Twistel

Meet Twistel, a metal disc hanging from a thin wire who twists one way, then the other, again and again.

Twist a disc that hangs from a wire and the wire twists back. The restoring torque is proportional to the angle, τ = −κθ, where κ is the torsion constant of the wire. Like a spring, this gives simple harmonic motion, but the motion is rotation.

The period is T = 2π√(I/κ), where I is the moment of inertia of the disc. A heavier or wider disc twists more slowly. A thicker or shorter wire is stiffer and twists faster.

A disc with I = 2 × 10⁻³ kg·m² on a wire with κ = 0.02 N·m/rad has a period of about 2.0 s.

Torsion balances can measure extremely small forces. Henry Cavendish used one in 1798 to measure the gravitational pull between lead balls and weigh the Earth. Anniversary clocks under glass domes keep time with a slowly turning torsion pendulum.