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.
"Wiring" is a oomprehensivo term, including all methods of conveying electricity through wires to and from the various points at which it is to be utilized. Before the proa and cons of the different methods can be discussed, it is necessary to understand the fundamental principle, that the electric current requires a circuit or path, from the point at which it is generated, to the point at which the work is to be done, and thence back to the original point at which it was generated.
The sire of the wires must be in proportion to the current conveyed and the distance traversed, in order that very little of the energy may be consumed in them, leaving nearly all the original energy to be usefully absorbed in the work in view; for instance, if we take an incandescent lamp, - which is merely a thin strip of carbon in a vacuum, preserved by a light glass globe, - it is in that strip of carbon (or filament, as it is termed) that the energy should be absorbed; so the wires leading to it should offer very little resistance to the current, and the pressure be such as will force the required amount of electricity through the high resistance of the carbon to make it incandescent, and so give light.
The pressure of the current should be decided upon according to the area or distance through which the current has to pass. In domestic lighting by electric, incandescent lamps, such pressures as 60, 100, or 110 volts are usually employed, because these have been found convenient for such work. That the theory, however, may be correctly understood, I would point out that 5, or 10, or any other number of volts, would be equally effective, as will be seen by the following explanation. It has been already explained that a given quantity of electrical energy can be divided into whatever pressure and current may be found necessary, - that is to say, 1000 watts of energy is represented alike by 100 volts and 10 amperes, 500 volts and 2 amperes, or any other combination the product of which is 1000. If it is decided to use an incandescent lamp having such a resistance of filament that 60-volts pressure is necessary to force one ampere through it, that lamp would be said to require 6O watts, and if the one ampere is found to make the filament hot to such an extent as to emit a light of sixteen candles, the sixteen-candle-power lamp is said to take 60 watts. The filament of the lamp, however, might be of such reduced size that half an ampere, on being forced through it by the 60-volts pressure, would be sufficient to make it hot enough to give the same light of sixteen candle-powers.
To understand this proportioning of the filament, it is only necessary to imagine a lamp with such a large diameter of filament that the one ampere would not render it hot at all, but would pass easily through it, just as it does through the copper circuit-wire, in which, of course, it does not appreciably raise the temperature. The length, however, of a filament of such an increased diameter would have to be extended, if it were required to offer sufficient resistance to the 60 volts to permit only one ampere to pass.
From these two examples, since an equal amount of light is obtained in each case, one would be apt to say that it would be more economical to take the smaller amount of current, and this would undoubtedly be true as far as first cost is concerned, but the smaller filament has to be raised to a higher -state of incandescence to make it give the same light, and it is consequently not likely to stand the wear and tear so long. By actual experience, a standard has been fixed, at which it is reasonable to work filaments.
A sixteen-candle-power lamp is considered to have a sufficiently long life when absorbing 60 watts, at which amount of energy the filament is not unduly heated, and it will be seen that a light of this intensity, absorbing a total of 60 watts, absorbs for each candle-power a little under 4 watts, which is the method used to describe the light-giving efficiency required; - that is to say, a lamp-manufacturer, offering lamps of sixteen candle-powers at 4 watts per candle, states directly the class of article, leaving the purchaser to order them at 100 volts or 60 volts, according to the pressure of his source of current.
At the present day a light-giving efficiency of one candle-power for every 2 ½ watte consumed, which would be equivalent to a sixteen-candle lamp absorbing 40 watts, is considered very high, but, by improvements in manufacture, the light-giving efficiency of lamps will no doubt be from time to time increased, the great problem of production being to find a process to render a filament so strong as to stand an increased state of incandescence, by which it will give more light for the same energy expended. The expression "light-giving" efficiency has been used to prevent confusion with "life" efficiency, which is the length of time the filament will bear being raised to the stated degree of incandescence.
It is an easy matter for a lamp-maker to guarantee any light-giving efficiency, but when this is coupled with life-efficiency, it is another matter; for instance, the 60-watt lamp could be made to give a light of (say) 200 candles, but only perhaps for a few minutes, as the filament could not stand such a very high degree of incandescence much longer. A 60-watt lamp is generally supposed to stand burning for 1000 hours, and a 40-watt lamp perhaps only 600 hours.
The efficiency in watts and life, then, are important points to stipulate for. All makers of incandeeoent lamps. 1 believe, retuse to guarantee any particular life, as the article is a very treacherous one to make, and they are uuable to feel satisfied that the lamp has received proper treatment.
To show how a lamp may be improperly treated, and at the same time to help to explain the above matter, we will suppose we have a 100-volt circuit, and put a 60-volt lamp in contact. The result will be an intense light, since the lamp has been made to take (say) one ampere with a pressure of 60 volts, whereas the increased voltage of 100 will send through it far more than one ampere, and will raise the degree of incandescence much higher than that for which the filament was intended. This would probably have the effect, during the short time the lamp could stand such treatment, of permanently blackening the inside of the glass by the volatilization of the carbon.