Chapter XIII: Electricity (1)

The adaptability of electricity to household use for lighting, heating and the generation of power has brought into use a host of mechanical devices that have found a permanent place in every community where electricity may be obtained at a reasonable rate, or where it can be generated to advantage in small plants.

Because of its cleanliness and convenience, electricity is used in preference to other forms of lighting, even though its cost is relatively high. Electric power for household purposes is constantly finding new applications and will continue to increase in favor because its use as compared with hand power is remarkably inexpensive. Small motors adapted to most of the ordinary household uses are made in convenient sizes and sold at prices that are conducive to their greater use. Human energy is far too precious to be expended in household drudgery where mechanical power can be used in its place and often to greater advantage.

Electric heating devices compete favorably with many of the established forms of household heating appliances, the electric flat-iron being a notable example. In all applications where small amounts of heat are required for short periods of time, electricity is used at a cost that permits its use, in competition with other forms of heating.

The remarkable advance that has taken place in electric transmission in the past few years tends to an enormous increase in its use. The constant increase in its use for lighting, heating and power purposes is due in a great measure to the development of efficient electric generating plants from which this energy may be obtained at the least cost. In those communities where hydro-electric generation is possible its field of application is almost without end.

=Incandescent Electric Lamps.=--Anything made in the form of an illuminating device, in which the lighting element is rendered incandescent by electricity, may properly be called an incandescent lamp, whether the medium is incandescent gas as in the Moore lamp, an incandescent vapor as the Cooper Hewitt mercury-vapor lamp, or the incandescent filament of carbon or metal such as is universally used for lighting.

From the year 1879, when Mr. Edison announced the perfection of the incandescent electric lamp, until 1903, when for a short period tantalum lamps were used, very little improvement had been made in the carbon-filament lamp. Immediately following the introduction of the tantalum lamp came the tungsten lamp, which because of its wonderfully increased capability for producing light has extended artificial illumination to a degree almost beyond comprehension. The influence of the tungsten lamp has induced a new era of illumination that has affected the entire civilized world. The development of the high-efficiency incandescent lamp has brought about a revolution in electric lighting. Its use is universal and its application is made in every form of electric illumination.

Regardless of the immense number of tungsten lamps in use, the carbon-filament lamp is still employed in great numbers and will probably continue in use for a long time to come. In places where lamps are required for occasional use and for short intervals of time, the carbon filament still finds efficient use. In one form of manufacture the carbon filament is subjected to a metalizing process that materially increases its efficiency. This form, known commercially as the GEM lamp, fills an important place in electric lighting.

Of the rare-metal filament lamps, those using tungsten and tantalum are in general use, but the tungsten lamps give results so much superior in point of economy in current consumed that the future filament lamps will beyond doubt be of that type unless some other material is found that will give better results.

The filaments of the first tungsten lamps were very fragile and were so easily broken that their use was limited, but in a very short time methods were found for producing filaments capable of withstanding general usage and having an average life of 1000 hours of service. These lamps give an efficiency of 1.1 to 1.25 watts per candlepower of light, as will be later more fully explained. This, as compared with the carbon-filament lamps which average 3.1 to 4.5 watts per candlepower, gives a remarkable advantage to the former. The tungsten lamp has a useful life that for cost of light is practically one-third that of the carbon-filament lamp.

The metal tungsten, from which the lamp filament is made, was discovered in 1871. It is not found in the metallic state but occurs as tungstate of iron and manganese and as calcium tungstate. Up to 1906 it was known only in laboratories and on account of its rarity the price was very high. As greater bodies of ore were found and the process of extraction became better known, the price soon dropped to a point permitting its use for lamp filaments in a commercial scale.

Pure tungsten is hard enough to scratch glass. Its fusing point is higher than any other known metal; under ordinary conditions it is almost impossible to melt it and this property gives its value as an incandescent filament. One of the laws that affect the lighting properties of incandescent lamps is: “the higher the temperature of the glowing filament, the greater will be the amount of light furnished for a given amount of current consumed.” The high melting point permits the tungsten filament to be used at a higher temperature than any other known material. Tungsten is not ductile, and in ordinary form cannot be drawn into wire. Because of this fact, the filaments of the first lamps were made by the “paste” process, which consisted of mixing the powdered metal with a binding material, in the form of gums, until the mass acquired a consistency in which it might be squirted through a minute orifice in a diamond dye. The resulting thread was dried, after which it was heated, and finally placed in an atmosphere of gases which attacked the binding material without affecting the metal. When heated by electricity in this condition, the particles of metal fused together to form a filament of tungsten. While the “paste” filaments were never satisfactory in general use, their efficiency as a light-producing agent inspired a greater diligence in the search for a more durable form.

