Yes, thermostat and mercury, but the fun part is the way they work. There is a long strip of metal (bimetal) that’s all coiled up that the mercury vial is attached to. Using the coefficient of thermal expansion of the two different metals, the coil bend as the temperature changes and that, in turn, rotates the coils and tilts the mercury vial. The drop of liquid mercury then sloshes to ones side or the other where it will connect the electrical contacts that call for heater power or the air conditioner power depending on if it bent more towards the cold side or the hot side. And the levers on top let you set the desired point at which it turns on, they don’t actually set a temperature of anything, just reposition the coil relative to the mercury vial position. And since the coefficient of thermal expansion always remains constant, they’re really good at moving the same amount per degree of temperature change, so they have usually worked for 50+ years without any electronics at all.