Pete Clerx was a Philips Semiconductor engineer in the early 1950s — one of the people who actually had to measure sheet resistance for a living. The state of the art was four loose tips on springs in a machine‑shop fixture: usable, fiddly, slow.
In 1955 he packaged the geometry into one self‑contained head. Equal spacing, equal force, swappable mount. It's a small piece of engineering — but every analytical lab in the Western world adopted it within a decade, and Clerx founded Signatron (later Signatone) to build them at volume.
Seventy years later, the same SP4 head ships from our shop, with the same equal‑spacing geometry and the same swappable mount. Some engineering doesn't need to change.