This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
This term is used to denote the force which moves or tends to move electricity from one point in a conductor to another. The analogy of the water pipes will again be useful in explaining the nature of electromotive force, or E.M.F., as it is usually abbreviated. The rate of flow of water in pipes depends not only upon the pressure or head of water, but upon the resistance offered by the pipes. Thus if the pipes are choked with dirt, they will not deliver the same amount of water as when the pipes are clear, although the head of water is the same in each case. So with electricity in a conductor, the quantity that passes any point depends upon the electromotive force and the resistance of the conductor. Just as water flows from a cistern at a high level to a tap at a low level, so electricity may be regarded as flowing from the positive pole of a battery or dynamo towards the negative pole, the flow being maintained by the difference of pressure, or the difference of potential as it is called, between the two poles.
Unit Electromotive Force, or unit difference of potential, exists between two points of a conductor when an expenditure of one erg is required to force one unit of positive electricity from one point to the other against the electric force. This is the absolute unit of electromotive force.
A conductor is said to have one absolute unit of resistance when it requires unit difference of potential to cause unit current to flow through it.
The practical unit of resistance is the ohm, which is 109 of the absolute unit of resistance, and is represented practically by the resistance of a column of mercury 106.3 centimetres long, and 14.4521 grammes in mass, at o° C. A unit of 1000 ohms is called a megohm.
The practical unit of electromotive force is called the volt, and is that electromotive force which, applied to a resistance of one ohm, will cause a current of one ampere to flow.
Ohm's Law states that the current flowing between two points of a conductor is directly proportional to the electromotive force and inversely proportional to the resistance. This, expressed algebraically, is
C=E / R or, E = C R or, R = E / C where C = current in amperes; E = electromotive force in volts, and R = resistance of conductor in ohms. One thousand ohms is called a megohm. These formulae are necessary for most practical calculations.
The term power as applied to electricity represents the rate at which electrical energy is produced or consumed.
The practical unit of electrical power is the watt, which is the power produced by a current of one ampere flowing at a pressure of one volt. It should be noted that a watt is equivalent to ten times the absolute unit of mechanical power, namely, ten ergs per second. The value of the watt may be expressed as follows: -
1 ampere x 1 volt = 1 watt or, C x V = W
This formula is of great value in practical calculations. A unit called the kilowatt, consisting of 1000 watts, is often used for convenience in practice, as, for instance, to indicate the rate at which a dynamo can produce electrical energy, or the rate at which energy is consumed.
To indicate the actual amount of work done in a given time, the unit known as the watt-hour is employed. For convenience sake, a unit consisting of 1000 watt-hours is used in practice, and is known as a Board of Trade Unit. It is this unit which is referred to upon a householder's electric light or power bill as so many "units" at such and such a price each.