Of electric lamps three kinds are common in household lamp costs one-half cent an hour. This is true of any 50-watt lamp without regard to the kind of filament it has. The amounts of light produced by different kinds of filaments are, however, decidedly different (Fig. 47). If the voltage supplied is lower than the lamps are intended for, the watts taken by the lamps are reduced somewhat, but the light is reduced a great deal more. The amount of light obtained from a given amount of electric

Inverted manfle Open flame Upright mantle use. These are the ordinary carbon, the metallized carbon or "Gem," and the tungsten lamp. Nearly all of the last kind sold in this country bear the trade name "Mazda." All three kinds are commonly marked with the number of watts (power) they take when used at the number of volts (electrical pressure) also marked on the lamp. When electricity is paid for at a certain rate a kilowatt hour, the cost of current for any lamp is easily calculated. The kilowatt hour is 1000 watt hours, and the number of watt hours used by any electrical device is simply the watts times the number of hours burned. For example, a 50-watt lamp in twenty hours uses 1,000 watt hours or 1 kilowatt hour; at 10 cents a kilowatt hour current for such a Fig. 46.* - Comparison of amount of light given by different gas lamps. Each lamp is supposed to burn 5 cubic feet an hour, costing 1/2 cent if gas is \$1.00 a 1000 cubic feet. Note that the mantle lamp gives four or five times as much light as the open flame. The inverted mantle gives more light downward and less upward than the upright mantle.

* U. S. Bur. Standards, Circ. 55. ≪--------- 20 Candlepower energy depends, therefore, on the voltage. Under fair conditions the amount of light and the corresponding cost for current (at 10 cents a kilowatt hour) would be about as follows:

Fig. 47. - Comparison of amount of light given by different electric lamps. For comparison a consumption of 50 watts is assumed, although tungsten lamps are not usually made in this size. Fifty watts at 10 cents a kilowatt hour costs 1/2 cent an hour, and gives 12 to 16 candles in a carbon lamp and 40 to 45 in a tungsten lamp. All lamps give different candle-power at different angles. Each arrow in the figure is proportional to the candle-power in its direction.

 Table XII Kind of lamp Candles per watt Cost for 1,000 candle hours in cents Carbon.............................. 0.25 to 0.33 30 to 40 Gem................................. About 0.40 25 Tungsten................. ........... 0.80 to 1.00 10 to 12.5

From a 60-watt lamp, for example, the candle power obtained is 15 to 20 for a carbon lamp, 24 for the Gem, and 56 for the tungsten.

Comparative cost of various common lights * (Fig. 48).

For the production of light a great variety of lamps are available, and in some kinds remarkable inprovements have been made in the last few years. These improvements have made it possible in many cases either to improve the lighting of the home without increasing the cost or to reduce the cost.

Kerosene flame--Kerosene Mantle Gas Open Flame Gas Mantle ---Carbon Electric -'Gem' Electric---Tonwteh Electric