factory, round wire is pressed flat, and a machine wraps it around a rod, coil after coil, then cuts it: about ten seconds for each one. Each end is crimped onto its coil so it is smooth. Richard James invented it in 1943, after a spring he was working on fell and walked.

A slinky is one long steel wire, pressed flat and wound into coils. These pages follow it from the spring that fell off a shelf in 1943 to the factory that still makes it. Here is how it is made, a few numbers, why it walks, where the pictures stop being true, what to ask, and how to try it at home.

How a slinky is made

Round wire. Steel wire arrives at the factory round, on big spools. It is spring steel, the kind that springs back when it bends.
Flat. A flattener presses the round wire into a flat ribbon. Richard James built the machine himself.
Coiled and cut. A coiler wraps the flat wire around a rod, coil after coil, then cuts it. One slinky takes about ten seconds.
Pinched ends. Each end is crimped: pressed tight onto the coil next to it, so nothing pokes out. The factory began doing this in 1973.
Boxed. The finished slinkies ride a belt, one behind the other, and a machine pushes each one into its box.

Why flat?

Richard James's patent explains it. Round wire is much stiffer to twist than flat wire: more than nine times as stiff, in his example. A flat ribbon, about four times as wide as it is thick, makes a soft, slow spring whose coils stack and unstack easily. That softness is what lets it walk.

A few numbers

Each slinky holds about 80 feet of wire. The national rules for school buses allow no more than 45 feet, so the wire is nearly twice as long as the longest bus. A 2008 visit to a slinky factory gave other figures, 62 feet in about 80 coils, one slinky every 15 seconds, so the numbers have changed over the years.

The wire runs into the flattener at about 12 miles an hour.

The first 400 slinkies went on sale at Gimbels department store in Philadelphia on November 27, 1945, for a dollar each. They sold out in 90 minutes.

Metal slinkies are still made in Hollidaysburg, Pennsylvania, on the machines Richard James built in 1945: about 3 million a year.

Why it walks

Start a slinky over the edge of a step and the top end flips over and lands below. Then the coils pour across one at a time, and the last ones swing over. Gravity pulls it down, and the speed it gathers carries it into the next flip. On a slope that is too gentle, gravity is not enough to keep it going, and it stops. On a steeper slope it walks faster.

Where it came from

In 1943 Richard James, an engineer at the Philadelphia Navy Yard, was working on springs to keep instruments steady on ships. A spring fell and, instead of landing in a heap, stepped away end over end. He spent about two years getting the steel and the size right. His wife, Betty, found the name in the dictionary, where slinky means sleek and graceful in the way it moves. The toy was patented in 1947.

Where the pictures stop being true

Fewer coils: The slinkies on these pages have about 30 coils, so each one can be seen. A real one has about 80.
The shelf and the books: The accounts differ on how the spring fell: off a shelf, off his desk onto a stack of books, or onto the floor. The pictures put them all in one room. The plan of a ship on his wall is made up.
The rollers: Nobody outside the company has shown the inside of the slinky flattener. Flat wire is usually made by squeezing round wire between two rolls, so that is what the pictures show.
The flat wire: It is drawn much bigger than it is. In the patent's example it is a tenth of an inch wide and a quarter as thick.
The wheel: On the page a child turns the coiler by hand. The real machine runs by itself.
The pliers: At the factory the ends are crimped by workers at the machine, not with a pair of pliers.
The bus: On the page the wire is a little less than twice as long as the bus. Buses come in many sizes, and 80 feet of wire is longer than any of them.
The ramp: On the page the slinky needs five books, a slope of about 14 degrees. The patent says a slope of about 2 inches for every foot (about 10 degrees) is close to the least that keeps one walking. A real slinky, board and floor will each be a little different.

Questions to ask

Before:

“What do you think a slinky is made of?”
“How can a toy walk down the stairs with no batteries?”

After:

“What would a slinky look like if you unwound it?”
“Why does the factory squish the wire flat?”
“Why did the slinky stop on the flat ramp?”
“Richard James was making something else when he found the slinky. Can you think of a toy you found by accident?”

