The past few years which have witnessed a rapid improvement in the arc and incandescent forms of electric lighting have also seen the commercial application of a new and distinct type of lamp; that is, the enclosed vapor or vacuum-tube lamp. Three forms of this type have become generally known. The first of these is the Cooper Hewitt, which is now widely used and well understood; a second is the Bastian, which like the former is a mercury vapor arc lamp, but of smaller dimensions and provided with an incandescent carbon filament lamp to provide red light rays; the third is the Moore vacuum tube. Since the latter has recently given evidences of commercial practicability and is perhaps not thoroughly understood, a brief description of it and consideration of its salient features will be given here. It will be shown, also, that certain phenomena observed in this tube may have an important bearing on the question of the nature of gaseous luminosity in general and one which has not been previously recognized.
Mr. D. McFarlane Moore began to experiment about twelve years ago for the purpose of producing a "cold light," that is, one in which all the electrical energy would be converted into light without any loss in the form of heat. At that time the maximum lighting efficiency was obtained with the arc lamp, consuming 1 watt per candle power. Dividing this value into the theoretical figure for the conversion of electrical energy into light as given by Dr. E. F. Roeber - one spherical candlepower per 0.115 watt - we see that only about 10% of the electrical energy is converted into light in this form of lamp, the other 90% is dissipated and lost in the undesirable form of heat.
Since then, however, illuminating art has been much improved, so that we now have a maximum lighting efficiency of 50% in the flaming arc lamp, which consumes only 0.23 watt per candle power. The most efficient form of the Moore light, that is, the one in which rose-red light rays predominate, has a maximum efficiency of about 3/4 watt per candle, and is but little better than that of the older form of are light. The heat loss is, therefore, about 85% in the most efficient form of the vacuum tube lamp.
Description of the Moore Vacuum Tube
Although the "cold light" problem therefore is still unsolved, Mr. Moore's experiments have finally produced a novel and fairly efficient form of illumination, and one which possesses distinct advantages for some purposes. As it is now being installed it consists of a continuous stretch of 154-inch glass tubing of any desired length; this is supported near the ceiling by suitable brackets and encircles the room or space to be illuminated.
This continuous length of tubing is made in situ by joining together 6 or 8-foot lengths, and at the corner, previously shaped angle pieces. The joints of the tube are made in the manner long used for joining large bore glass tubing in making-chemical and physical apparatus; the blow-pipe employed is a slightly modified and movable form of that, having two impinging flames, which has long been used for that purpose.
The ends of the one-piece tube thus formed come together in a box about 2 feet square, which is suitably and inconspicuously placed. These ends are constructed of slightly larger bore tubing for a foot or so, and are, of course, rounded and sealed at their extremity. They each contain an electrode of carbon electrically connected with the exterior by platinum wires sealed in the end of the tube.
These wires in turn are in electrical connection with a transformer, which raises the voltage of the alternating-current supply to 10,000, at which pressure the current is delivered to the tube.
Another essential part of the system, also placed within the box, is an ingenious vacuum regulator. This consists of a cone-shaped carbon pencil, sealed point upwards in the end of a narrow glass tube, sealed in turn to the main tube. Surrounding this carbon pencil and sealed to the narrow inlet tube which carries it, is a glass tube of larger diameter. The annular space thus formed contains enough mercury to cover the entire pencil, and in this mercury a metal tube displacer floats, attached at its upper end to the core of a solenoid placed above the regulator and in the lighting circuit.
The tube thus completed is evacuated by a Geryk or other mechanical vacuum pump to a pressure stated to be about 1/40,000 of an atmosphere. With the passage of the high-tension alternating current the tube immediately becomes luminous throughout, the light being a soft one with rose-red rays predominating.
A suitable manometer was first attached to similar tubes several years ago, which, however, were not then provided with a pressure regulator. It was then found that the rare faction of the gases in the tube slowly increased, and this was accompanied by perplexing resistance changes, which finally culminated in the failure of the light.
In order to secure a permanent light it has been found necessary, therefore, to admit very small portions of air to the tube at intervals. This is accomplished by the vacuum regulator which was devised subsequent to the measurement of pressure variations. When the air pressure decreases the internal tube resistance also decreases; more current flows through the solenoid of the regulator, and the core thereupon rises a trifle, withdrawing the displacer so that the mercury falls enough to uncover the tip of the carbon pencil.
