Mechanics of the Household / A Course of Study Devoted to Domestic Machinery and Household Mechanical Appliances

by E. S. Keene

110 = 220 × 0.5

From this it is seen that any change in the voltage will produce a corresponding change in the current to keep an equality in the equation. If the voltage increases, the current also increases and the lamp burns brighter. Should the voltage decrease the current will decrease and the lamp will burn dim. This dimming effect is noticeable in any lighting system whenever there occurs a change in voltage.

The quantity of electricity used up in such a lamp is expressed in watts, which is the product of the volts and amperes of the circuit. In the lamp described, the product of the voltage (110) by the amount of passing current (½ ampere) is 55 watts. With the above conditions the 16 candlepower of light will require 3.43 watts in the production of each candlepower. The best performance of carbon-filament lamps give a candlepower for each 3.1 watts of energy.

The filament of the tungsten lamp must offer a resistance sufficient to prevent only enough current to pass as will raise its temperature to a point giving the greatest permissible amount of light, and yet not destroy the wire. The high fusing point and the low specific heat of tungsten permits the filament to be heated to a higher temperature than the carbon filament and with a less amount of electric energy. These are the properties that give to the tungsten lamp its value over the carbon lamp.

The exact advantage of the tungsten lamp has been investigated with great care and its behavior under general working conditions is definitely known. In light-giving properties where the carbon-filament lamp requires 3.1 watts to produce a candlepower of light, in the tungsten filament only 1.1 watts are necessary to cause the same effect. The tungsten lamp therefore gives almost three times as much light as the carbon lamp for the same energy expended. The manufacturers aim to make lamps that give the greatest efficiency for a definite number of hours of service. It has been agreed that 1000 working hours shall be the life of the lamps and in that period the filament should give its greatest amount of light for the energy consumed.

The Mazda Lamp.

—The trade name for the lamp giving the greatest efficiency is Mazda. The term is taken as a symbol of efficiency in electric incandescent lighting. At present the Mazda is the tungsten-filament lamp, but should there be found some other more efficient means of lighting, which can take its place to greater advantage, that will become the Mazda lamp.

Candlepower.

—The incandescent lamps are usually rated in light-giving properties by their value in horizontal candlepower. This represents the mean value of the light of the lamp which comes from a horizontal plane passing through the center of illumination and perpendicular to the long axis of the lamp. Candlepower in this connection originally referred to the English standard candle which is made of spermaceti. The standard candle is 0.9 inch in diameter at the base, 0.8 inch in diameter at the top and 10 inches long. It burns 120 grains of spermaceti and wick per hour. This candle is not satisfactory as a standard because of the variable conditions that must surround its use. The American or International standard is equal to 1.11 Hefner candles. The Hefner candle (which is the standard in continental Europe and South American countries) is produced by a lamp burning amylacetate. This lamp consists of a reservoir and wick of standard dimensions which gives a constant quantity of light. The light from this lamp has proven much more satisfactory as a means of measurement of light than the English standard and therefore its use has been very generally adopted.

The light given out by an incandescent lamp is not the same in all directions. In making comparisons it is necessary to define the position from which the light of the lamps is taken. The horizontal candlepower affords a fairly exact means of comparing lamps which have the same shape of filament, but for different kinds of lamps it does not give a true comparison. The spherical candlepower is used to compare lamps of different construction as this gives the mean value at all points of a sphere surrounding the lamp. The candlepower is measured at various positions about the lamp with the use of a photometer, and the mean of these values is taken as the mean spherical candlepower.