Part 17
The next great improvement came when W. von Bolton produced the tantalum lamp in 1904. There are certain metals known to have a melting point about 2000° C. or upwards, and of these tantalum is one. It can be produced from the potassium tantalo-fluoride in a pulverulent form. By carefully melting it _in vacuo_ it can then be converted into the reguline form and drawn into wire. In this condition it has a density of 16.6 (water = 1), is harder than platinum and has greater tensile strength than steel, viz. 95 kilograms per sq. mm., the value for good steel being 70 to 80 kilograms per sq. mm. The electrical resistance at 15° C. is 0.146 ohms per metre with section of 1 sq. mm. after annealing at 1900° C. _in vacuo_ and therefore about 6 times that of mercury; the temperature coefficient is 0.3 per degree C. At the temperature assumed in an incandescent lamp when working at 1.5 watts per c.p. the resistance is 0.830 ohms per metre with a section of 1 sq. mm. The specific heat is 0.0365. Bolton invented methods of producing tantalum in the form of a long fine wire 0.05 mm. in diameter. To make a 25 c.p. lamp 650 mm., or about 2 ft., of this wire are wound backwards and forwards zigzag on metallic supports carried on a glass frame, which is sealed into an exhausted glass bulb. The tantalum lamp so made (fig. 19), working on a 110 volt circuit takes 0.36 amperes or 39 watts, and hence has an efficiency of about 1.6 watts per c.p. The useful life, that is the time in which it loses 20% of its initial candle power, is about 400-500 hours, but in general a life of 800-1000 hours can be obtained. The bulb blackens little in use, but the life is said to be shorter with alternating than with direct current. When worked on alternating current circuits the filament after a time breaks up into sections which become curiously sheared with respect to each other but still maintain electrical contact. The resistance of tantalum increases with the temperature; hence the temperature coefficient is positive, and sudden rises in working voltage do not cause such variations in candle-power as in the case of the carbon lamp.
Patents have also been taken out for lamps made with filaments of such infusible metals as tungsten and molybdenum, and Siemens and Halske, Sanders and others, have protected methods for employing zirconium and other rare metals. According to the patents of Sanders (German patents Nos. 133701, 137568, 137569) zirconium filaments are manufactured from the hydrogen or nitrogen compounds of the rare earths by the aid of some organic binding material. H. Kuzel of Vienna (British Patent No. 28154 of 1904) described methods of making metallic filaments from any metal. He employs the metals in a colloidal condition, either as hydrosol, organosol, gel, or colloidal suspension. The metals are thus obtained in a gelatinous form, and can be squirted into filaments which are dried and reduced to the metallic form by passing an electric current through them (_Electrician_, 57, 894). This process has a wide field of application, and enables the most refractory and infusible metals to be obtained in a metallic wire form. The zirconium and tungsten wire lamps are equal to or surpass the tantalum lamp in efficiency and are capable of giving light, with a useful commercial life, at an efficiency of about one watt per candle. Lamps called osram lamps, with filaments composed of an alloy of osmium and tungsten (wolfram), can be used with a life of 1000 hours when run at an efficiency of about 1.5 watts per candle.
Tungsten lamps are made by the processes of Just and Hanaman (German patent No. 154262 of 1903) and of Kuzel, and at a useful life of 1000 hours, with a falling off in light-giving power of only 10-15%, they have been found to work at an efficiency of one to 1.25 watts per c.p. Further collected information on modern metallic wire lamps and the patent literature thereof will be found in an article in the _Engineer_ for December 7, 1906.
Mention should also be made of the Helion filament glow lamp in which the glower is composed largely of silicon, a carbon filament being used as a base. This filament is said to have a number of interesting qualities and an efficiency of about 1 watt per candle (see the _Electrician_, 1907, 58, p. 567).
Mercury vapour lamps.
The mercury vapour lamps of P. Cooper-Hewitt, C. O. Bastian and others have a certain field of usefulness. If a glass tube, highly exhausted, contains mercury vapour and a mercury cathode and iron anode, a current can be passed through it under high electromotive force and will then be maintained when the voltage is reduced. The mercury vapour is rendered incandescent and glows with a brilliant greenish light which is highly actinic, but practically monochromatic, and is therefore not suitable for general illumination because it does not reveal objects in their daylight colours. It is, however, an exceedingly economical source of light. A 3-ampere Cooper-Hewitt mercury lamp has an efficiency of 0.15 to 0.33 watts per candle, or practically the same as an arc lamp, and will burn for several thousand hours. A similar lamp with mercury vapour included in a tube of _uviol_ glass specially transparent to ultra-violet light (prepared by Schott & Co. of Jena) seems likely to replace the Finsen arc lamp in the treatment of lupus. Many attempts have been made to render the mercury vapour lamp polychromatic by the use of amalgams of zinc, sodium and bismuth in place of pure mercury for the negative electrode.
