There are several types of thermometers used in health care today. The choice of thermometer usually depends on the patient and their condition, and the resources on hand at the time.
In the Australian health system today the most commonly seen thermometers are digital. Some are in the form of a probe which is covered by a plastic sleeve which is changed for each patient. These types can be placed under the tongue (sub lingual), in the armpit (per axilla), or gently into the anus (per rectum). Common sense often determines which of these locations is best. For a routine temperature reading it is inappropriate to ask someone to drop their trousers! Usually the sublingual or per axilla route is preferable for obvious reasons. A rectal temperature may be required if a more accurate reading is necessary, eg if an extremely low or high temperature is suspected. Given that the other two methods are quite external they can be misleading due to environmental factors - the patient may have recently had a hot or cold drink, or may have just taken off a warm shirt. A rectal temperature is more indicative of the patient's core temperature. In a gravely ill patient this is vital information and needs to be as accurate as possible.
The other commonly used digital thermometer is placed in the external ear opening. Once again a plastic cover is placed over the sensor for each patient to preserve infection control. This is known as a tympanic thermometer as temperature from the tympanic membrane in the ear is measured. To obtain the most accurate reading possible the ear should be gently retracted - up and back for adults, and down and back for children. This helps straighten the ear canal and thus create more direct exposure to radiated tympanic membrane temperature. (The directional difference in retraction between adults and children is simply due to a slight anatomical difference between adult and child ear canals.) This is also a reasonably accurate measure of internal temperature due to the internal nature of the membrane. This method can be influenced by battery life in the machine, as can the digital probe method, and any obviously unusual readings should be double checked with new batteries if practical. Both these methods are reasonably accurate, quick, easy, and non invasive.
Past thermometers were also probes. These were glass and originally mercury-filled, then later alcohol-filled. The contents were housed mostly in a bulb at the bottom of the probe and upon temperature contact the liquid either rose or fell in the shaft. Graduations marked on the probe side indicated the temperature level. These forms of thermometers were discarded many years ago in Australia due to the hazardous nature of the contents, and the risk of injury to a patient in the event of the glass shaft breaking at some point during the procedure. A patient with a mouthful of glass after biting the probe is less than ideal regardless of his or her temperature!
One other method of temperature measurement comes to mind is a long insulated wire that is inserted via the nose or mouth into the back of the throat, or top of the airway. It is then plugged into a monitor where the temperature is measured electronically, constantly. This is an extremely invasive procedure and is only used in an operating theatre or a critical or intensive care type unit. Usually the patient is unconscious, may be ventilated (artificially breathing) and very sick. For obvious reasons this is not a common method.
Different types of thermometers are available today "over the counter" for home use. They are mostly of the first two types - digital probe or tympanic. These are ideal for home use as they are simple to use and not too scary for children. It is important to explain to anyone, adult or child, what you are about to do to them. If possible with children it helps to show them the machine and let them hear any noises it may make. A demonstration on the user or the parent is always convincing that it will be ok! It is also important to remember that if you don't have a high tech gizmo on hand, always rely on your own senses. If someone is very hot to touch (good old hand on the forehead) and flushed looking, chances are they have an elevated temperature. Likewise if they are cold and clammy, pale, and shivering - you guessed it - they're probably darn near freezing! Don't get too caught up with the technology.