Also, a rectal thermometer can be used on adult patients who have suffered an oral injury or have undergone surgery in mouth or patients unable to take commands. Most commonly used rectal thermometers are made of glass and contains mercury. Mercury further adds to accuracy element of these thermometers. Dec 21, · A rectal thermometer is one means of taking body temperature. It is usually thought to give an extremely accurate reading, but since it is inserted into the anus, not all people prefer this method of taking temperatures.