titanium oxide. These lamps for a time won considerable favor for lighting large spaces and for advertising purposes. One disadvantage was the formation of objectionable fumes which precluded their use in confined spaces, and of an opaque deposit on the enclosing glass-ware. This latter difficulty was met either by having both carbons in the upper case and inclined toward each other in the form of a V, or by the usual vertical construction with special provision for ventilation. In the former arrangement the carbons pass through holes in a shield, the arc being formed on its lower side in an inverted shallow cup and blown downward by an electromagnet.

In the magnetite-arc, which in large measure replaced the carbon-arc for street lighting, coming into particular prominence in some of the earlier 'white-way' lighting, the negative electrode, or cathode, consists of a thin tube of iron filled with magnetite, an oxide of iron, while the anode is a block of solid copper. The anode is stationary and lasts a long time. As the cathode is consumed it is fed upward toward the copper block. Most of the light comes from the arc itself and not from the electrodes as in the carbon-arc. The efficiency is much greater and the light is brilliant white. Usually the arc is enclosed in a globe of diffusing glass-ware. These lamps require direct current; in localities supplied with alternating current, mercury-arc rectifiers are installed to make the necessary conversion, fifty or more lamps being connected in series to each rectifier.

The older forms of arc lamp were operated on direct current, from a battery, but this source of energy was too costly to justify their use for general lighting, which did not become feasible until after the invention of the dynamo. After the development and general introduction of the latter, they became common, especially for street lighting, in which case the lamps were generally operated in series, at first on direct current, later on alternating current. The excep

tions for other than carbon lamps have been stated. For historical completeness mention should be made of an early form of lamp called Jablockkoff's candle, consisting of two narrow vertical strips of carbon separated by kaolin or plaster. The arc played across the top; uniform consumption of strips was achieved by operating the device on alternating current. This form does not appear to have been widely adopted.

the

In recent years arc lamps for

general lighting have been largely superseded by large incandescent lamps, the arc being retained to some extent for powerful projection lanterns, but more particularly for search-lights. A special form of the latter is the 'high-intensity arc'; in the 150ampere size the positive carbon is about 5% inch in diameter and has an exceedingly hard shell; the core is of half that diameter, softer, and carries a large percentage of cerium fluoride. The negative is smaller, but also consists of a hard shell and softer core. In operation, the positive is white hot from the crater back to the contact brush through which the current is brought to it. The fluorides are driven off as a gas which becomes intensely bright and is, in fact, the chief source of light. Its temperature probably exceeds 5,500° absolute on the centigrade scale. The arc is centered at the focus of a large parabolic reflector by automatic mechanism. In the high-intensity arcs the vapor is evolved in a small crater in the positive electrode. The very high temperature and intense light emission are due to the concentration of several kilowatts of energy in about a cubic centimeter of vapor, the brightness of which is stated to be 690 candles per sq. mm.

Incandescent Lamps.. The story of the incandescent lamp is centered around the discovery of certain physical facts and of the laws governing them, followed by successive technical advances in applying them practically. Primarily its operation is based on the following phenomena:-(1) When an electric current is made to flow in a piece of conducting material the latter becomes heated; the heat developed depends on its electrical resistance and the strength of the current. Electrical energy is consumed, being converted into radiant heat and, if the temperature is high enough, light. (2) As the temperature is raised above the point at which light begins to be given off, the color changes from a dull red and becomes more and more nearly white. At the same time the amount of light given off increases more than the energy consumed. Hence it would be best, on the score of efficiency, to operate an incandescent filament at the highest possible temperature. This restricts the choice of materials to those having a high melting or high vaporizing point. (3) Since practically all the available materials will oxidize they must be heated in a vacuum or in an inert gas such as nitrogen or argon. (4) The presence of an

inert gas reduces the tendency of the filament material to vaporize, and thus permits of operation at a higher temperature with consequent gain in efficiency, but introduces a heat loss by convection which becomes more serious the thinner the filament.

