【正文】 and further effort in developing personnel to engineer, design, construct, maintain, and operate these facilities will continue to improve the reliability of the electric power supply. The cost of electric power is a prince consideration in the design and operation of electric power is a prime consideration in the design and operation of electric power system. Although the cost of almost all modities has risen steadily over the past many years, the cost per kilowatthour of electrical energy has actually declined. This decrease in cost has been possible because of improved efficiencies of the generating stations and distribution systems. Although franchises often grant the electric power pany exclusive rights for the supply of electric power to an area. There is keen petition between electric power and other forms of energy, particularly for heating and for certain heavy load industrial processes. The power supply requirements just discussed are all well known to most electric power users. There are, however, other specifications to the electric power supply which are so effectively handled by the power panies that consumers are seldom aware that such requirements are of importance. The frequency of electric power supply in the United States is almost entirely 60 hertz (formerly cycles per second). The frequency of a system is dependent entirely upon the speed at which the supply generator is rotated by its prime mover. Hence frequency control is basically a matter of speed control of the machines in the generating stations. Modern speedcontrol systems are very effective and hold frequency almost constant. Deviations are seldom greater than hertz. In an ac system the voltage continually varies with time, at one instant being positive and a short time later being negative, going through 60 plete cycles of change in each second. Ideally a plot of the time change should be a sine wave. In poorly designed generating equipment, harmonics may be present and the wave shape may 8 be somewhat. The presence of harmonics produces unnecessary losses in the customer’s equipment and sometime produces hum in nearby telephone lines. The voltage wave shape is basically determined by the construction of the generation equipment. The power panies put specification limitations on the harmonic content of generator voltages and so require equipment manufactures to design and build their machines to minimize from this effect. ENVIRONMENT POLLUTION The existence of pollution in the environment, as a national and a world problem, was not generally recognized until the many people regard pollution as a problem that will not go away, but one that could get worse in the future. It is increasingly being appreciated that the general effects of pollution produce a deterioration of the quality of the environment. This usually means that pollution is responsible for dirty streams, rivers and sea shorts, atmospheric contamination, the dissociation of the countryside, urban dereliction, affecting the environment in which people reside, work, and spend their leisure time. The present increasing emphasis upon pollution may create the impression that there has been a relatively sudden deterioration of the environment, that was not apparent twenty or thirty years ago. This is not the case. Pollution must have started at the time when man began to use the natural resources of the environment for his own benefit. At he began to develop a settled life in small munities, the activities of clearing trees, building shelter, cultivating crops, and preparing and cooking food must have altered the natural environment. Later, as the human population increased and became concentrated into large munities which developed craft skills. There were increasing quantities of human and animal waste and rubbish to be disposed of in the early days of man’s existence the amount of waste was small. It was disposed of locally and had virtually no effect upon the environment. Later, when large human settlements and towns were established, waste disposal began to cause obvious pollution of streets and water courses. In the thirteenth century the prevalence of cholera, typhus, typhoid and bubonic plague was associated with the lack of proper waste disposal methods. By themednieenth century the population of the UK had increased to 22 million, and many canals and rivers were grossly polluted with sewage and industrial waste. Some sewerage systems existed in towns, but the collected sewage was discharged into the nearest river without ant treatment. Salmon had pletely disappeared from the River Thames and outbreaks of cholera still occurred in London. A Royal Commission on the Prevention of River Pollution was established in 1857, and eventually the first preventive river pollution legislation was passed in 1876 and 1890. However, there was little significant improvement in pollution until after the First World War, and the condition of rivers had deteriorated again by the end of the Second World War. Even today, a number of British and continental coastal towns discharge almost untreated sewage into nearshore waters. The increasing pollution of land water was acpanied by air pollution. This must have begun as soon as man started to use wood fires to provide ‘space hosting’ and a means of cooking food. Later surface, soft coal was discovered and used as a fuel, and records shown that coal smokes was a nuisance in London in the thirteenth century. In 1273,Edward I made the first ever antipollution law to prevent the use of coal for domestic heating, so smoke pollution has been recognized for at least 700 years. However, smoke pollution in London continued and is 9 recorded in both the sixteenth a