Oscilloscope

Watches the current in a cable and shows it on a Phosphor Screen.

Crafting

Shapeless: a Galvanometer, an Iron Plate, Copper Wire and Ceramic.

Using it

Put it against a cable and against a screen. It joins no line; it only reads. Every tenth of a second it samples the cable and drives the screen to match (four levels). Right-click it to see the last 16 samples as a row of bars, to the nearest 0.25 A, signed by the scope.

Above 4 A the trace sits pegged at the top, exact and wrong. A pegged screen is also at full brightness, which is what burns it in. It has no use but to look; it changes nothing in the line.