Although tungsten in ordinary condition is not at all ductile, methods were soon found for making tungsten wire and the wire-filament lamps are now those of general use. One process of producing the drawn wire is that of filling a molten mass of a ductile metal with powdered tungsten after which wire is drawn from the mixture in the usual way. The enclosing metal is then removed by chemical means or volatilized by heat.

Of the difficulties encountered in the use of metal-filament lamps that of the low resistance offered by the wire was overcome by using filaments very small in cross-section and of as great length as could be conveniently handled. The long tungsten filament requires a method of support very different from the carbon lamp. The characteristic form of tungsten lamps is shown in Fig. 217, in which the various parts of the lamp are named.

The filament of an incandescent lamp is heated because of the current which passes through it. The electric pressure furnished by the voltage, forces current through the filament in as great an amount as the resistance will permit. A 16-candlepower carbon lamp attached to a 110-volt circuit requires practically 1/2 ampere of current to render the filament incandescent; the filament resistance must, therefore, allow the passage of 1/2 ampere. With a given size of filament, its length must be such as will produce the desired resistance. A greater length of this filament would give more resistance and a correspondingly less amount of current would give a dim light because of its lower temperature. Likewise, a shorter filament would allow more current to pass and a brighter light would result. When the size and length of filament is once found that will permit the right amount of current to pass, if the voltage is kept constant, the filaments will always burn with the same brightness. This is in accordance with Ohm’s law which as stated in a formula is

_E_ = _RC_

that is _E_, the electromotive force in volts, is always equal to the product of the resistance _R_, in ohms, and the current _C_, in amperes.

In the incandescent lamp, if the electromotive force is 110 volts and the current is 1/2 ampere, the resistance will be 220 ohms and as expressed by the law

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 (1/2 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_.

At their best, carbon-filament lamps require in electricity 3.1 w.p.c. (watts per candlepower). As the lamp grows old the number of watts per candle power increases, until in very old lamps the amount of electricity used to produce a given amount of light may become excessively large. According to a bulletin issued by the Illinois Engineering Experiment Station on the efficiency of carbon-filament incandescent lamps, the amount of electrical energy per candlepower varied from 3.1 w.p.c., when new, to 4.2 w.p.c., after burning 800 hours.

A common practice in the use of carbon-filament lamps is to consider that the period of useful life ends at a point where the amount of electricity, per candlepower, reaches 20 per cent. in excess of the original amount. This point (sometimes termed the smashing point) would be reached after 800 working hours, according to the Illinois Station, and at about 1000 hours as stated by the bulletins of the General Electric Co. If a carbon-filament lamp burns for an average period of 3 hours a day for a year, it ought to be replaced.

The Edison screw base as shown in Fig. 217 is now generally used in all makes of incandescent lamps for attaching the lamp to the socket. When screwed into place this base forms in the socket the connections with the supply wires, to produce a circuit through the lamp. One end of the filament is attached to the brass cap contact; the opposite end connects with the brass screw shell of the base. When the current is turned on, the contact made in the switch is such as to form a complete circuit between the supply wires; the voltage sending a constant current through the lamp produces a steady incandescence of the filament.

In Fig. 218 is shown a carbon-filament lamp attached to an ordinary socket. The lamp base and socket are shown in section to expose all of the parts that comprise the mechanism. The insulated wires of the lamp cord enter the top of the socket and the ends attach to the binding screws _A_ and _B_, which are insulated from each other and form the brass shell which encases the socket. The lamp base is shown screwed into the socket, the brass cap contact _F_ making connection at _G_; the screw shell joins the socket at _D_. To the key _S_ is attached a brass rod _R_, on which is fastened _E_, the contact-maker. The rod _R_ passes through a supportary frame which is secured to the lamp socket at _G_. As shown in the figures the piece _E_ makes contact with a brass spring attached to _A_, and this completes a circuit through the filament. The brass cap contact of the lamp base makes connection at one end of the filament _H_, the other end of the filament _K_ is attached to the brass screw shell of the base, which in turn connects with the screw shell of the socket and this shell is connected with the piece containing the binding screw _B_ by the rod _C_ to complete the circuit. When the key _S_ turns, the contact above _E_ is broken and the lamp ceases to burn.

Fig. 118 shows the use of an adapter that is sometimes encountered in old electric fixtures, the use of which requires explanation. Mention has already been made of the various forms of lamp sockets in use before the Edison base became a standard. In order to use an Edison lamp in a socket intended for another form of base an adapter must be employed to suit the new base to the old socket. In the figure the piece _P₁_, is the adapter. This is intended to adapt the standard lamp base to a socket that was formerly in use on the Thompson-Houston system of electric lighting. The adapter is joined to the old socket by the screw at _G_ and the circuit formed as already described.

