# Useless Machine

How a useless box works: a motor\-driven arm flips its own switch back off, and the parts inside each show their job, from batteries to limit switch\.

By Bertil Chapuis

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/x7fZZyEvB0OE)

## Transcript

### 1. A box with one job

This little wooden box has a single silver switch on top\. Its one and only job is to switch itself off\. Let's look inside at the parts no one usually sees\.

Tap the switch on the box at any time to try it\. Drag anywhere else to look around\.

### 2. Switch it on

Flip the switch to ON and watch\. A lid pops open, a little arm reaches out and pushes the switch back to OFF\. Then the arm hides and the lid drops shut\.

A whole cycle takes about two seconds\.

### 3. Take the front off

Now the front wall fades away\. There is no computer in here, just six simple parts\. Each one glows in its own colour as we meet it\.

Battery, switch, wires, motor, arm and a tiny limit switch\.

### 4. The battery pack

Four AA batteries lie on the floor of the box\. Together they give about six volts\. Volts measure how hard the electricity pushes\.

This is the only power the machine has\.

### 5. The toggle switch

The silver lever on top is a toggle switch\. Underneath, it has two sets of contacts\. ON sends power to the motor one way, and OFF sends it the other way\.

Builders call this a double\-pole, double\-throw switch\.

### 6. The wires

Thin red and black wires join the parts in a loop called a circuit\. Electricity flows from the battery, through the switch, to the motor and back\.

Red usually marks the plus side and black the minus side\.

### 7. The motor and gearbox

The yellow block is a small electric motor with a gearbox\. The motor spins very fast, and the gears trade that speed for strength\. The arm is fixed to the slow, strong shaft\.

Reverse the power and the motor spins the other way\.

### 8. The arm and finger

The arm swings up in an arc, lifts the lid from below and reaches the switch\. Its bent tip, the finger, pushes the lever back to OFF\.

The finger must be stiff, or it bends instead of pushing\.

### 9. The limit switch

When the arm comes home it presses a tiny limit switch\. A limit switch is a button that tells a machine where to stop\. Here it cuts the power completely, so the box is truly off\.

Watch the metal tab under the arm in slow motion\.

### 10. Boxes with a personality

Some modern useless boxes hide a tiny computer that makes the arm hesitate, peek out and then strike\. Pick a mood, then flip the switch to see the difference\.

A box with a computer stays slightly awake\. The classic box truly switches itself off\.

### 11. Why build something useless?

Marvin Minsky dreamed up this box in 1952 and called it the ultimate machine\. It is a joke you can hold, and a lesson in feedback: a machine that senses what it is doing and reacts\.

Thermostats and robots use the same idea\. Try the switch once more\.