The Snap-Action Switch: A Spring That Jumps Past Its Own Centre

Fig. 1 of US 1,960,020 shows the switch in section: a base 11 of wood, hard rubber or Bakelite; the thin leaf spring 12 bolted at one end over a thick metal shim 14; two parallel spring strips 20 and 21, joined to the leaf's free end by a U-shaped stirrup 22 and pivoted in notches in the shim; and a plunger 15 pressing the leaf near its anchored end. What the specification specifies: the leaf "preferably is a thin beryllium copper strip" (or phosphor bronze); the strips are in longitudinal compression, putting the leaf in tension; contacts "preferably made of silver … which minimizes the damage resulting from arcing"; "the bottom contact 25 is placed above the neutral or dead-center position" so the arm returns when released; and the leaf is widened ahead of its anchorage so it does not fatigue there.

Cut a strip of thin spring steel 60 mm long and 12 mm wide. Cut a slot 3 mm wide down its middle for 40 mm, leaving the two outer legs and a solid end. Bend the centre tongue slightly so that, when you clamp the solid end and push the tongue's tip back into line with the legs, the legs bow — now they are in compression and the tongue is in tension, as in McGall's switch. Clamp the solid end to a plywood base with a machine screw. Set two machine-screw stops either side of the free end, the lower one just above the point where the strip flips over, and solder a wire to the strip and to each stop. Press slowly near the clamped end with a spring scale: the force rises, then the free end jumps to the lower stop with a click. Release it and it jumps back. Wire the lower stop through a buzzer: it sounds cleanly, with no chatter, however slowly you press. Note the operating force and compare with the notebook's curve.

**Philip Kenneth McGall** filed **US 1,960,020** on 29 March 1933, assigned to C. F. Burgess Laboratories of Madison, Wisconsin; it was granted 22 May 1934. The company Micro Switch was formed later in the 1930s, and the name came to stand for the small snap-action switch itself. **Honest limits.** The notebook's two-spring model shows the snap-through and the role of the stops; McGall's real geometry, with a leaf in tension and strips pivoted near its middle, has a different force curve with the same character. Spring steel strip has sharp edges: wear gloves when cutting and bending it.