=Lamp Labels.=--For many years all incandescent lamps were rated in candlepower and were made in sizes 8, 16, 32, etc., candlepower. On the label was printed the voltage at which the lamp was intended to operate, and also the candlepower it was supposed to develop. Thus 110 v., 16 cp. indicated that when used on 110-volt circuit, the lamp would give 16 candlepower of light. This label in no way indicated the amount of energy expended. With the development of the more efficient filaments came a tendency to label lamps in the amount of energy consumed. This has resulted in all lamps being labeled to show the voltage of the circuit suited to the lamp, and the watts of electricity consumed when working at that voltage. At present a lamp label may be marked 110 v., 40 w., which indicates that it is intended to develop its best performance at 110 volts and will consume 40 watts at that voltage.

Commercial lamps are now manufactured in sizes of 10, 15, 25, 40, 60, 75, and 100 watts capacity for ordinary use. Of these the 40-watt lamp probably fulfills the greatest number of conditions and is most commonly used. Besides these there are the high-efficiency lamps of the gas-filled variety that are made in larger sizes and the miniature lamps in great variety. All are labeled to show the volts and the watts consumed.

=Illumination.=--The development of high-efficiency lamps has caused a radical change in the methods of illumination. With cheaper light came the desire to more nearly approximate the effect of daylight in illumination. This has brought into use indirect illumination, in which the light from the lamp is diffused by reflection from the ceiling and walls of the room. Illuminating engineering is now a business that has to do with placing of lamps to the greatest advantage in lighting any desired space. In large and complicated schemes of lighting professional services are necessary, but in household lighting the required number of lamps for the various apartments are almost self-evident. The lighting of large rooms, however, requires thoughtful consideration and in many cases the only definite solution of the problem is that of calculation.

=The Foot-candle.=--The amount of illumination produced over a given area depends not only on the number of lamps and their candlepower, but upon their distribution and the color of the walls and furnishings. In the calculation of problems in illumination, units of measure are necessary to express the amount of light that will be furnished at any point from its source. The units adopted for such purposes are the foot-candle and the lumen.

=The Lumen.=--A light giving 1 candlepower, placed in the center of a sphere of 1 foot radius illuminates a sphere, the area of which is 4 × 3.1416 or 12.57 square feet. The _intensity_ of light on each square foot is denoted as a candle-foot. The candle-foot is the standard of illumination on any surface. The _quantity_ of light used in illuminating each square foot of the sphere is called a lumen. A light of 1 candlepower will therefore produce an intensity of 1 candle-foot over 12.57 square feet and give 12.57 lumens. Therefore, if all of the light is effective on a plane to be illuminated, a lamp rated at 400 lumens would light an area of 400 square feet to an average intensity of 1 candle-foot.

To find the number of lamps required for lighting any space, the area in square feet is multiplied by the required intensity in foot-candles, to obtain the total necessary lumens, and the amount thus obtained is divided by the effective lumens per lamp.

The bulletins of the Columbia Incandescent Lamp Works gives the following method of calculating the number of lamps required to light a given space:

Number of lamps = (_S_ × _I_)/(Effective lumens per lamp)

_S_ (square feet) × _I_ (required illumination in foot-candles)
= total lumens.

The total lumens divided by the number of effective lumens per lamp gives the number of lamps required. In using the formula the effective lumens per lamp is taken from the following table:

Watts per lamp 25 40 60 160 150 250

Effective lumens per lamp 95 160 250 420 630 1090

Lumens per watt 3.8 4.0 4.2 4.2 4.2 4.3

The size of the units is a matter of choice since six 400-lumen units are equal to four 600-lumen units in illuminating power, etc. In deciding upon the proper size of lamps to use, consideration must be taken of the outlets if the building is already wired. In general the fewest units consistent with good distribution will be the most economical. The table shows the lumens effective for ordinary lighting with Mazda lamps and clear high-efficiency reflectors with dark walls and ceiling. Where both ceiling and walls are very light these figures may be increased by 25 per cent.

To illustrate the use of the table, take an average room 16 by 24 to be lighted with Mazda lamps to an intensity of 3.5 foot-candles. If clear Holoplane reflectors are used, the values for lumens effective on the plane may be increased 10 per cent. due to reflection from fairly light walls. The lamps in this case are to be of the 40-watt type which in the table are rated at 160 lumens. To this amount 10 per cent. is added on account of the reflectors and walls. This data applied to the formula gives:

_s_ = 16 by 24 feet
_I_ = 3.5
Lumens per lamp = 160
((16 × 24) × 3.5)/176 = eight 40-watt lamps.

