Meat probes contain heat-reactive materials and measure temperature by gauging the reaction of the material when it comes in contact with the heated layers of meat. There are several different types of meat thermometers, including dial, bulb and digital models. Each of these types works differently.
A dial thermometer contains two different metals, and each expands at a different rate when exposed to heat. A needle made from these two metals probes the meat, and a coil of the same metals attaches to a pointer in front of a temperature dial. When the metals expand, the pressure in the coil turns the pointer, which stops in front of the marked temperature on the dial.
A digital thermometer works on the principle that electricity has more difficulty passing through heated metal than cooled metal. A metal probe sticks into the meat, and then an electric current runs through the probe while a microchip measures the level of resistance. The digital screen shows this resistance as a temperature reading.
A bulb thermometer for kitchen use usually contains alcohol and red dye. The thermometer probes the meat, and the liquid reacts to the heat by expanding. The red liquid rises beside a vertical line marked with temperature readings and measures the expansion.