Thermometers come in various types to suit different purposes. Common types include: 1) Digital – for oral, rectal, or underarm use. 2) Mercury – less common now due to safety concerns. 3) Infrared – measures surface temperature from a distance. 4) Ear – uses infrared technology to measure the inner ear's temperature. 5) Pacifier – for infants. 6) Basal – for tracking ovulation.

The working principle of a thermometer depends on its type. Generally, they measure temperature based on the expansion or change in properties of a substance or electronic sensors. For example, mercury thermometers use the expansion of mercury, while digital ones use electronic sensors that convert the temperature into a digital readout.

Mercury was traditionally used in thermometers because of its unique properties: it remains liquid across a wide temperature range, expands and contracts uniformly with temperature changes, and doesn't stick to glass. This allowed for consistent and accurate readings. However, due to potential health risks if the thermometer breaks, many modern thermometers have shifted away from mercury.

To use a digital thermometer, start by cleaning the tip using soap and water or alcohol wipes. Then, turn on the device and place the probe under the tongue, in the armpit, or in the rectum (depending on the thermometer design). Wait until you hear a beep or until the reading stabilizes. Remove the thermometer, read the temperature, and clean the tip before storing it.

Yes, an oral thermometer can be used under the arm, a method known as axillary measurement. However, it tends to read slightly cooler than oral readings. If you're using an oral thermometer this way, it's recommended to add approximately 0.5°F to 1°F to the reading to get a more accurate body temperature.

Infrared forehead thermometers, also known as non-contact thermometers, measure the infrared energy emitted from the forehead. Their accuracy is generally within ±0.4°F. However, the accuracy can vary based on the brand, how it's used, and the environmental conditions. For best results, follow the manufacturer's guidelines and ensure the forehead is clean and dry.

Digital thermometers are designed to be durable. With proper care and maintenance, including periodic battery replacements, they can last for several years. However, their lifespan may vary based on usage frequency, the brand, and how well they're taken care of.

Ear thermometers, or tympanic thermometers, are safe for babies and offer a quick way to gauge a child's temperature. However, for accurate readings, the thermometer must be positioned correctly in the ear canal. It's important to read the user manual and perhaps consult with a pediatrician for best practices.