=Reflectors.=--The character and form of reflectors have much to do with the effective distribution of the light produced by the lamp. The most efficient form of reflectors are made of glass and designed to project the light in the desired direction. The illustration in Fig. 219, marked open reflector, shows the characteristic features of reflectors designed for special purposes. They are made of prismatic glass fashioned into such form as will produce the desired effect and at the same time transmit and diffuse a part of the light to all parts of the space to be lighted. The greater portion of the light is sent in the direction in which the highest illumination is desired. The reflectors are made to concentrate the light on a small space or to spread it over a large area as is desired. They are, therefore, designated as intensive or extensive reflectors and made in a variety of forms.

=Choice of Reflector.=--Where the light from a single lamp must spread over a relatively great area, it is advisable to use an _extensive_ form of reflector. This reflector is applicable to general residence lighting, also uniform lighting of large areas where low ceilings or widely spaced outlets demand a wide distribution of light. Where the area to be lighted by one lamp is smaller, the _intensive_ reflector is used. Such cases include brilliant local illumination, as for reading tables, single-unit lighting or rooms with high ceilings as pantries or halls.

Where an intense light on a small area directly below the lamp is desired, a _focusing_ reflector is used. The diameter of the circle thus intensely lighted is about one-half the height of the lamp above the plane considered. Focusing reflectors are used in vestibules or rooms of unusually high ceilings.

Type Height above plane to be lighted
Extensive 1/2 _D_
Intensive 4/5 _D_
Focusing 4/3 _D_
_D_ = distance between sides of room to be illuminated.

The various other fixtures of Fig. 219 that are designated as reflectors are in some cases only a means of diffusion of light. In the use of the high-efficiency gas-filled lamps the light is too bright to be used directly for ordinary illumination. When these lamps are placed in opal screens of the indirect or the semi-indirect form the light produced for general illumination is very satisfactory. Considerable light is lost in passing through the translucent glass but this is compensated by the use of the high-efficiency lamps and the general satisfaction of light distribution.

=Lamp Transformers.=--Lamps of the Mazda type, constructed to work at the usual commercial voltages, are made in low-power forms to consume as little as 10 watts; but owing to the difficulty of arranging a suitable filament for the smaller sizes of lamps, less voltage is required to insure successful operation. The lamps for this purpose are of the type used in connection with batteries and require 1 or more volts to produce the desired illumination. When these little lamps are used on a commercial circuit, the reduction of the voltage is accomplished by small transformers, located in the lamp socket. The operating principle and further use of the transformers will be explained later under doorbell transformers. The lamp transformer, although miniature in design, is constructed as any other of its kind but designed to reduce the usual voltage of the circuit to 6 volts of pressure. The socket is that intended for the use of the Mazda automobile lamp giving 2 candlepower. This lamp used with electricity at the average rate per kilowatt can be burned for 10 hours at less than half a cent. In bedrooms, sickrooms and other places where a small amount of light is necessary but where a considerable quantity is objectionable, the miniature lamp transformer serves an admirable purpose in adapting the voltage of the commercial alternating circuit to that required for lamps of small illuminating power. Such a transformer is shown in Fig. 220.

The figure shows in _A_ the assembled attachment with the lamp bulb in place. The part _B_, the transformer, changes the line voltage to that of a battery lamp. A line voltage of 110 may be transformed to suit a 6-volt miniature lamp. The parts _C_ and _D_ compose the screw base and the cover, in which is fitted the transformer _B_.

=Units of Electrical Measurement.=--The general application of electricity has brought into common use the terms necessary in its measurement and units of quantity by which it is sold. The volt, ampere and ohm are terms that are used to express the conditions of the electric circuit; the watt and the kilowatt are units that are employed in measuring its quantity in commercial usage. The use of these units in actual problems is the most satisfactory method of appreciating their application.

As already explained the volt is the unit of electric pressure which causes current to be sent through any circuit. The electric circuits of houses are intended to be under constant voltage--commonly 110 or 220--but the voltage may be any amount for which the generating system is designed. Independent lighting systems such as are used in house-lighting plants--to be described later--commonly employ 32 volts of electric pressure.

Opposed to the effect of the volts of electromotive force is the resistance of the circuit, which is measured in ohms. Resistance has been called electric friction; it expresses itself as heat and tends to diminish the flow of current. Every circuit offers resistance depending on the length, the kind and the size of wire used. Since the wires of commercial lighting systems are made of copper, it can be said that the resistance of the circuit increases as the size of the conducting wire decreases. In large wires the resistance is small but as the size of the wire is reduced the resistance is increased. A long attachment cord of a flat-iron, may offer sufficient resistance to prevent the iron from heating properly.

The ampere is the unit which measures the amount of current. The amperes of current determine the rate at which the electricity is being used in any circuit. The wires of a house must be of a size sufficient to carry the necessary current without heating. Any house wire which becomes noticeably warm is too small for the current it carries and should be replaced by one that is larger.

