What is a oil tempera thermometer bulb in aircraft?

If you were more specific about the type of aircraft that you are talking about then you will get a more precise answer. There are many different types of aircraft out there, eg jet engined aircraft, piston engined, helicopters. My main experience is on 747-400, A380 ,helicopters. I have never heard of a oil temperature thermometer bulb, only a oil temperature sensor, . I'm not saying they don't exist but they may be specific to piston engined aircraft (which i have never worked on)or it could be the nomenclature that is applied to a certain make of engine by the manufacturer. The very name oil temp thermometer bulb sounds like old terminology,the bulb part of the name could be the lamp or filament that is in the cockpit which warns the pilots of over temp conditions. I will ask my work colleagues about this and get back to you. Regards. S.