South-Pointing Chariot

The secret is in the gears between the wheels. When the cart turns, the outer wheel travels farther than the inner one, and that difference is fed through a train of gears to the figure on top, rotating it by exactly the amount needed to cancel the turn. Modern engineers recognise this as a differential gear, the same principle used in the rear axle of every car, and the chariot is often cited as the first mechanical differential in history. Look at the model from the side to see the vertical gear wheels and the way they engage. The museum's version follows the reconstruction proposed by historians of Chinese technology, since no original chariot survives.

Legend credits the device to the Yellow Emperor, who used it to find his way through fog in battle, and later dynasties built south pointing chariots as symbols of imperial power to lead processions. The model belongs to the museum's galleries on Chinese science, which also include the famous astronomical clock tower and models of early seismographs and looms. A memorable detail is how sensitive the mechanism is: on a real road the wheels would slip, so the chariot was more ceremonial than practical, but on the museum's smooth floor it works beautifully.

Did You Know? The chariot has no magnet at all; it keeps its bearing with a differential gear, the same idea that lets a car's wheels turn corners at different speeds.