The watt is the unit of electric quantity. The quantity of electricity being used in any circuit is the product of the volts of pressure and amperes of current flowing through the wires. The amount of current--in amperes--sent through the circuit is the direct result of the volts of pressure; the quantity of electricity is therefore the product of these two factors. A 25-watt lamp on a circuit of 110 volts uses 0.227 ampere of current.

25 watts = 110 volts × 0.227 amperes.

Ten such lamps use

10 × 0.227 amperes = 2.27 amperes.

The product of 110 volts and 2.27 amperes is 250 watts.

In order to express quantity of energy, it is necessary to state the length of time the energy is to act and originally the watt represented the energy of a volt-ampere for one second. For commercial purposes this quantity is too small for convenient use and the hour of time was taken instead. The watt of commercial measurement is the watt-hour and in the purchase of electricity the watt is always understood as that quantity.

Even as a watt-hour the measure is so small as to require a large number to express ordinary amounts and a still larger unit of 1000 watt-hours or the kilowatt-hour was adopted and has become the accepted unit of commercial electric measurement. Just as a dollar in money conveniently represents 1000 mills so does a kilowatt of electricity represent a convenient quantity.

In the purchase of electricity, the consumer pays a definite amount, say 10 cents per kilowatt. This represents an exact quantity of energy, that may be expended in light, in heat, or in the generation of power, all of which may be expressed as definite quantities.

As light, it indicates in the electric lamp the number of candle-power-hours that may be obtained for 10 cents. At this rate a single watt costs 0.01 cent an hour. A 25-watt electric lamp will therefore cost 0.25 (1/4) cent for each hour of use; a 60-watt lamp costs 0.6 cent per hour; the ten 25-watt lamp mentioned above using 250 watts costs 2.5 cents per hour.

As heat, it is expressed in English-speaking countries as British thermal units, 1 kilowatt-hour representing 3412 B.t.u. per hour. One cent’s worth of electricity at the rate given yields 341.2 B.t.u. of heat.

As power, it represents an exact amount of work. So expressed, a watt represents 1/746 horsepower; therefore a kilowatt is represented in power as 1000/746 = 1.3 horsepower. Since the kilowatt purchased for 10 cents is a kilowatt-hour, the equivalent horsepower is for the same length of time. At the assumed rate, 10 cents buys 1.3 horsepower for one hour. When used as work it represents 2,544,000 foot-pounds or 255,400 foot-pounds of work for 1 cent. This work when expended in a motor, to do the family washing or perform any other household drudgery, represents the greatest value to be derived from its use. A 1/2-horsepower motor is amply large to operate a family washing machine. Even though the motor is only 50 per cent. efficient its cost of operation is less than 7 cents per hour.

=Miniature Lamps.=--Miniature electric lamps include all that are not used for general illuminating purposes. The term applies more particularly to the form of the base than to the voltage or candlepower of the filament. There are three general classes of these lamps: candelabra and decorative, that operate on lighting circuits of 100 to 130 volts and are usually intended for decorative purposes; general battery lamps used for flash lights; and lamps for automobiles and electric-vehicle service.

Candelabra screw base
Miniature screw base
Double-contact bayonet candelabra base
Single-contact bayonet candelabra base

FIG. 221.--Miniature lamp bases.]

The term miniature lamp applies more particularly to the base than to the voltage or candlepower. The style of base is characteristic of the service for which the lamp is designed rather than the size or number of watts consumed. There are two general styles of bases: the screw type of the Edison construction of which there are two sizes; and the bayonet type of which there are two styles of construction.

Bases for miniature lamps are made in form to suit the conditions of their use. The styles at present are shown in Fig. 221. Of these the screw bases at the left are those attached to small flash-lamp bulbs and others of the smaller sizes of lamps. The two at the right of the figure are the bayonet style used under conditions not suited to the screw contact. In the case of automobile lamps and in places where vibration will cause loss of contact the bayonet base is generally in use. The lamp is held in place by the projecting lugs that engage with openings in the socket and kept in place by the pressure of a spring. The contact with the lamp filament is made by two terminals that make connection directly with the terminals of the lamp filament. The single contact base is kept in place similarly to that of the other but makes a single contact at the end of the socket while the other but makes a single contact at the end of the socket while the circuit is completed through the pressure exerted between the projecting lugs and the socket.

