Soldering Iron: Temperature Controlled by a Curie-Point Tip and a Magnet Switch (Weller, US 3,188,448, 1965)

Turn the tip from tellurium copper: a cylindrical body, a taper and a chisel end (any shape suits the work). Plate it first with iron, which keeps the copper out of the molten solder, then with nickel, which shuts out the air. Bore its rear end and fit tightly into it the shank of a slug of thermomagnetic nickel-iron alloy, such as 48 % nickel, 3 % molybdenum and the rest iron, or about 43 % nickel and the rest iron. The alloy sets the control point: make tips for 600, 700 and 800 °F. Not given: the plating baths, sizes.

Make the barrel of two stainless steel tubes: an inner spool the tip slides into snugly, and an outer cover tube joined to it at the front, with a thread and screw cap to hold the tip. In the space between them wind the heater from one length of resistance wire: an inner layer, then an outer layer wound the opposite way so their fields cancel, with insulating sheet inside, between and outside the layers. Silver-solder heat-resistant leads to its ends. Ours: nichrome wire. Not given: the insulating sheet, the turns.

In a stainless steel sleeve about 2.4 in long, close the front with a non-magnetic thermal bushing. Behind it slides a cylindrical Alnico V magnet, then a non-magnetic spacer about 0.032 in thick, then a cold-drawn steel bushing about 0.50 in long held by dents in the sleeve; the magnet's stroke is about 0.015 in. A push rod runs from the magnet through the bushing to an insulator. On a melamine plate, hinge a moving contact between a thin conducting strip and a holding strip, facing a flexible contact; a spring of fourteen coils of 0.010 in music wire, 0.210 in across, presses the moving contact toward the flexible one. Rivet the spring bracket to the holding strip. Not given: the contact metals.

Mould the handle in two plastic halves with a socket at the front whose bosses lock the cover tube. Lay in the switch unit, held only by the bowed tail of its holding strip on an anchor pin with a speed nut, so it floats and its thermal bushing presses on the tip's slug. Wire the heater, the switch and a ground lead from the cover tube to the cord. Rivet the halves near the butt and fit a split ring near the front. In use, below the control point the slug pulls the magnet forward and the spring closes the contacts; above it the steel bushing pulls the magnet back and the push rod opens them. At idle the heater runs only about 30 to 40 % of the time. Not given: the plastic.