Try it

Make a ramp. Lean a board, a tray or a big hardcover book on a pile of books. Bend a metal slinky into an upside-down U at the top, with its front end just touching the ramp, and let go. If it stops, add a book and try again. Which pile is the lowest one that works? Does it walk faster when the ramp is steeper?
Count the coils. Count them together, or count by tens with a finger on every tenth coil. A metal slinky has about 80.
Walk out 80 feet. Take about 30 big grown-up steps along a sidewalk or a hallway. That is how long the wire inside one slinky is.
Round to flat. Roll a snake of play dough, then press it flat with a rolling pin, the way the rollers do. Wrap the flat strip around a pencil to make a coil.

Where this comes from

Lemelson-MIT Program, Richard James: springs to steady instruments on ships, the spring knocked off a shelf that stepped down to the floor, Betty James naming it, machines that coil 80 feet of steel wire into a slinky in about 10 seconds, and 400 sold at Gimbels in 90 minutes.
Smithsonian magazine, The accidental invention of the Slinky: 1943, the name chosen from the dictionary in 1944, 80 feet of wire in a two-inch spiral, and the 1947 patent's toy that walks down an inclined plane or a set of steps with only a starting push and gravity.
Richard T. James, US Patent 2,415,012, Toy and process of use (1947): spring steel, a flat section about four times as wide as thick (example 0.100 by 0.025 inch), round rod more than nine times as stiff in twisting, and a slope of about 2 inches per foot to keep it walking.
The Fabricator, a visit to the slinky factory (2008): round wire fed into a flattener at 12 miles an hour, a coiler that wraps the wire around a spindle, ends crimped since 1973 so they do not jab, a conveyor belt and automatic boxing, with the figures of 62 and 98 feet and one every 15 seconds.
How Products Are Made, Slinky: the machine flattens the wire, winds it and cuts it to length; a worker crimps one end to the next coil, then the other; a boxing machine.
The Philadelphia Inquirer, on Betty and Richard James and the Slinky (2026): a Navy Yard engineer, the spring falling from his desk onto a stack of books and walking down it, 80 feet of steel, 10 seconds a slinky, about two years of refining, and the sale at Gimbels on November 27, 1945, at a dollar each.
6abc Philadelphia, at the Hollidaysburg slinky factory (2026): the 11 machines from 1945 still making metal slinkies in Hollidaysburg, about 3 million a year.
Wonderopolis, How was the Slinky invented?: the name from the dictionary, meaning sinuous and graceful.
Ulbrich, on flat wire: flat wire is made by rolling round wire flat between two rolls.
Science Buddies, Slinking Slinkies, and Scientific American, Bring Science Home: a board on a stack of books, the slinky held in an upside-down U, and gravity and momentum making it walk faster on a steeper slope.
National Congress on School Transportation, National School Transportation Specifications and Procedures (2015): a school bus may be no longer than 45 feet.

Every word, page by page

Look! A slinky, walking down the stairs. Step, step, step! How is a slinky created? Let's find out!
Long ago, a man named Richard James was making springs for ships. One day, he bumped a spring off a shelf. Tap it! Boing! Boing! The spring walked down the books, all by itself! Richard had an idea. A toy that walks! His wife, Betty, found a name for it in the dictionary. Slinky!
A slinky is one long steel wire, wrapped around and around. Pull the end, and unwind it! Whoa! It's longer than a school bus! How does a wire become a slinky? Let's go to the factory!
At the factory, the wire starts out round, like a play dough snake. Push it into the rollers! Squish! The rollers press it flat, like rolling pins. Round... flat! Flat wire makes a soft, floppy spring.
Next, a machine wraps the flat wire around a rod, around and around. Spin the wheel! Each loop is called a coil. Keep spinning! More coils! Snip! The machine cuts the wire. One whole slinky, in about ten seconds!
Now the ends. Each end gets pinched tight, so it's smooth and safe. Pinch both ends! Pinch! Pinch! Into a box it goes, and off to a store!
Your turn! Here is a ramp. Give the slinky a little push! Oh! It stopped. This ramp is too flat. Lift the ramp up, then push again! Flip, flip, flip! Gravity pulls it down the ramp, one flip at a time. Squish! Spin! Snip! Pinch! That's how a slinky is made. Got a slinky? Make a ramp at home!