=Effect of Voltage Variations.=--Voltage variation may be temporary, due to changing load in the circuit, or in constantly overloaded circuits the voltage may be constantly below normal. The change in electric pressure affects in a considerable degree the amount of light given by the lamp. As an example, a 5 per cent. drop from the normal voltage will cause a decrease of 31 per cent. in the amount of light given. This means that if a lamp is working on a circuit of 110 volts and the voltage from any cause were to drop to 104-1/2 volts, the light would decrease 6.8, almost 7 candlepower. Drop in voltage may also be due to the resistance of wires that are too small for the service. Lamps attached to such a circuit will constantly burn dim.

=Turn-down Electric Lamps.=--The ordinary incandescent lamp lacks the flexibility of gas and oil lamp, in that the amount of light cannot be varied at will. This feature is attained in the electric turn-down lamp either by resistance added to the lamp circuit or by the use of two separate filaments in a single globe; one of ordinary lamp size and the other of such size that it consumes only a fraction as much energy as the normal lamp.

Turn-down lamps of the latter form are made in several styles, the chief points of difference being in the method of changing the contact from the high-to the low-power filament. In Fig. 222 a sectional view shows the “pull-string” form of lamp in which the parts are exposed. The long filament _H_ and the smaller one _L_ represent two individual lamps of different lighting power. The change in light is made from one to the other by pulling the string which is attached to a switch in the socket and which changes the contact to send the current through the filament giving the desired amount of light. The figure shows a carbon-filament lamp, but tungsten lamps are made to accomplish the same purpose. The difficulty of manufacturing a 1-candlepower tungsten lamp for direct operation on a 110-volt circuit requires the filaments to work in series. The figure is arranged on the same plan as for a tungsten lamp.

The lamp base when screwed into the socket makes contact with the two service wires of the circuit at _A_ and at _E_, which are part of the screw base. To light the lamp the current is switched on as in any lamp. The current enters at _A_ and passes down the connecting piece to the contact _B_. The piece _B_ is moved by the cord to light either the large or the small filament. In the position shown the current enters the small filament at _C_ and in order to complete the circuit to _E_ must traverse both the large and the small filament. The resistance of the small filament is such that the passing current raises it to a temperature of incandescence but the large filament does not heat sufficiently to give an appreciable amount of light. When the cord is pulled to light the large filament, the contact is made at _D_ and the current passes directly through the large filament to complete the circuit at _E_.

Turn-down lamps are especially adapted to the home. Their use in a child’s bedroom or sick chamber is a great convenience. The lamps are often constructed with a long-distance cord extending from a fixture to the bedside. By this means a dim or bright light is given as desired, with the least inconvenience. Turn-down lamps are made in a variety of sizes. The large filaments are arranged to give 8, 16, and 32 candlepower. With the 8-candlepower lamp the small filament gives 1/2 candlepower and with the 16-and 32-candlepower the small filament gives 1 candlepower.

With the lamps described, the variation in amount of light is attained by changing the contacts, to bring into action filaments of different resistances. They admit of only two changes, either the lamp burns at full capacity or at the least light the lamp will give. The heat liberated by the large filament, when the small light is in use, takes place inside the lamp globe.

=The Dim-a-lite.=--In another form of turn-down lamp the change in amount of light is produced by external resistance in the circuit. The resistance is furnished by a coil of wire which is enclosed in a special lamp socket. It possesses the advantage as a turn-down lamp in a number of changes of light. The added resistance in a socket decreases the flow of current and, therefore, the filament gives less light. The resistance wire is divided into a number of sections and contact with the terminals of these sections decreases the light with each addition of resistance. The heat generated in the resistance coils is dissipated by the brass covering of the socket.

An illustration of a turn-down lamp using a separate resistance is that of Fig. 223, known commercially as the Dim-a-lite, which is an excellent example. The Dim-a-lite attachment is a lamp socket in which is enclosed a miniature rheostat or resistance unit. The lamp, when placed on the Dim-a-lite, makes electrical contact as in an ordinary socket but with the difference that in series with the lamp filament is the rheostat, by means of which additional resistance may be added to change the current flowing in the lamp. The rheostat is so arranged that contact may be made at four different points in the resistance coil, through which the electricity may be varied from 100 to 20 per cent. of the normal quantity. The resistance in any case permits current to pass through the filament in amounts of 70, 30 and 20 per cent. of the normal amount. In use, the variation is made by pulling one string to add resistance and thus dim the light; or by pulling the other string, the resistance is decreased and more electricity passes through the filament to produce a brighter light. The quantity of light given out by the filament does not vary in the ratio of the added resistance but a variable light is obtained at the expense of a small amount of electricity which is changed into heat. When the light is burning at its dimmest only 20 per cent. of the normal current is used. Under this condition the light given out by filament does not express the high efficiency attained when the lamp is burning at its full power but it does give a convenient form of light regulation with the minimum waste of energy.

