Constant differential air conditioning thermostat with SPDT snap action switch and straight capillary tube or bulb type temperature sensor. The "cooling" contacts of the double throw switch close on temperature rise, and the "heating" contacts open on a temperature rise. The capillary tube or sensing bulb is normally positioned in the return air stream of an air conditioner to sense the temperature of the air in a room. Variations in temperature of the capillary cause the snap action switch to make or break the electrical circuits to the heating and cooling functions. When a cross ambient condition exists in an application, a straight capillary tube may satisfactorily work with a heater attached to the bellows housing. A charcoal filled bulb will eliminate the effects of the cross ambient condition.