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Life of Lamps. — Since all of the above statements refer to initial candle-power, it is necessary to specify the useful life of a lamp, or the time it will burn before falling to a certain candlepower, usually 80 % of the initial candle-power. For lamps having an initial efficiency of 3.1 watts per candle-power, the useful life is about 400 to 150 hours. At 3.5 watts it is about 800, and at 4 watts about 1600 hours.

Candle-hours — The true measure of a lamp's value is the product of its useful life in hours and its average candle-power during that time. The latter is usually about 90 % of the initial, hence a 3.1 watt 16 candle-power lamp should give at least 400 X 16 x .90=5760 candle-hours.

Efficiency. — The number of watts consumed per candle-power is another important point in lamp specifications. It refers usually to initial values, the specified useful life being a check upon the fall in candle-power and indirectly upon the efficiency. The standard efficiencies are 3.1, 3.5, and 4 watts per candle-power. Each lamp at rated voltage should take within 6% of the watts specified, and the average for a large number should be within 4 % of the specified figure. If the efficiency is high (i.e., small consumption of power) the life is shortened, and vice versa, a fair compromise being adopted in practice, as explained on pp. 415 and 418. If the cost of energy is low, as for example in some water-power plants, a lower efficiency lamp may be used, but it is seldom economical to use 4 instead of 3.5 watt lamps. The useful life of the former is about 1600 hours compared with 800 for the latter, which would save one lamp costing about 20 cents every 1600 hours. The energy consumed in 1600 hours costs } to cent per lamp-hour at ordinary central station rates, or $8 to $12; and a 3.5-watt lamp would use one-eighth less energy than a 4-watt lamp, the saving being $1 to $1.50, which is 5 to 74 times the cost of a lamp. Isolated electric-lighting plants in hotels, factories, etc., involve very little extra expense for engineers and other labor, or for coal when the exhaust steam is used for heating ; hence the electrical energy may be produced at .15 to 25 cent per lamp-hour. For 1600 hours it amounts to $2.40 to $4.00, and one-eighth of this is 30 to 50 cents, which is also greater than the cost of a lamp, so that even then 4-watt lamps are less economical than those using 3.5-watts per candle-power. It may happen that lamps are located in some inaccessible place, such as the ceiling of a large hall or railway station, and in that case it might be better to use the long-lived 4-watt lamps to save the trouble of frequent renewals. Where the regulation is poor (i.e., voltage varies considerably) the life is shortened, and it may be desirable to use 4watt lamps.

Bulbs and Bases. — The former are specified to be uniform in size and of best quality glass, clean and free from flaws or blemishes. The metallic parts of the base should be of good quality brass, uniformly and accurately fitted to the bulb so as to be impervious to moisture. When placed in the socket no live metallic part (i.e., connected to the circuit) should be exposed.

Vacuum. — All lamps must have a practically perfect vacuum, and show no glow when tested with an induction coil giving a halfinch spark.

For further information regarding Incandescent Lamps, reference may be made to the following:

The Incandescent Lamp and Its Manufacture, by Gilbert S. Ram, pp. 218, London, 1893.

A Life and Efficiency Test of Incandescent Lamps, by Professor B. F. Thomas and Messrs. Martin and Hassler, Transactions of the American Institute of Electrical Engineers, vol. ix., p. 271, 1892.

The Most Economical Age of Incandescent Lamps, by Carl Hering, ibid., vol. x., p. 65, 1893.

Conductivity of Incandescent Carbon Filaments and of the Space Surrounding Them, by John W. Howell, ibid., vol. xiv., p. 27, 1897.

The Incandescent Lamp (Manufacture), by Manning K. Eyre, The Electrical World, Jan. 5, 1895.

Incandescent Lamps, by Francis W. Willcox, Journal of the Franklin Institute, April, 1900.

CHAPTER XVIII.

LAMPS NOT EMPLOYING CARBON.

All forms of electric lamp in successful use prior to 1900 employed carbon as the light-giving body. This applies to arc lamps, which in all cases are provided with carbon electrodes, and to incandescent lamps, which employ carbon filaments. There are, however, two other interesting classes of lamps which do not use carbon : one includes the so-called vacuum tubes, in which all the light is emitted by a gas or vapor; and the other comprises incandescent lamps, in which the filament is composed of some material other than carbon, the Nernst lamp being a prominent example. The use of vacuum tubes as sources of light is a very old idea, being described by Hauksbee in a treatise published about two hundred years ago.* He employed glass vessels containing rarefied air, made luminous by frictional electricity, and to quote his own words, the light was “so great that large print, without much difficulty, could be read by it."

