The degree of "doneness" of meat or bread correlates closely with the internal temperature, so that a thermometer reading indicates when it is cooked as desired.
Some show the temperature only; others also have markings to indicate when different kinds of meat are done to a specified degree (e.g., "beef medium rare").
Some use a bimetallic strip which rotates a needle which shows the temperature on a dial; the whole thermometer can be left inside the oven during cooking.
Another variety commonly used on turkey is the pop-up timer, which uses a spring held in by a soft material that "pops up" when the meat reaches a set temperature.
[2] For beef, lamb, or veal insert the meat thermometer away from bone, fat, or cartilage.
[2] Pork needs to reach the same temperature 71 °C (160 °F) as beef, lamb, or veal and the same rules for use of the thermometer apply.