Ruchardta

Ruchardta, a silver-curled experimenter, holds a glass flask with a glowing ball bobbing in its neck.

The Physics

In 1929 Eduard Rüchardt measured the heat-capacity ratio of a gas with a simple trick (Phys. Z. 30, 58). A ball fits snugly in the neck of a large flask. Push it and it bounces up and down on the springy gas below. The squeezes are quick, so no heat flows and the gas acts adiabatically. Timing the oscillation gives γ.

γ = 4π²mV/(A²pT²)

• γ: ratio of heat capacities C_p/C_v
• m: mass of the ball (kg)
• V: volume of gas (m³)
• A: cross-section of the neck (m²)
• p: gas pressure under the ball (Pa)
• T: oscillation period (s)

For air γ ≈ 1.40; for argon or helium ≈ 1.67, as kinetic theory predicts.

Honest caveat: Friction and leaks around the ball damp the motion and bias the result; p includes the weight of the ball over the neck area.

On the sticker

Ruchardta holds a glass vessel with a glowing orange ball in its neck. Printed text: RUCHARDTA with the line γ = 4π²mV/(A²pT²). Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.