Photometry of glow lamps.
An important matter in connexion with glow lamps is their photometry. The arrangement most suitable for the photometry and testing of incandescent lamps is a gallery or room large enough to be occupied by several workers, the walls being painted dead black. The photometer, preferably one of the Lummer-Brodhun form, is set up on a gallery or bench. On one side of it must be fixed a working standard, which as first suggested by Fleming is preferably a large bulb incandescent lamp with a specially "aged" filament. Its candle-power can be compared, at regular intervals and known voltages, with that of some accepted flame standard, such as the 10 candle pentane lamp of Vernon Harcourt. In a lamp factory or electrical laboratory it is convenient to have a number of such large bulb standard lamps. This working standard should be maintained at a fixed distance on one side of the photometer, such that when worked at a standard voltage it creates an illumination of one candle-foot on one side of the photometer disk. The incandescent lamp to be examined is then placed on the other side of the photometer disk on a travelling carriage, so that it can be moved to and fro. Arrangements must be made to measure the current and the voltage of this lamp under test, and this is most accurately accomplished by employing a potentiometer (q.v.). The holder which carries the lamp should allow the lamp to be held with its axis in any required position; in making normal measurements the lamp should have its axis vertical, the filament being so situated that none of the turns or loops overlies another as seen from the photometer disk. Observations can then be made of the candle-power corresponding to different currents and voltages.
The candle-power of the lamp varies with the other variables in accordance with exponential laws of the following kind:--
If A is the current in amperes through the lamp, V the voltage or terminal potential difference, W the power absorbed in watts, _c.p._ the maximum candle-power, and a, b, c, &c., constants, it has been found that A and _c.p._ are connected by an exponential law such that
c.p. = aA^x
For carbon filament lamps x is a number lying between 5 and 6, generally equal to 5.5 or 5.6. Also it has been found that c.p. = bW³ very nearly, and that
c.p. = cV^y nearly
where c is some other constant, and for carbon filaments y is a number nearly equal to 6. It is obvious that if the candle-power of the lamp varies very nearly as the 6th power of the current and of the voltage, the candle-power must vary as the cube of the wattage.
Sir W. de W. Abney and E. R. Festing have also given a formula connecting candle-power and watts equivalent to c.p. = (W - d)² where d is a constant.
In the case of the tantalum lamp the exponent x has a value near to 6, but the exponent y is a number near to 4, and the same for the osmium filament. Hence for these metallic glowers a certain percentage variation of voltage does not create so great a variation in candle-power as in the case of the carbon lamp.
Curves delineating the relation of these variables for any incandescent lamp are called its _characteristic-curves_. The life or average duration is a function of W/c.p., or of the _watts per candle-power_, and therefore of the voltage at which the lamp is worked. It follows from the above relation that the watts per candle-power vary inversely as the fourth power of the voltage.
From limited observations it seems that the average life of a carbon-filament lamp varies as the fifth or sixth power of the watts per candle-power. If V is the voltage at which the lamp is worked and L is its average life, then L varies roughly as the twenty-fifth power of the reciprocal of the voltage, or
L = aV^(-25).
A closer approximation to experience is given by the formula
V V² log10L = 13.5 - -- - ------. 10 20,000
(See J. A. Fleming, "Characteristic Curves of Incandescent Lamps," _Phil. Mag._ May 1885).
Ageing of lamps.