Carbon Lamp.-In 1810 Sir Humphrey Davy discovered that thin strips of metal could be made white hot and give light by the passage through them of an electric current obtained from a voltaic battery. In a practical sense this method of producing light did not become feasible until after the invention of the dynamo, many years later, whereby the necessary energy was obtained from the combustion of coal rather than from the consumption of a relatively costly substance like zinc. In October, 1879, Thomas A. Edison made a lamp with a carbonized thread for a filament, sealed in an air-tight glass bulb from which the air had been exhausted. In the patent application filed November 4, 1879, it is indicated that various substances such as cotton fibres, linen, paper strips, and threads of tar and lampblack could be carbonized and used for the filament. In the first commercial lamps bamboo fibres were used. Subsequent improvements included changes in the base, resulting finally in the well known screw-shell type; the production of the filament by dissolving absorbent cotton in a zinc chloride solution and squirting the syrup - like mixture through a die into alcohol to harden it, this being followed by suitable shaping and carbonizing; the process of 'flashing' or 'treating' to make the filament uniform, by heating it in a hydrocarbon vapor and causing a deposition of graphitic carbon; finally in 1905, 'metallizing' the filament by heating it to a very high temperature in an electric furnace. By the last-named process the filament was given the property of increasing in resistance with heating (positive temperature coefficient) thereby tending to reduce the fluctuations of light when operating on a circuit with slightly unsteady voltage, whereas the characteristic of the unmetallized filament is to decrease in resistance (negative temperature coefficient) with rising temperature. By these changes the efficiency of the lamp was gradually increased from an energy consumption of nearly 100 watts for a 16 candle power lamp, or 5.8 watts per candle, in 1881 to 40 watts, or 2.5 watts per candle. Since an increase in the voltage applied to a lamp increases its

output of light more than in direct proportion to the rise in voltage, but causes it to burn out more quickly, there is an inverse relation between efficiency and life. The normal rating is based on an average life for a number of lamps of about 1,000 hours of burning.

Metallic Filament Lamp. - In some of the early experiments in the development of the incandescent lamp, platinum had been tried as a filament material, but it was too expensive for practical use. With progress in chemical technology, including the availability of electrical methods of treatment, other rare metals could be considered. Filaments of osmium, tantalum and tungsten were successively developed. The first of these metals was too rare for large commercial use, and the introduction of the tantalum filament in 1906 marked the first important step from carbon to metallic filament lamps. The efficiency was higher, 2 watts per candle, and the light was white; unfortunately the material crystallized when used on alternating current, which materially shortened its life, so that lamps having an average life of 1,000 hours on direct current failed in perhaps 600 hours on alternating current.

Tungsten Lamp.-The commercial career of the tantalum

Tungsten-Filament Lamp

lamp was cut short by the appearance, in 1907, of a metalfilament lamp using tungsten, which soon superseded the for

mer. Several processes were devised for making the new filaments, differing greatly in detail, but in general involving the formation of a paste carrying finely divided tungsten or compounds thereof, forcing it through a die, and as a final stage sintering or welding the particles of tungsten together. The efficiency, 1.25 watts per candle, was far in advance of that of any other incandescent lamp then on the market but the extreme fragility of the filaments was a serious drawback.

About 1910 the difficulty was solved by the introduction of a process developed by Coolidge for converting tungsten from a brittle, commercially infusible and unworkable metal to a ductile material that could be drawn into fine wires. of high tensile strength. The metal is obtained chemically in the form of a fine powder. Under high pressure this is formed into bars, heated in ovens above 1,000° c., sintered by the passage of a current that raises the temperature to about 2,850° c.,whereby the particles are welded together, and subsequently rapidly hammered in special machines. These operations at high temperatures are all carried on in hydrogen, to prevent oxidation of the metal. The bars are thus reduced from 4 mm. to less than 1 mm. thickness before they are ductile enough to be drawn into wire. The drawing process has to be repeated many times, each pass making only a small reduction, before the wire is fine enough for use in the lamps. The resulting filament is stronger than steel, and very rugged. It is then mounted on spreaders consisting of several fine wire arms supported on a glass stem. Until 1913 the filament was operated in a vacuum. In that year a gain in the efficiency of the lamps Iwas obtained by using an inert gas in the bulbs. At first this was done only with the larger sizes, 750 and 1,000 watts, the gas being nitrogen. Progressively the invention was applied to smaller lamps, down to the 50-watt size; in these, the bulbs were filled with argon. The change was accompanied by a change in the filament shape to a closely coiled spiral. Like the carbon lamp, the early tungsten lamps had pear-shaped bulbs. Approximately the same shape has been retained for the small lamps, designated in the United States as 'Mazda B' lamps, but the larger, gas-filled lamps known as 'Mazda C' spherical bulbs with straight tubular necks, the overheating of the latter by rising convection currents being prevented by a circular mica shield or baffle.