Similar, but not much more successful, attempts have been made repeatedly during the succeeding two centuries. The development of the Geissler and other improved forms of vacuum tube, and of the induction coil, during the past fifty years or more, has facilitated and encouraged such investigations.

Mr. Nikola Tesla, in a paper on “Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination,” † gave prominence to this subject, and has since investigated and written further in connection with it, but has not yet advanced beyond the experimental stage. A paper on “Recent Developments in Vacuum Tube Lighting,” \ by Mr. D. McFarlan Moore, describes the methods employed and results obtained by him. In his laboratory and at the New York Electrical Exhibition of 1896 he showed a room of considerable size lighted fairly well in this way, but no commercial applications have yet been made. In another series of investigations, Mr. Cooper Hewitt of New York City has succeeded in making a vacuum tube lamp of several hundred candlepower, and having a very high efficiency of about watt per candlepower ; but these very promising results have not been published, and his methods up to the present time have not been applied commercially.

* Physico mechanical Experiments, etc., London, 1709. † Transact. Amer. Inst. Elec Eng. vol viii., p. 267. May, 189' | Ibid., vol. xiii., p. 85, April, 1896

The chief advantages to be expected from vacuum tube lamps are high efficiency, long life, and distribution of light. The last is due to the large volume from which the light is given off ; for example, a tube one foot long and an inch in diameter, or even larger, is luminous throughout. In the ordinary incandescent lamp the light is emitted from a filament six to ten inches long and a few thousandths of an inch in diameter. This is practically a line, and produces too sharp an image upon the retina, as shown by the fact that it persists after the eye is shut or turned away from the light.

The vacuum tube should have a long life, since the lightgiving body being a gas, and not a solid, is not worn away. On the other hand, the degree of vacuum may rise or fall owing to absorption of the gas or leakage of air, in either case changing the resistance of the tube and interfering with constancy of action. The high efficiency of a vacuum tube results from the fact that a gas or vapor may be raised to a much higher temperature than a solid. The consequence is, the quantity of light emitted is increased in comparison with the emission of heat. In fact, such sources are often said to give “light without heat,” but in most cases heat is given off with the light. Nevertheless, it is true that a glow-worm, for example, or some phosphorescent body, radiates a large part of its energy within the visible spectrum, the proportion of the longer, non-visible waves, called radiant heat, being far less than with ordinary sources of light.

There appears to be a discrepancy between the statements that the temperature in a vacuum tube is high, and yet the heat given off is small, but these are easily reconciled. If a 110-volt, 16 candle-power lamp is supplied with about 125 volts, it will give 32 candle-power. The power consumed is increased in about the ratio 1102 : 1252 = 12100 : 15625, or about 30 per cent, as shown on page 421. Hence the rate of the total emission of energy is raised 30 per cent, but the light emitted is doubled. Thus the quantity of heat for the same amount of light would be only 130 = 2 = 65 per cent as great as before. By carrying this still further, the proportion of heat to light can be reduced very greatly, and what is called “ light without heat ” may be produced. It is also a fact that the temperature of the filament is increased at the same time, but in order to give the same candle-power its mass may be diminished. This applies exactly to a vacuum tube lamp in which the mass is very small, but the temperature of the individual particles is raised to a high point by the passage of electric current or discharge. It is possible that the electrical effect upon the atoms or ions may be somewhat different from what is ordinarily called high temperature; but it amounts to the same thing, since high rates of vibration or short wave lengths are produced.

In the experiments of Tesla, luminous discharges were created in vacuum tubes or even in the open air by a high frequency generator (10,000 to 20,000 periods per second) connected to the primary of an induction coil, the secondary of which gave very high voltage. He also employed a form of induction coil in the primary of which electrical oscillations are set up by sudden breaking of the circuit, producing a much higher frequency (100,000 or more periods per second), and therefore giving an extremely high voltage with only a few turns of wire. In this way vacuum tubes were made to glow by holding them near the terminals, but without any electrical connection to them. Such forms of apparatus are hardly suitable for practical use, and they involve considerable losses from leakage.

Moore employed induction tubes with connections made to them in the usual manner and operating at comparatively low voltage obtained from a self-induction coil with an electromagnetic make-and-break in the circuit. The latter was placed in a vacuum in order to give a sudden break and to avoid burning the contact points, but even with this precaution such a device is likely to give trouble. The Wehnheldt interrupter may be substituted, but it is doubtful if any form of break yet devised can be relied upon to act for the long periods of time demanded in lighting service.

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