This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
At the outset it is necessary to define a few of the technical terms in use among electricians.
Ampere and Current both signify quantity; as in speaking of water, one would say so many gallons, so in speaking of electricity, one would say so many amperes, and to combine the expressions of pressure and quantity, one would say, for instance, 10 amperes at l00 volts.
Watts are the product found by multiplying amperes and volts together; for example, 100 volts multiplied by 10 amperes represent 1000 watts, which is certain amount of electrical energy. If a battery were described as giving 1000 watts at (say) 50 volts, the current could be ascertained by dividing the 1000 by 50, the result in amperes being 20. So any quantity of energy, described by so many watts, can be expressed in volts and amperes, provided the quantity of either of these latter is known.
Ohm is the term used to express the resistance offered in any electrical circuit. This resistance - to use again the analogy of water - may be compared to the stance offered by a pipe to a quantity of water passing through it.
In order to illustrate the meaning of these terms, we will take one example comprising them all, using the analogy of water again to prevent any misunderstanding. If 100 gallons of water at a pressure of 50 lbs. are passing through a pipe offering a certain resistance, the 100 gallons multiplied by the 50 lbs. give a force of 5000 pound-gallons, or a definite amount of energy, from which, however, the resistance offered by the pipe remains to be taken; so, with electricity, we can have 100 amperes at a pressure of 50 volts, passing through a certain resistance, and the 100 amperes multiplied by the 50 volts show a force of 5000 watts, or a definite amount of energy, while the positive side of the resistance will be that at which the current enters, and the negative side that at which it leaves, or to which the energy of the current has been reduced or exhausted. All calculations, such as those above given, follow a rule known as Ohm's law, wliich is a rule for ascertaining any one of the three quantities, if the oth«r two are known. Let C = current, E = electro-motive force (usually abbreviated to E.M.F.), and R = resistance, then
C = "R' R=C' and -E= R x C.
As an example, we will suppose that a current at an E.M.F. of 50 volts is passing through a resistance of 5 ohms, and that it is desired to know the quantity of that current; this is ascertained by merely dividing the 50 volts by the 5 ohms, which gives the result of 10 amperes. Similarly, the resistance or the electro-motive force can be ascertained if, in each case, the other two quantities are known.
The terms "positive" and "negative" refer to the direction of the current, the former being often designated by the sign +, and the latter by the sign - .