Digital Caliper: Capacitive Scale and Slider with Two-Chip Electronics (Baxter and Buehler, Starrett, 1986)

A six-inch caliper that reads out on a display. Under the slide, a strip of etched copper bars runs along the beam. Electrodes on the slider pass a signal through the bars by capacitance, and the slider's electronics count the bars crossed and work out the position in between. Result: a digital caliper reading in inches or millimetres, repeatable to a ten-thousandth of an inch, with floating zero, peak hold and high/low limits. It runs on two 1.5 V cells at 10-50 microamps. Source: L. K. Baxter and R. J. Buehler (L. S. Starrett Co.), US Patent 4,586,260 (1986). Unknown: the metal of the beam, slide and rod, their dimensions, the processor's program, and how the chips and display are made. Ours: the etchant, photoresist and glue. Picture: Wikimedia Commons.