The Tremor Catcher

The shelf in the corner of the lab is rattling. It has been rattling for three weeks. Every time the truck goes past outside — and trucks go past every twenty minutes — the small bottles on the top shelf tap together, and the pen cup has migrated two centimetres to the left since the term began. Last Tuesday a marble fell off and nobody could say where it went. The lab technician says: "It's not rattling." The lab teacher says: "It shook. I saw it." Nobody can prove anything, because by the time anybody looks, the truck is gone and the shelf is perfectly still. What you need is a witness. Not a person — persons go home, go to lunch, stop paying attention. A machine. A machine that stays exactly where the shaking is, with infinite patience, no lunch breaks, and no imagination. A machine that feels the shelf move and writes down what it felt, right then, in a form nobody can argue with. A line. A wiggly line. The shakier the shake, the wigglier the line. Today, your team is going to build one.