Finger probe: presses the button and pulls it sidewaysGlass button on a spring-mounted carriage, read by a dial gaugeA 120 × 32 glass button on a test rig, as a three.js scene: a carriage on two guide rods held by four springs, a finger probe on a gantry behind it, and a dial gauge riding on the carriage whose feeler reads how far the button's top leans. Pressed, the button swells 8% across; pulled, the carriage takes 15% of the pull against its springs and the rubber top leans the rest, up to 14 px. Pulled 80 px it leans 13.9 px; one turn of the needle is 20 px.