=88. Electrical Measurements.= A volt is the unit of electric pressure which compares with the pound as the unit of water pressure. An ampere is the unit of electricity flowing thru a wire which compares to the gallon as the unit of water per minute flowing thru a pipe. A watt is the unit of electrical power. It is determined by multiplying the volts by the amperes. A kilowatt equals 1000 watts. A kilowatt hour equals 1000 watt-hours. A watt-hour is the amount of energy needed by a device which uses one watt and is operated for one hour. For example, a 25-watt lamp uses 25 watts, and if it is operated one hour, it uses 25-watt hours of electricity. The cost of burning an electric lamp is the number of watts marked on the lamp multiplied by the hours the lamp is burned, and then translated into kilowatt hours and multiplied by the price per kilowatt hour. [Illustration: FIG. 45. Direct light.] =89. Carbon Lamps.= Few carbon lamps are being made now, but they may still be obtained in some stores. The carbon lamp can be distinguished from Mazda lamps (Fig. 45) by the appearance of the filament. The carbon lamp gives about 0.40 candles of light per watt of electricity consumed. Carbon lamps burn, making a yellow or reddish light, and consume fully twice as much current as Mazda lamps of the same candle power. =90. Mazda or Tungsten Lamps.= Tungsten lamps are the ones in common use. They give 0.80 to 1.00 candle of light to one watt of electricity used. They have a filament of tungsten and may now be used in any position. Less electricity is required to bring tungsten to a glowing white heat than other materials used in lamps. To compare the brightness of two lamps, do not look at the filament, but hold pieces of white material like paper at an equal distance from each lamp and compare the brightness of the surfaces; or put an opaque object in front of the light and let a shadow be cast on another object. The brighter light will cast a heavier shadow. When substituting a new tungsten lamp for a carbon lamp, select one about one-half the number of watts, unless more light is wanted. In houses, it is a common practice to substitute a 40-watt Mazda for a 50-watt gem carbon lamp, thus saving ten watts per hour and getting more light. =91. Selecting Lamps for a Room.= There are so many possibilities for the use of electricity in lighting a house, that it becomes a fine art. When buying lights for a room, consider (1) the size of the room, (2) the use of the room, and (3) the color of walls, floors, ceilings, furnishings and decorations. For lighting purposes, lamps may be obtained ranging from 10 or less to more than 100-candle power. There are colored, transparent and frosted globes. There are reflectors and shades of various colors and patterns. To obtain the same degree of illumination, smaller lamps are needed in small rooms than in large ones.