frosting, by etching the glass, was introduced; the efficiency of lamps thus treated is only about 2 per cent. lower than that of clear lamps of the same rating, while the light is emitted almost uniformly from the entire area of the glass. In addition, many special forms are available but the tendency is to reduce their number and, in the interest of reducing the cost, to concentrate on the manufacture of a limited number of standard types and sizes. The light from tungsten lamps is whiter than that from the old carbon-filament lamps, especially in the larger sizes, but is nevertheless appreciably yellower than ordinary daylight. In order to approach the latter more closely, lamps known as 'Mazda C2' are made with bulbs of bluish glass, by which the excess of red and yellow is absorbed; the efficiency is, however, reduced by about 30 per cent. The choice in a given case depends, therefore, on the relative importance of true daylightcolor effects and of economy in securing the desired intensity of illumination.

Tungsten lamps for use on ordinary lighting circuits are made in size ranging from 10 to 1,000 watts, the larger sizes being suitable only for large spaces and with adequate shades and reflectors. The efficiency has meanwhile been greatly increased.

Nernst Lamp.-Shortly before the introduction of metal-filament lamps the Nernst lamp was developed and often used. The light-giving element consisted of a thin rod of rare earth oxides called a glower, material similar to that used in the Welsbach gas mantle. In efficiency, 2 to 2.3 watts per candle, it was better than the carbon lamp and its light was whiter but it was more complicated. The glower was a non-conductor when cold, and needed to be heated when the lamp was switched on; to meet

Ohms Amperes

Per Cent Lumens, Lumens per Watt, Watts, Amperes, and Ohms

has a high resistance, the current is feeble, the power consumed and the light emitted are small; contrariwise for lamps of high candle power. By far the most common voltage for these circuits is around 115 volts. The rate of a lamp. i.e., the voltage which it is designed to be operated, is determined by a compromise between efficiency and length of life. The latter is commonly taken at or somewhat over 1,000 hours. The performance varies with change of voltage, and in certain respects, more rapidly the greater the departure from rated voltage. Approximately, a 10-per cent. increase raises the resistance 4 per cent.. the current 6, the power consumed 16, the efficiency (lumens per watt) 20 and the total luminous output 39 per cent., but shortens the life by half.

Aside from those for ordinary lighting, tungsten incandescent

Tungsten Arc Incandescent Lamp.-A lamp of moderate candle power giving its light from practically a point source makes use of the incandescence of a tungsten globule mounted on a stem and constituting the positive terminal of an arc, which differs from the carbon arc in the important feature that it requires no adjustment, mechanical starting or feeding. The negative electrode is a tungsten wire, part of it formed into a spiral and part straight, the latter encased in a tube of tungsten and thorium oxide. The two electrodes are mounted close together in a glass bulb containing nitrogen at low pressure. In operation the hot negative, serving as a source of electrons, the ionized gas, and the positive electrode provide the necessary conditions for the maintenance of an arc. Starting is accomplished by a simple switching arrangement whereby the tungsten spiral is temporarily connected across the line so as to bring about the initial heating and ionization. The physical characteristics of the arc make it necessary to have a certain amount of resistance in circuit as a stabilizer.

The lamps are made in sizes from 30 c.p. up, the construction of the larger units and those for operation on alternating current differing somewhat from that here described. Because of the concentrated light source they are useful for projection and other optical purposes.

Pointolite Lamp suitable for Small Projection Work (From Biddle's Pointolite Lamps, Bulletin 1,100)

Efficiencies.-Lamp efficiencies were formerly expressed in watts per candle. Since the candle power in all practical cases varies with the direction, it was necessary to add a qualifying term.

The average of candle power measurements in all directions in a horizontal plane, the lamp being held tip-up or tip-down, gave mean horizontal candle power; the average in all directions gave mean spherical; in all from the horizontal downward it gave mean lower hemispherical. In modern practice the entire output of the lamp is measured in lumens (see ILLUMINATION), and the efficiency is expressed in lumens per watt.

