Melissa Jacobs, age 8, of Salt Lake City, Utah, for her question:

WHAT MAKES A REFRIGERATOR COLD?

Many thousands of years ago man discovered that he could preserve his food by keeping it cold. Since the middle of the 7800s a system of refrigeration has been under development that will keep many kinds of food from spoiling. Today stores and homes have the problem solved with refrigerators and freezers, cold storage warehouses and refrigerated trucks, railroad cars and display cases.

Refrigeration is based on the second law of thermodynamics which states that heat flows only from warmer bodies to colder bodies, or from a substance at a certain temperature to a substance at a lower temperature. Refrigeration, to put it simply, removes heat from solids, liquids and gases.

A mechanical refrigerator works on the principle that liquids absorb heat when they vaporize. An easy demonstration of this can be seen when you set your hands and wave them rapidly. The water evaporates quickly and causes a cooling feeling as well as a lowering of the skin temperature.

Compression and absorption systems refrigerate by changing a refrigerant from a liquid to a gas and back to a liquid again. When this operation is repeated many times, it is called the refrigeration cycle.

Electric refrigerators use a compression type unit powered by an electric motor. The refrigerator in your home probably has five basic parts: the receiver, the refrigerant control device, the evaporator, the compressor and the condenser.

A refrigerant, in most cases a product called Freon 12, leaves the receiver to begin the refrigeration cycle. It then travels through pipes to the refrigerant¬control device. This is a unit which reduces the pressure of the refrigerant as it enters the evaporator. The evaporator is the coldest spot in the refrigerator and serves as the freezing unit.

A refrigerator's evaporator is made up of pipes or coils on the walls or sides of the cabinet or surrounding the ice tray compartment. At a low pressure, the liquid refrigerant evaporates inside these coils and absorbs heat. This causes refrigeration to take place.

The compressor pumps the refrigerant from the freezing unit as a vapor and raises its pressure. It then discharges high pressure gas into the air cooled condenser. There the gas loses the heat it gained in the evaporator and condenses into a liquid at the high pressure. The liquid refrigerant then blows back to the storage tank.

A gas refrigerator works on the absorption principle. Heat energy is used as a source of power and it has no moving parts. A home gas refrigerator is made up a generator, a separator, a condenser, an evaporator and an absorber. Liquid ammonia is used as the refrigerant in most gas refrigerators.

Defrosting is vitally important in caring for a refrigerator. Many units in homes today have either automatic or semiautomatic defrosting devices.