If we file a soft iron nail for a moment and then feel the file surface, we find that it is warm or hot; that is, the surface of the file is warmer than the body. Another way of expressing the same idea is to say that the temperature of the surface of the file is higher than that of the body. There is then a transfer of heat from the warmer body to the colder body, until both are equally warm. Then both bodies are said to have the same temperature. A hot frying pan when plunged into a bucket of water gives off heat to the water, until the temperatures of the water and the frying pan become equal. Temperature is a measure of the tendency of a body to give up its heat to other bodies.
The surface of the file becomes warm or hot because of friction. The same effect is produced on the surface of a saw in sawing wood, in rubbing a metal surface on cloth, in the bearings of moving car-wheels, etc. Heat is generated also when a piece of lead or other metal is hammered and when a rifle bullet strikes a wall. This heat is caused by percussion.
Heat is also produced by compression, chemical means, and electricity. For example, the temperature of air is raised when it is compressed in a bicycle pump; when muriatic acid is added to zinc the chemical reaction which takes place produces heat; a current of electricity passing through a piece of platinum raises the temperature of the platinum.
Two very common effects of heat noticed in every-day life are the changes in length, surface, or volume of materials, and the changes of state - from solid to liquid and from liquid to gaseous. Since heat is due to the motion of the particles that compose a body, it will expand as the rate of motion is increased. This principle is utilized when the blacksmith first heats a tire before putting it on a wheel so that when the tire contracts as it cools it fits closely. For the same reason, rivets are made red-hot before they are put into boilers, bridges, or steel structures. When cool they contract and draw the parts tightly together.
Heat travels in three distinct ways: by conduction, by convection, and by radiation.
When a poker is placed in a fire, the heat passes along the poker from the hot to the cold part; this action illustrates conduction. Heat passes through some materials more readily than through others; materials of the first class are called good conductors and those of the second class, poor conductors. Iron, for instance, is a good conductor and wood a poor conductor of heat.
The heat from a stove passes through the air without any apparent motion; movement of heat in this manner is called convection.
Heat comes to us from the sun; this method of transmission of heat is called radiation.