Comparative efficiencies of a number of light sources are shown in the following table; temperatures, where given, are in degrees Kelvin, i.e., on the absolute scale starting at -273° centigrade.

Light-source

Sun, outside of earth's atmosphere... Black body at 6500° K.

Crater of solid carbon arc..

istic whereby it experiences momentary increases of resistance of sufficient magnitude to break the arc. It has been found necessary to introduce inductance into the circuit so that the magnetic energy stored opposes and overcomes the tendency to reduce the current. The arc may be started in two ways. In one arrangement the tube is tilted and the mercury in the bulb end is poured in a small stream between the two electrodes. When this stream is broken, by returning the tube to its normal inclined position, the arc is started.

The second method of starting causes a short local high-tension discharge from the mercury,

Vacuum Tube Lamps.-When the atoms of a gas or vapor are made luminous, by electrical means, heat is also produced, but the apparent temperature is not related to the quality of the light except as a certain temperature may be necessary for the existence of metallic vapor in quantity. This is the reverse of the relation of temperature to luminescence in solids. Vapors may be made luminous in electric arcs as with the calcium and titanium lamps. The mercuryvapor lamp develops an arc, but in a vacuum tube. The complete lamp for direct current comprises an inclined tube and holder and a small set of inductance and resistance coils connected in series with the tube. The tube has an iron electrode at the upper or positive end, and a small amount of mercury__in the bulb or negative end. tubes are made in lengths of about 21 and 45 inches, with a diameter of about 1 inch, for candle powers of 300 and 700, respectively, for the general groups of voltages about 110 and 220, respectively.

The

The mercury tube, considered alone, has a peculiar character

which causes some of the metal to vaporize. Then the true arc is established through the vapor path. As the candle power of the mercury arc varies markedly with variations of voltage, a 'ballast' resistance similar to that of the Nernst lamp is utilized.

The alternating-current mercury tube carries two positive electrodes and a small starting electrode at the upper end. These two positive electrodes are connected to the ends of a small autotransformer, while the negative, or bulb end, is connected to the centre. The arc stream then passes from the negative to each of the positive electrodes, alternately, as in the mercury-arc rectifier. The luminous effect is continuous. The light emitted by this lamp is peculiar and characteristic, the almost complete absence of red rays robbing it of its utility for æsthetic purposes, causing objects appearing yellow. by daylight to assume a greenish tint. The lamp may be employed to great advantage for purposes where a considerable illumination is required, and where the practically monochromatic light serves as an aid to acuity of vision.

Moore Tube.-By sending a discharge through a gas at a suitably low pressure, on the order of 1/10,000 of an atmosphere, contained in a glass tube, a luminous glow is obtained that seems to fill the entire tube, the color depending on the gas used. In the practical form known as the Moore light, tubes about 134 inch in diameter and many feet long are used. An automatic device keeps the pressure right, and a small transformer connected to the ordinary lighting circuit supplies current at the necessary voltage, which may be several thousand volts. The efficiency compares favorably with that of the carbon lamp, the only form of incandescent lamp in use when it was developed, and the light, coming from a large surface, is soft.

Less efficient than the tungsten lamp, however, it was unable to compete with the latter for general lighting, but because of its whiteness the carbon-dioxidefilled tube, which gives a close approach to daylight, is suitable for color matching and therefore still finds practical application.

Neon Light.-A form of illuminant, thus far used chiefly for novelty and display lighting and substantially similar in principle to the Moore light, makes use of the rare gas neon as the light source. This gas, which constitutes about 1/66,000 of the atmosphere, is obtained as a by-product of the oxygen industry, and purified. A small quantity is admitted to a highly evacuated glass tube provided with a metal electrode at each end, and glows throughout its length with an orange-red color when a current is passed through it. Compared with incandescent lamps the applied voltage is high and the current small. The tubes are therefore not served directly from the ordinary 60-cycle supply circuits, but by way of small step-up transformers, which raise the voltage to a suitable value depending on their dimensions. For a tube 1⁄2 inch in diameter and 15 feet long it is about 6,000 volts, the current being around 0.025 ampere.

