What variables may affect the time taken to cook a potato?

Directions for cooking meat usually state the time of cooking in terms of minutes per pound. At best this can only serve as a guide, for several factors may cause variation in the length of time required to cook a piece of meat. The only way of knowing when the interior of a piece of meat has reached a definite temperature and thus a definite stage of cookery is to insert something in the center of the meat by which the interior temperature may be read. For this purpose a thermometer or a thermocouple may be used. The easiest method for household use is the insertion of a thermometer into the meat. Two types of thermometers may be used: a right-angled thermometer with the temperature scale on the horizontal arm or a very short tube-type thermometer about 6 or 8 inches long with the temperature scale on the upper half. The latter must be short to keep the top from touching the upper part of the oven, particularly the small gas ovens and many electric ovens. Thus the graduated scale is condensed, which makes it more difficult to read. The short thermometer is preferable for small roasts of meat and for meat with soft fibers. The right-angled thermometer is suitable for beef roasts, ham, leg of veal, and lamb. It is convenient for taking temperatures of custards, cakes, and other foods, for it can be supported by the horizontal arm from a shelf in the oven.