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 | ” |
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 Voltage regulators. 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.
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 Edison effect. 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.
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 Domestic use. 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
