Fig 9Wilhelmy tensiometerHover to raise the stagewater · γ 73 mN/m · θ 40° · F 1.14 mN
Meniscus and dial needle: F = pγcosθPlate, trough, jack: stage raises the liquid to the plateClick a bottle or press 1 to 3: choose the liquidA platinum plate hangs from the balance with its bottom edge at the liquid level, so buoyancy is zero (L33 p.11). The balance is tared with the plate hanging in air. When the jack lifts the liquid to the plate the meniscus forms at the contact angle θ and the force becomes p γ cosθ, with p = 2(10 + 0.2) mm = 20.4 mm. γ at 20 °C is ethyl ether 17, ethanol 23, water 73 mN/m (L32 p.11). The lecture gives no contact angles for platinum, so θ = 0°, 20° and 40° are illustrative.