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will set fire to inflammable material near it. It is not the total amount of heat generated that determines the temperature, for the wire may be long, and the heat is then distributed, or it may take so long to generate the heat that it is dissipated as fast as it is produced. The temperature is determined by the rate of generation per foot of wire. Thus though the whole of the resistance in an electric circuit is what determines the total heat generated by a given current, it is the amount of resistance per foot that, with a given current, determines the temperature to which the wire will be raised. On the other hand, since all heat generated means lost power, it is the total heat and not the temperature that affects the available force at the lamp or motor.
It is partly because of these facts, that wires of inadequate size are so often used. The total resistance may not be sufficient to absorb an appreciable part of the force and so affect the lamps or motor, but the resistance for a given length of wire may be sufficient to cause more heat than can be dissipated by that length without great rise in temperature.
The boards of fire underwriters have, accordingly, determined upon proper "carrying capacities" for the different sizes of copper wire, and wiremen generally now follow the tables provided. As inflammable material is often brought near electric wires and as excessive heat destroys the insulating properties of the covering, but little rise in temperature is allowed.
The compliance with the underwriters' rules in this regard has done a great deal to make incandescentlighting service satisfactory. Whatever energy is lost in heating the wire is, of course, unavailable for heating the filament in the lamp. The effect of the rules is to limit the loss, or heat, per foot. There might be a very small loss per foot and yet a serious total loss, but for short distances the minimum sizes of wires allowed by the underwriters are often larger than would be necessary if consideration were given only to the effect upon the lights. In places where a small number of lights are used and where, ordinarily, guesswork would be resorted to, the rules thus do much toward securing good service.
As soon as the wiremen get away from compulsion, however, they are apt to make the wires too small for good results. Since every foot of wire has its definite amount of resistance depending upon its size, a definite amount of force depending upon the volume of current is spent in overcoming the resistance in each foot. The amount of force or pressure in incandescent-lighting is ordinarily kept constant, and if all the lights in a building are to burn with equal brilliancy, the loss of force in the wires leading from the mains to the lamps must be made negligible. To this end the wire must be chosen with regard to both the distance and the number of lamps, for it is the distance that determines the total resistance, and the number of lamps or "load" that determines the volume of current.
The analogy between the distribution of electrical energy and the distribution of water or gas is useful here. In the holiday season, when the volume of gas drawn off is very great, the lights are often noticeably poor. The pipes offer so much resistance that the usual pressure at the gas-works is not sufficient to force along the great volume of gas and still give the ordinary pressure in the buildings. The same effect is seen in long lengths of water-pipe. If water be drawn from cocks near the supply, much less will come from a cock farther off, although the pressure at the pump may remain the same. The pressure at the gas-jet or water-cock cannot be kept up, and in the same way the electrical pressure falls before it reaches the lamp terminals, if too small wires are used. Each incandescent lamp requires its own small but definite amount of current. Turning on the current is like turning on water or gas, but the filament of the lamp offers so much resistance that the force at the terminals can send only a very fine stream, so to speak, through it, just as a very small pipe will allow but little water to pass.
The lamp itself is a good illustration of the heating a
effect of the current. There is a great resistance per foot in the filament. Even the small current passing through generates heat so fast that it can be dissipated at an equal rate only when the whole is incandescent. The construction of the lamp gives a conductor of considerable resistance in small space, and it is designed to give the filament "long life," but any conductor will be heated to incandescence provided sufficient current be passed through it.
A lamp is made to be used at a certain electrical pressure, and it is only at this pressure that the proper amount of current will go through it and thus cause the proper amount of light. It is for this reason that it is so important to calculate the sizes of the wires and thus give every lamp as nearly as possible the same pressure. Every branch of a line requires a separate calculation, for there are different volumes of current in each branch, and the lamps are at different distances. It is at this point that rule-of-thumb methods fail, and some degree of engineering ability becomes necessary to prevent excessive loss in the wires and on the other hand to prevent unnecessary expenditure for copper in the conductors. In buildings improperly wired, it will frequently be noticed that lamps in one part will be much brighter than those in other parts, or it will be found that when all lamps are turned on the light is poor, and that it improves when some of the lamps are switched off. In these cases it will almost invariably be found that some of the wires, usually the longer ones, are too small.
There is no excuse for this sort of work except the saving it effects in the cost of material. It is not a matter of judgment nor of experience, but simply of calculation. It is known how much current each lamp requires, and exactly how much the pressure will be lessened when the necessary current is forced over a given length of wire of a certain size. With no other force that we use can results be so easily and so exactly predicted.
Of course, with wires of reasonable size there must always be some difference in the lighting effects of any two lamps, but it is not a difficult matter to make it so small that the eye cannot detect it. It is now ordinarily specified that the electrical pressure at any two lamps in a building shall not differ by more than two per cent under any conditions of use. This difference is within reasonable limits, and is sufficiently small to prevent noticeable change in lights when others are turned on or off. Not infrequently it is required that the pressure at no two lamps shall differ by more than one per cent.
The prediction of results in electric circuits and the laying out of systems of distribution are greatly facilitated by the simplicity of the law that controls the