The thermometer is humanity's oldest heat-measuring tool, dating all the way back to the 1600s. Today, different kinds of thermometers can measure everything from outdoor temperature to the temperature of cooked meat. Other tools can measure the heat of entire buildings or even the amount of energy in food by producing heat reactions.
TL;DR (Too Long; Didn't Read)
A variety of instruments can measure heat for different purposes. Thermographs produce images of heat using infrared light. These images can pinpoint the hot and cold areas of people, buildings and more. Thermometers can measure different temperatures. Calorimeters can measure the amount of calories present in food by measuring the heat that the food gives off.
Measuring Heat with Thermographs
Unlike a thermometer, a thermograph does not simply produce a number to explain the heat that it measures. Thermographs use built-in infrared cameras to produce images of heat. Because the infrared light produced by an object or organism is directly proportional to its temperature, the colors present on an infrared picture can accurately convey a full temperature "scan" of whatever image it captures. The images produced by thermographs show up in detailed colors or in black and white. Either way, lighter parts of the image indicate warmer temperatures than darker parts.
Thermographs range in size and shape, depending on what type of job they need to accomplish. Thermography, or the practice of using thermographs to measure heat, can measure the body heat of subjects undergoing medical trials. It can even determine the draftiest parts of a building, by showing which areas are the coldest. The U.S. Department of Energy offers thermography readings to U.S. homeowners to help them insulate their homes more efficiently.
Measuring Heat With Thermometers
Invented centuries ago, the first thermometers had alcohol, encased in glass, to measure air temperature. Mercury replaced alcohol because it expands and contracts quickly in response to temperature. Today, digital thermometers, which use screens and number displays to communicate temperature, can measure heat in a variety of situations.
Medical thermometers can measure body heat. The most common type of medical thermometer goes into the ear and stays there until the thermometer gives a reading. These thermometers measure the infrared energy near a person's eardrum. Unlike a thermograph, however, you do not get a picture. Instead, the temperature shows up as numbers on a small screen.
Meat thermometers, used to measure the temperature of cooking meat, have oven-safe components, which allow them to withstand high temperatures. A current of electricity flows through the metal tip of a meat thermometer, while a microchip keeps track of the current. The more the metal tip heats up, the more difficult it is for the current to flow. The microchip keeps track of these changes in current resistance and converts that information into a readable temperature.
Measuring Heat with Calorimeters
Most packaged foods sold in U.S. stores have to list the food's calories on their nutrition labels. Calories are units of heat. One calorie describes the amount of heat needed to raise 1 liter of water by 1 degree Celsius. To determine the amount of calories in food, you can use an instrument called a calorimeter.
First, you place about 1 gram of food in a sealed metal container, which is inside the calorimeter. You fill the rest of the calorimeter with water and seal it shut. The food inside of the metal container ignites via a cotton thread, which sticks out of the calorimeter. The burning food inside the metal container heats up the water surrounding it. The calorimeter measures this change in water temperature. By measuring how much the water temperature rises, the calorimeter can determine the calories present in the food.