Зображення сторінки
PDF
ePub

CHAPTER II.

THE PROPORTIONING OF WIRES. — THE HEATING

-

EFFECT OF THE CURRENT. —LOSS OF FORCE.— OHM'S LAW.-WIRE CALCULATION.

A MATTER of great importance in electrical work, with regard to both safety and satisfactory service, is the correct proportioning of wires to the currents they have to carry. Conductors always offer a resistance to the passage of the current, and this not only causes heating of the wire and waste of energy, but it diminishes the pressure at the lamp, motor, or other device operated.

A current of electricity, however small, will heat in some degree a wire of any size, and if the current be large and the wire small this heat may become of great intensity. The effect is not different from that in any case where resistance is overcome by force. When a nail is driven into lead it is very plain that the energy is transformed into heat. Meteors falling through the atmosphere are heated to incandescence, and even when water is forced through a pipe, a part

of the energy is transformed into heat because of the resistance of friction. We may not know exactly how a current of electricity sets the molecules of a conductor in motion and thus produces heat, but we do know that it requires a certain force to make a current flow in a wire, and that if we make the wire smaller it takes more force to send the same current through it. We know, too, that if the heat generated is measured it is found to be equal to the energy expended. By making the wire smaller, the resistance is increased and more force must be applied so that this resistance may be overcome.

If a pump is to deliver 100 gallons of water per minute through a pipe, it will take a certain amount of force even though the pipe be a foot in diameter, but if it be reduced to one-tenth the size, it will take enormously more force and the additional amount will all be used simply in overcoming friction—in heating the pipe and water. In this case the heat is carried away so fast by the water itself that it is of course only the waste of power that is serious. In forcing electric currents through wires, however, not only is energy wasted, but the heat is left in the wire to dissipate as best it can. The wire goes on heating until the generation of heat is balanced by the dissipation, and if the current be much too large the wire, in the meantime, may become of such a temperature that it

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 exces

sive 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

« НазадПродовжити »