A thermometer (from the Greek θερμός, thermos, meaning "hot" and μἐτρον, metron, "measure") is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury-in-glass thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer).
There are many types and many uses for thermometers, as detailed below in sections of this article.
Read more about Thermometer: Temperature, Development, Physical Principles of Thermometry, Primary and Secondary Thermometers, Calibration, Precision, Accuracy, and Reproducibility, Uses, Various Types of Thermometer