Constant differential air conditioning thermostat with SPST snap action switch and straight capillary tube or bulb type temperature sensor. The 3ART28 is a higher rated thermostat than the 3ART17. The contacts of the 3ART28 close 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 circuit to the compressor. 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.