=Gas-filled Lamps.=--Until 1913 the filaments of all Mazda lamps operated in a vacuum. The vacuum serving the purpose of preventing oxidation and at the same time it reduced the energy loss to the least amount. It was found, however, under some conditions of construction that lamps filled with inert gas gave a higher efficiency and more satisfactory service than those of the vacuum type. In this construction, the filament is operated at a temperature much higher than that of the vacuum lamp and as a consequence gives light at a less cost per candlepower. Mazda vacuum lamps are now designated by the General Electric Co. as Mazda B lamps, Fig. 224, and those of the gas-filled variety, Fig. 225, are designated as Mazda C lamps.

The filaments of the gas-filled lamps are intensely brilliant and where they come within the line of vision should be screened from the eyes. The high efficiency of these lamps permit the use of opal shades to produce a desired illumination at a rate of cost that compares favorably with the unscreened light of the vacuum lamps.

=Daylight Lamps.=--The color of the light from an incandescent electric lamp depends on the temperature of the filament. In the case of the gas-filled Mazda lamp the high filament temperature produces a light that differs markedly from the vacuum lamps in that it contains a greater amount of blue and green rays. It is therefore possible to produce light that is the same as average daylight. Gas-filled lamps with globes colored to produce light of noonday quality are produced at an expenditure of 1.2 watts per candlepower.

In the matching of colors, it should be kept in mind that the tint of any color is influenced by the kind of light by which it is viewed. Colors matched by ordinary incandescent light containing a large percentage of red rays cannot produce the same effect when the same articles are seen in light of different quality. The daylight lamps are therefore intended to be used under conditions that require daylight quality.

=Miniature Tungsten Lamps.=--The wonderful light-giving properties of tungsten has made possible the use of miniature incandescent lamps for an almost infinite variety of usages. The miniature lamps are similar in action to other incandescent electric lamps except that they are operated on voltages lower than is used on commercial circuits. When used on commercial circuits, incandescent tungsten lamps of less than 10 watts capacity require filaments that are too delicate to withstand the conditions of ordinary use. The properties of tungsten are such that the passage of only a small amount of current is required to render the filament incandescent. In the case of a 110-volt circuit, a 10-watt lamp requires only 0.09+ ampere to produce the desired incandescence. It will be remembered that the watt is a volt-ampere and the 10-watt lamp will then require

110 volts × 0.09 + ampere = 10 watts.

Since 10-watt lamps are the smallest units that may be used on 110-volt circuits, their employment in smaller sizes must be such as will give more stable filaments. This is possible when the lamps are used at lower voltage. A 10-watt lamp on a 10-volt circuit will require an ampere of current.

10 volts × 1 ampere = 10 watts.

A filament suitable for an ampere of current is shorter and heavier than that of the 110-volt lamp and therefore furnishes a good form of construction. Still lower voltages may be used with filaments suited to the quantity of light desired.

In the case of battery lamps that are intended to operate on 1 or more volts, the filaments are made in size and length to suit the condition of action. In all cases the product of the volts and amperes give the capacity of the lamp in watts.

Miniature lamps are ordinarily marked to show the voltage on which they are intended to operate. A 6-volt battery lamp is intended to be used with a primary battery of four to six cells depending on the condition of usage, or three cells of storage battery, each cell of which gives 2 volts of pressure.

=Flash Lights.=--These are portable electric lamps composed of a miniature incandescent bulb, which with one or more dry cells are enclosed in a frame to suit the purpose of their use. They are made in pocket sizes or in form to be conveniently carried in the hand and are convenient and efficient lamps wherever a small amount of light is required for a short time. The electricity for operating the lamp is supplied by a battery of dry cells (to be described later), or by a single dry cell. In each case the incandescent bulb is suited to the voltage of the battery.

In replacing the bulbs care must be taken to see that the voltage is that suited to the battery. The voltage is usually stamped on the lamp base or marked on the bulb. In case a lamp intended for a single cell is used with a battery of three or four cells, the lamp filament will soon be destroyed. The reverse will be true should a lamp intended for a battery be used with a single cell. The single cell giving not much more than a volt of electromotive force will not send sufficient current through the lamp filament to render it incandescent.

=The Electric Flat-iron.=--The changes that have been made in domestic appliances by the extended use of electricity have brought many innovations but none are more pronounced than the improvements made in the domestic flat-iron. It was the first of the household heating devices to receive universal recognition and its place as a domestic utility is firmly established.

The relatively high cost of heat as generated through electric energy is in a great measure counterbalanced in the flat-iron by high efficiency in its use. In the electric iron, the heat is developed in the place where it can be used to the greatest advantage, and transmitted to the face of the iron with but very little loss. Because of this direct application the cost of operation is but slightly in excess of the other methods of heating.

