Abstract: A temperature-adjustable electric soldering iron manipulable with a single hand to change the temperature of the iron comprises a handle, a circuit board, a Nichrome tip heating wire, a cover for the wire and a soldering tip. On the circuit board are two mercury switches, a diode, and five resistances, connected in a manner to provide four circuits connected with the Nichrome wire so that the tip can be heated to four different temperatures dependent upon which circuit is connected in series with the Nichrome wire. The circuit board is placed inside the handle and the mercury switches are arranged so that holding the handle in a completely horizontal position shuts off the electric circuit, but tilting it forward or backward, or turning it around to the left or to the right actuates the mercury switches by means of the function of the mercury drop which connects respective circuit contacts at either end of the switches to place a different one of the four circuits in series with the Nichrome wire.