All forms of incandescent or glow lamps are found to deteriorate in light-giving power with use. In the case of carbon filaments this is due to two causes. As already explained, carbon is scattered from the filament and deposited upon the glass, and changes also take place in the filament which cause it to become reduced in temperature, even when subjected to the same terminal voltage. In many lamps it is found that the first effect of running the lamp is slightly to increase its candle-power, even although the voltage be kept constant; this is the result of a small decrease in the resistance of the filament. The heating to which it is subjected slightly increases the density of the carbon at the outset; this has the effect of making the filament lower in resistance, and therefore it takes more current at a constant voltage. The greater part, however, of the subsequent decay in candle-power is due to the deposit of carbon upon the bulb, as shown by the fact that if the filament is taken out of the bulb and put into a new clean bulb the candle-power in the majority of cases returns to its original value. For every lamp there is a certain point in its career which may be called the "smashing-point," when the candle-power falls below a certain percentage of the original value, and when it is advantageous to replace it by a new one. Variations of pressure in the electric supply exercise a prejudicial effect upon the light-giving qualities of incandescent lamps. If glow lamps, nominally of 100 volts, are supplied from a public lighting-station, in the mains of which the pressure varies between 90 and 110 volts, their life will be greatly abbreviated, and they will become blackened much sooner than would be the case if the pressure were perfectly constant. Since the candle-power of the lamp varies very nearly as the fifth or sixth power of the voltage, it follows that a variation of 10% in the electromotive force creates a variation of nearly 50% in the candle-power. Thus a 16 candle-power glow lamp, marked for use at 100 volts, was found on test to give the following candle-powers at voltages varying between 90 and 105: At 105 volts it gave 22.8 c.p.; at 100 volts, 16.7 c.p.; at 95 volts, 12.2 c.p.; and at 90 volts, 8.7 c.p. Thus a variation of 25% in the candle-power was caused by a variation in voltage of only 5%. The same kind of variation in working voltage exercises also a marked effect upon the average duration of the lamp. The following figures show the results of some tests on typical 3.1 watt lamps run at voltages above the normal, taking the average life when worked at the marked volts (namely, 100) as 1000 hours:
At 101 volts the life was 818 hours. " 102 " " 681 " " 103 " " 662 " " 104 " " 452 " " 105 " " 374 " " 106 " " 310 "
Voltage regulators.
Self-acting regulators have been devised by which the voltage at the points of consumption is kept constant, even although it varies at the point of generation. If, however, such a device is to be effective, it must operate very quickly, as even the momentary effect of increased pressure is felt by the lamp. It is only therefore where the working pressure can be kept exceedingly constant that high-efficiency lamps can be advantageously employed, otherwise the cost of lamp renewals more than counterbalances the economy in the cost of power. The slow changes that occur in the resistance of the filament make themselves evident by an increase in the watts per candle-power. The following table shows some typical figures indicating the results of ageing in a 16 candle-power carbon-filament glow lamp:--
+----------+-------------+-------------+ |Hours run.|Candle-Power.| Watts per | | | |Candle-Power.| +----------+-------------+-------------+ | 0 | 16.0 | 3.16 | | 100 | 15.8 | 3.26 | | 200 | 15.86 | 3.13 | | 300 | 15.68 | 3.37 | | 400 | 15.41 | 3.53 | | 500 | 15.17 | 3.51 | | 600 | 14.96 | 3.54 | | 700 | 14.74 | 3.74 | +----------+-------------+-------------+
The gradual increase in watts per candle-power shown by this table does not imply necessarily an increase in the total power taken by the lamp, but is the consequence of the decay in candle-power produced by the blackening of the lamp. Therefore, to estimate the value of an incandescent lamp the user must take into account not merely the price of the lamp and the initial watts per candle-power, but the rate of decay of the lamp.
Edison effect.
The scattering of carbon from the filament to the glass bulb produces interesting physical effects, which have been studied by T. A. Edison, W. H. Preece and J. A. Fleming. If into an ordinary carbon-filament glow lamp a platinum plate is sealed, not connected to the filament but attached to a third terminal, then it is found that when the lamp is worked with continuous current a galvanometer connected in between the middle plate and the positive terminal of the lamp indicates a current, but not when connected in between the negative terminal of the lamp and the middle plate. If the middle plate is placed between the legs of a horse-shoe-shaped filament, it becomes blackened most quickly on the side facing the negative leg. This effect, commonly called the _Edison effect_, is connected with an electric discharge and convection of carbon which takes place between the two extreme ends of the filament, and, as experiment seems to show, consists in the conveyance of an electric charge, either by carbon molecules or by bodies smaller than molecules. There is, however, an electric discharge between the ends of the filament, which rapidly increases with the temperature of the filament and the terminal voltage; hence one of the difficulties of manufacturing high-voltage glow lamps, that is to say, glow lamps for use on circuits having an electromotive force of 200 volts and upwards, is the discharge from one leg of the filament to the other.
Domestic use.
