They really don't have a name, so they are just called thermometers.
Unless your refrigerator has a built-in thermometer, you would need to check it with a thermometer. Purchase of a refrigerator thermometer is highly recommended.
You can measure the refrigerator temperature using a refrigerator thermometer placed inside the unit. Allow the thermometer to sit for a few hours before checking the temperature. The recommended temperature for a refrigerator is between 35-38 degrees Fahrenheit.
Not usually. An actual "meat" thermometer is not designed to be used in the refrigerator, nor are they generally of the appropriate temperature range.
To get the most accurate reading, place the calibrated thermometer in the middle of the refrigerator, away from the walls and any food items. This location will provide a more average temperature reading, giving you a better sense of the overall temperature inside the refrigerator.
A RV absorption refrigerator uses a thermistor as a thermostat. The thermistor is moved up or down to regulate the refrigerator temperature.
There are several things you can use to guestimate the temperature, but nothing else will give you a decent value for it.
the sixth thermometer
The units on a Fahrenheit thermometer are called degrees Fahrenheit.
Mercury the liquid inside thermometer is mercury.. but it is called thermometric liquid.
In the center of the the refrigerator
A thermometer
The liquid in a clinical thermometer is called mercury. It is used to measure body temperature by expanding and rising within the thermometer.