The difference between oral and anal thermometers is the shape of the mercury bulb. The mercury bulb in an anal meter is round so that it doesn’t sting the anus by being too sharp. The mercury bulb of the oral meter, is long and skinny so that the mouth is able to warm up the mercury quicker. Both meters have the same scale and have the same meaning. With most thermometers, it only takes a minute to be able to take a temperature in the anus. Taking the temperature in the mouth, on the other hand, takes longer, about two minutes. This is because it takes time for the mouth to warm up when it is open and there is some air around the mercury bulb in the mouth.