Since reference will be made to the various forms of energy later on, a brief discussion of the most important forms of energy is given below.

(a) Electrical Energy. The generation of electricity by mechanical means is accepted by all of us as a fact. The ease with which electrical energy can be converted into other forms, and the ease with which it can be transmitted from one place to another had led to its wide use in our modern times. Electricity plays a very important part in the recording and reproduction of sound pictures, and a fundamental knowledge of the behavior of electricity is necessary for a clear understanding of the processes involved. Therefore a brief discussion of the principles of electricity will be given later.

(b) Light Energy. Light is anything which affects the sensation of sight. It may seem peculiar to consider light as a form of energy, since it was said that energy was anything which arose from work, or which could be converted into work, or was work itself. It cannot be shown very easily that light is work, but it is easy to see that light may arise as a result of work, as, for example, the red hot sparks that fly from a high speed saw in cutting steel. In this case a great deal of heat is generated. The heated material emits light depending on the degree of temperature. Another interesting feature of light is the fact that practically all of the energy which exists on the earth came here as light from the sun. It is this light energy from the sun which produces our winds, and gives us rain and the resultant water power.

These effects are commonly attributed to the heat of the sun's rays, but heat does not travel through a vacuum as evidenced by the "Thermos" bottles in every day use. Practically no heat arrives at the earth directly from the sun, but is produced at the surface of the earth by the effect of the light rays. This gives us a striking example of the transformation of energy from one form to another. When the light from the sun strikes the earth, the surface of the earth is heated, which, in turn, heats the surrounding air. Heated air is lighter than cold air and rises, thus setting the air in circulation and producing winds. The warm air evaporates water from the surface of the earth (oceans, lakes, rivers, etc.), and rising warm air currents carry the evaporated moisture up with them until they come in contact with the cooler upper layers of air and are cooled. Cold air cannot hold as much water vapor as warm air so that, when the warm air is cooled, the water vapor contained in it condenses and falls as rain. Some of this rain falls on portions of the earth's surface of high elevation, and the energy of the water due to its elevated position is available for turning water wheels for the generation of electric power.

(c) Heat Energy. Heat, as pointed out above, is another form of energy, and a certain amount of it is produced whenever work is done in any form as a sort of by product of the work itself. This is usually considered as a loss of energy except where it is produced for a definite purpose.

Frictional losses in machinery result from the transformation of mechanical energy to heat energy. If the bearings are not kept lubricated so as to reduce this loss, the bearing surfaces will heat up and become pitted if not entirely destroyed.

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