The electric flat-iron has now become a part of the equipment of every commercial laundry, where electricity can be obtained at a reasonable rate. The popularity of the electric iron is due to its cleanliness and to the increased amount of work that may be accomplished through its use. Because of the time saved in changing irons and the comfort of the room by reason of its lower temperature, a sufficiently greater amount of work is accomplished to more than compensate for the greater cost of heat.

The electric current is conducted to the flat-iron from the house circuit by wires made into the form of a flexible cord. The cord attaches to the electric-lamp fixture by a screw-plug and connects with the iron by a special attachment piece as indicated at _P_ and _R_ in Fig. 226. Connection is made to an incandescent lamp socket at any convenient place. The only precaution necessary in attaching the iron is to see that the fuse and the wires, which form the circuit, are of size sufficient to transmit the amount of current the iron is rated to use. As explained later, the fuse which is a part of every electric house circuit, and the conducting wires which form the heater circuit, must be sufficient in size to transmit the necessary current without material heating.

The cord connects with the socket at _P_, and the current turned on. It is attached with the iron by a piece _R_, made of non-conducting and heat-resisting material and arranged to make contact with the heater terminals by two brass plugs that are insulated from the body of the iron and afford easy means of making electric contact. The contact plugs are shown in Fig. 227. To make electric connection, the contact piece is simply pushed over the plugs, where it is held in place by friction. Instructions which accompany a flat-iron when purchased advise that the attachment piece be used in turning off the current. The reason for this is because of the flash that accompanies the break in the circuit when disconnection is made in the socket. This flash is really a small electric arc, that forms as the circuit is broken and which burns away the switch at the point of disconnection. The arc so formed burns away the contact pieces in the switch and it is soon destroyed. The attachment piece will stand this wear more readily than the socket switch and hence is preferable for disconnecting. The irons are frequently provided with a special switch for the service required in the flat-iron.

A spiral spring connected to the attachment cord prevents it from kinking when in use and thus breaking the conducting wires. The attachment cord is made of stranded wires to make it flexible. The strands of fine copper wire are made to correspond to the gage numbers by which the various sizes of wire are designated. In use the constant movement of the iron tends to kink the cord and thus breaks the strands. This action is most pronounced at the point where the cord attaches to the iron. For this reason a spiral spring wire encloses the cord for a short distance above the attachment piece. After long usage the cord is apt to break in this vicinity. It may usually be repaired by cutting off the ends of the cords and new connections made in the attachment piece. When the iron is in use the slack portion of the cord is kept from interfering with the work by the coiled wire _S_, which connects with the cord at any convenient place.

Electric flat-irons are made in a variety of styles and forms, the mechanism of each possessing some particular advantage, but all are provided with the same essential parts, chief of which is the heater with its electric attachment piece. In Fig. 228 is shown very clearly the construction of an example in which attention is called to the points of excellence that are required in a particularly serviceable iron. The form of the heating element which is recognized in the iron is also shown in Fig. 228.

In the figure the heater is made of coils of resistance wire, wound on a suitable frame of mica. The heating element is insulated from the body of the iron with sheets of mica, this being a material that makes an excellent insulator and is not materially affected by the heat to which it is subjected. The resistance wire of which the element is composed is especially prepared to resist the corroding action common to metal when heated in air. The form of the element is such as to permit the least movement of the turns of wire--in their constant heating and cooling--that will allow the different spires to make contact and thus change the resistance. Should the spires of wire come together, the current would be shunted across the contact and the resistance of the element decreased. The effect of such a reduction of resistance would be an increased flow of current and a corresponding increase of heat. In this, as in the electric lamp and all other electric circuits, the current, voltage and resistance follow the conditions of Ohm’s law.

Different sizes of irons will, of course, require different amounts of current. A 6-pound iron, such as is commonly used for household work, will take about 5 amperes of current at 110 volts pressure. The amount of electricity the iron is intended to consume is generally stamped on the nameplate of the manufacturer. This is specified by the number of volts and amperes of current the iron is rated to use. As an example, the iron may be marked, Volts 105-115, Amperes 2-3. This indicates that the iron is intended to be used on circuits that carry electric pressure varying from 105 to 115 volts and that the heater will use from 2 to 3 amperes of current, depending on the voltage.

To estimate the cost of operating such an iron, it is necessary to determine the number of watts of electric energy consumed. The number of watts of energy developed under any condition will be the product of the volts times the amperes. Suppose that in the above example the iron was used on a circuit of 110 volts. Under this condition the current required to keep the iron hot would be 2.5 amperes. The product of these two qualities, 110 × 2.5 is 275 watts. If the cost of electricity is 10 cents per kilowatt-hour (1000 watts) the cost of operating the iron would be

275/1000 × 10 cents = 2-3/4 cents an hour.