The chapter discusses the effect of dimensions on resistance, resistivity, resistance calculations, effect of temperature on resistance, effect of temperature changes, and voltage drop in cables. A summary of formulas, and exercises are finally given.

In early childhood, we become familiar with the sensations of cold and warmth and are able to distinguish between them. We learn to estimate the degree of hotness or coldness of a body, which is known as the temperature. Heat is a form of energy. Heat added to a body makes it hotter, and heat taken away from a body makes it colder. It is possible, for instance, to increase the heat energy contained in a piece of metal, and hence to increase its temperature, by doing work such as cutting, bending or hammering it. Again, if work is done against friction, or if a fire is lit beneath the metal, it will become hotter as it absorbs part of the energy made available to it. Thus, when heat energy is produced, energy in some other form is expended. Most of the losses of energy which occur in machines appear as heat, which is usually lost to the process concerned, although not destroyed.

The book chapter covers the following topics: magnetic fields; units of magnetic flux; electromagnets; calculations for air-cored solenoids; effect of iron on magnetic circuit; permanent magnets; summary of formulas.

Most of the electrical power used is generated in rotating machines; a sufficient number of generators are necessary to provide the maximum load required, since power used at a particular time must be generated at that time. Many generators stand idle for long periods because they are needed only to meet peak demands, which usually occur for only a few hours a day. Fewer generators would be necessary if electricity could be stored during the night for use during the day. In one method, a pumped-storage system is used in which water is pumped up to a high-level reservoir during the period of low demand at night so that it can be used to drive water turbines at tunes when demand is high. Lack of suitable sites prevents the system from being widely applied. Another method of storage is to provide chemical energy which can be converted to electrical energy as required. Although the cost of such an operation on a national scale would be prohibitive, this method is very widely used. A unit for chemical to electrical energy conversion is called a cell, and there are two types of cell, primary cells and secondary cells.

Although direct-current systems and calculations are still indispensable to the electrical engineer, virtually all public supplies are now alternating-current mains. The reasons for the changeover from DC to AC supplies will be considered in Section 10.2, our purpose here being to indicate how the two systems differ The easiest method of portraying an alternating quantity is to draw a graph showing how it varies with tune. Any part of the graph which lies above the horizontal (or zero) axis represents current or voltage in one direction, and values below it represent current or voltage in the other direction. The pattern given by the graph is known as the waveform of the AC system, and this usually repeats itself. There is no need for the waveform above the zero axis to have the same shape as that below it although, in most AC systems derived from mains supplies, this is the case.

The purpose of this chapter is to show how the basic theory already considered is put into practice to provide electrical supplies and systems. There are a number of basic types of supply, and these will be considered in greater detail in the succeeding section. Electricity is dangerous, as its misuse can generate heat which results in fire. If the human body becomes part of an electric circuit, the resulting 'shock' can result in burns, or may even be fatal. This chapter will begin to show the principles to be followed if these dangers are to be avoided.

Having considered the principles of electrical technology, the precautions necessary for safety, and the cables which can be used for installations, we now have to con sider the electrical installations themselves. Electrical installations can vary widely in scope and complexity, from, for example, a single lighting point on the one hand to the complete installation of an oil refinery on the other All installations follow the same basic principles to ensure safety from fire and shock. However, the mains and distribution parts of large industrial systems are complicated and will not be considered here; domestic and small commercial installations form the majority of the total of electrical installations. Only this type of work is considered in the following sections.