A brief allusion may be made to the mode of use of incandescent lamps for interior and private lighting. At the present time hardly any other method of distribution is adopted than that of an arrangement _in parallel_; that is to say, each lamp on the circuit has one terminal connected to a wire which finally terminates at one pole of the generator, and its other terminal connected to a wire leading to the other pole. The lamp filaments are thus arranged between the conductors like the rungs of a ladder. In series with each lamp is placed a switch and a fuse or cut-out. The lamps themselves are attached to some variety of ornamental fitting, or in many cases suspended by a simple pendant, consisting of an insulated double flexible wire attached at its upper end to a ceiling rose, and carrying at the lower end a shade and socket in which the lamp is placed. Lamps thus hung head downwards are disadvantageously used because their _end-on candle-power_ is not generally more than 60% of their maximum candle-power. In interior lighting one of the great objects to be attained is uniformity of illumination with avoidance of harsh shadows. This can only be achieved by a proper distribution of the lamps. It is impossible to give any hard and fast rules as to what number must be employed in the illumination of any room, as a great deal depends upon the nature of the reflecting surfaces, such as the walls, ceilings, &c. As a rough guide, it may be stated that for every 100 sq. ft. of floor surface one 16 candle-power lamp placed about 8 ft. above the floor will give a dull illumination, two will give a good illumination and four will give a brilliant illumination. We generally judge of the nature of the illumination in a room by our ability to read comfortably in any position. That this may be done, the horizontal illumination on the book should not be less than one candle-foot. The following table shows approximately the illuminations in candle-feet, in various situations, derived from actual experiments:--
In a well-lighted room on the floor or tables 1.0 to 3.0 c.f. On a theatre stage 3.0 to 4.0 c.f. On a railway platform .05 to .5 c.f. In a picture gallery .65 to 3.5 c.f. The mean daylight in May in the interior of a room 30.0 to 40.0 c.f. In full sunlight 7000 to 10,000 c.f. In full moonlight 1/60th to 1/100th c.f.
From an artistic point of view, one of the worst methods of lighting a room is by pendant lamps, collected in single centres in large numbers. The lights ought to be distributed in different portions of the room, and so shaded that the light is received only by reflection from surrounding objects. Ornamental effects are frequently produced by means of candle lamps in which a small incandescent lamp, imitating the flame of a candle, is placed upon a white porcelain tube as a holder, and these small units are distributed and arranged in electroliers and brackets. For details as to the various modes of placing conducting wires in houses, and the various precautions for safe usage, the reader is referred to the article ELECTRICITY SUPPLY. In the case of low voltage metallic filament lamps when the supply is by alternating current there is no difficulty in reducing the service voltage to any lower value by means of a transformer. In the case of direct current the only method available for working such low voltage lamps off higher supply voltages is to arrange the lamps in series.
Additional information on the subjects treated above may be found in the following books and original papers:--
Mrs Ayrton, _The Electric Arc_ (London, 1900); Houston and Kennelly, _Electric Arc Lighting and Electric Incandescent Lighting_; S. P. Thompson, _The Arc Light_, Cantor Lectures, Society of Arts (1895); H. Nakano, "The Efficiency of the Arc Lamp," _Proc. American Inst. Elec. Eng._ (1889); A. Blondel, "Public and Street Lighting by Arc Lamps," _Electrician_, vols. xxxv. and xxxvi. (1895); T. Heskett, "Notes on the Electric Arc," _Electrician_, vol. xxxix. (1897); G. S. Ram, _The Incandescent Lamp and its Manufacture_ (London, 1895); J. A. Fleming, _Electric Lamps and Electric Lighting_ (London, 1899); J. A. Fleming, "The Photometry of Electric Lamps," _Jour. Inst. Elec. Eng._ (1903), 32, p. 1 (in this paper a copious bibliography of the subject of photometry is given); J. Dredge, _Electric Illumination_ (2 vols., London, 1882, 1885); A. P. Trotter, "The Distribution and Measurement of Illumination," _Proc. Inst. C.E._ vol. cx. (1892); E. L. Nichols, "The Efficiency of Methods of Artificial Illumination," _Trans. American Inst. Elec. Eng._ vol. vi. (1889); Sir W. de W. Abney, _Photometry_, Cantor Lectures, Society of Arts (1894); A. Blondel, "Photometric Magnitudes and Units," _Electrician_ (1894); J. E. Petavel, "An Experimental Research on some Standards of Light," _Proc. Roy. Soc._ lxv. 469 (1899); F. Jehl, _Carbon-Making for all Electrical Purposes_ (London, 1906); G. B. Dyke, "On the Practical Determination of the Mean Spherical Candle Power of Incandescent and Arc Lamps," _Phil. Mag._ (1905); the _Preliminary Report of the Sub-Committee of the American Institute of Electrical Engineers_ on "Standards of Light"; Clifford C. Paterson, "Investigations on Light Standards and the Present Condition of the High Voltage Glow Lamp," _Jour. Inst. Elec. Eng._ (January 24, 1907); J. Swinburne, "New Incandescent Lamps," _Jour. Inst. Elec. Eng._ (1907); L. Andrews, "Long Flame Arc Lamps," Jour. Inst. Elec. Eng. (1906); W. von Bolton and O. Feuerlein, "The Tantalum Lamp," _The Electrician_ (Jan. 27, 1905). Also the current issues of _The Illuminating Engineer_. (J. A. F.)
Methods of charging.