To calibrate a normally-closed thermostatic switch of the type having a bimetallic switch arm fastened at one end to the inside of the switch housing, the switch is placed in an oven beneath reciprocable calibrating tool with the switch arm connected in circuit with the tool operation means. When the switch reaches the temperature at which it is intended to open, the tool is lowered so that a pair of projections on its lower end deform the housing at two spaced points until the switch arm snaps to its open position, simultaneously terminating the calibrating operation and deenergizing the tool actuating means.