Because the conductivity increases with increasing current, the tube is unstable electrically on a constant potential supply; special transformers are used to meet this characteristic. The life is stated to be 4,000 to 5,000 hours, thus greatly exceeding that of incandescent lamps. The gas then has to be replenished; during use it is gradually absorbed or disappears.

The commonest present use of neon lamps in the United States is for signs. The tubes are formed into letters or other shapes; the striking color, the continuous

lines of light and an efficiency apparently comparable with that of the incandescent lamp, are the chief characteristics for that application. A color variation has been obtained by introducing into the tube a few drops of mercury. The latter vaporizes, and the light becomes blue.

The neon light was developed in France; numerous installations have been made in France and England. It was introduced into the United States about 1925; installations thus far have been made chiefly in the vicinity of New York City.

An entirely different form of neon lamp has recently been developed and was put on the market in the United States in 1926. It has a screw-base, like the ordinary incandescent lamp, but a much smaller bulb, and is designed for use on common lighting circuits. Like the tube light, this lamp makes use of the glow discharge, but the light is very feeble, the energy consumption being correspondingly small. The lamps are not intended for general illumination, but rather as markers in the dark. The light source is, again, the orange-red glow of neon. Two small cylindrical electrodes, separated by a few hundredths of an inch, are surrounded with a sort of corona when connected to a 110- or 115-volt circuit.

Electric Lighting, History of. Several centuries before the Christian era a Greek philosopher named Thales noted the fact that if amber is rubbed it will attract light objects. The Greek word for amber was 'elektron,' whence our word electricity. Some centuries later Aristotle wrote that the lodestone would attract iron; lodestone is an iron ore having magnetic qualities and is now known as magnetite. The best specimens of lodestone came from Magnesia, hence our word 'magnet.' In 1180 an English monk named Alexander Neckham described the compass and about 1600 William Gilbert wrote a book containing all the information then known on that subject. Some 50 years later Otto Von Guericke of Magdeburg made a machine consisting of a ball of sulphur mounted on a shaft which, when rotated by the hand, generated electricity. If revolved at a high rate of speed and rubbed by hand it would produce a glowing light. About 1745 Von Kleist of Pomerania devised the Leyden jar, the forerunner of the present condenser, and about 1785 Luigi Galvani (q.v.), an Italian scientist, propounded the theory of animal electricity.

But in 1800 came the epochmaking discovery of Volta, who found that electricity could be

generated by chemical means. To demonstrate his point he made a pile of silver and zinc discs with cloths wet with salt water between them. This was the forerunner of the present day primary battery. He soon found that as the cloths became dry the generation of electricity became weaker so that in order to overcome this he made his 'crown of cups,' which consisted of a series of cups containing salt water in which strips of silver and zinc were dipped. Each strip of silver in one cup was connected to the zinc strip in the next, the end strips of silver and zinc being terminals of the battery. In his honor the volt, the unit of electrical pressure, was named.

A few years later Sir Humphrey Davy demonstrated that electric current can heat carbon and metal strips to incandescence and give light and that the current will give a brilliant flame between the ends of two carbon pencils which are allowed to touch each other and then pulled apart, i.e. the arc light.

sor

André Marie Ampère, profesof mathematics in the Polytechnic School in Paris, in 1820 discovered that if an electric current is passed through a coiled wire, the coil obtains the properties of a magnet. The ampere, the unit of flow of electric current, is named in his honor. Ohm's Law (q.v.), one of the fundamental laws of electricity, was demonstrated in 1825 by George Ohm, who showed the relation between the voltage, amperage and resistance in an electric circuit, and for whom the unit of electric resistance is named. Michael Faraday, a British scientist, in 1831 discovered that electricity could be generated by means of a permanent magnet, a principle now used in all dynamos. Since that time various developments have been made and in the early seventies commercial dynamos were available for use in arc lighting. In 1866 Sir Charles Wheatstone invented the 'selfexcited' dynamo now universally used. In 1840 Sir William Grove demonstrated his incandescent lamp in which platinum was made incandescent by means of a current flowing through it and the following year Frederick de Moleyns obtained the first patent for an incandescent lamp, which consisted of a spherical glass globe in whose upper part was a tube containing powdered charcoal. The tube was open at the bottom and through it ran a platinum wire whose end was coiled. Another platinum wire coiled at its upper end came up through the lower part of the globe not quite touching the