This is thereby exposed to atmospheric pressure, and a small bubble of air, to which the pencil is not impervious as it is to mercury, filters through the carbon; the normal vacuum and resistance are thereby restored and the mercury rises again, shutting off the air supply. Practically normal tube conditions can be thus maintained indefinitely, and tubes thus automatically controlled have had a life of 1,000 hours or more.
The circuit is conducted through the Moore tube from terminal to terminal solely by the highly rarified gases, and these alone are the source of the light it emits. The Moore light, therefore, is only a lengthened Geissler tube; its light may at times exhibit striated effects similar to those of this tube, but ordinarily these are not noticed by the eye.
Mr. Moore has, therefore, made a practical application of a mode of producing luminous effects which has long been known; which, indeed, was one of the earliest forms of light produced by electricity. Now that this form of light bids fair to have a commercial value it is to be expected that the nature of it may be the subject of more frequent, or at least more successful, scientific investigation, for strange to say little is definitely known concerning the source of the luminosity of conducting rarified gases.
Theories of Gaseous Luminosity
It will be interesting to note here the theories regarding luminiferous gases recently advanced by different investigators in this field. The older theory is that the gaseous molecules send out vibrations of a visible wave length as a result of purely physical collision or heat effects consequent on electrical conductivity. Regarding this theory, Prof. H. A. Armstrong, in 1902, in an article on "The Conditions Determinative of Change and of Electrical Conductivity in Gases, and on the Phenomena Luminosity, stated: "An argument which I think will sooner or later be regarded as of weight in favor of the view that the phenomena are electrolytic in their origin is afforded by the luminous manifestations in vacuum tubes. These can scarcely be mere collision effects or mere heat effects. It has long seemed to me that luminosity and line spectra are the expression - the visible signs - of the changes attending the formation of molecules from their atoms, or, speaking generally, that they are the consequence of chemical changes, a chemical change being one which involves an alteration of molecular composition, or it may be of molecular configuration, as it is conceivable that even changes involving isodynamic (tautomeric) molecules - changes in molecular structure unattended with change in molecular size - may give rise to such manifestations." Prof. Armstrong believes gaseous luminosity to be the result of chemical activity therefore.
Prof. J. Stark, of Gottingen, on the other hand, who has made extended researches in this field, believes it to be a purely physical phenomenon, according to the following translated statement: "The line spectrum of a gas has its source of energy in the kinetic energy of the particles of the ionized gas, its conduction in the positive ion atoms. The band spectrum of a gas, on the other hand, probably has its source of energy in the potential energy which the positive and negative ions possess in respect to one another, and which on recombination can be changed, at least partly, into light energy."
But a third authority, Dr. C. P. Steinmetz, in an address before the American Institute of Electrical Engineers, on "The Transformation of Electric Power Into Light," in November, 1906, stated regarding Geissler and vacuum tubes "The mechanism of this light production does not seem to be known, but the light seems to be somewhat of the character of a by-product."
What May be Learned from the Moore Light Concerning Luminous Gases
It has not been recognized, so far as I am aware, that the observed constant decrease of gaseous pressure in the Moore tube may have an important bearing on the question as to the cause of the luminosity of the gases contained in the tube, and, indeed, of conducting gases in general. As has been already stated, it has been found necessary to feed the tube with air at frequent intervals. As a matter of fact very small portions are added every minute or so during use. This, of course, can mean nothing else but that chemical reactions are occurring which result in an increase of molecular size, and therefore in a decrease in the number of molecules and of the volume of gas.
The substances in the tubes in considerable amounts which might cause such reactions are, of course, nitrogen and oxygen and the carbon of the electrodes. Of the known reactions into which these might enter, those involving cyanogen formation, either as an intermediate or as an end product, cannot cause reduced pressure. The formation of ozone from oxygen, on the other hand, would cause reduction in the number of molecules and a higher vacuum. Ozone, however, is rapidly decomposed into oxygen again at temperatures below that of the interior of the Moore tube when in use, so that ozone cannot be a stable end product, nor can its formation cause the reduced pressure.
This leaves only the uniting of nitrogen and oxygen to form one of the oxides of nitrogen as the probable cause of the phenomena. The formation of NO and of N2Os as end products would not result in reduced pressure, while that of N20, N02 or of N203 would. Of these, NO2 is the more probable end product, but this investigation must, of course, verify.
It is true that solid brownish-colored deposits are frequently formed in the body of the tube; but is highly improbable that they are the products of reactions causing the observed gaseous contraction, for they are by no means invariably formed after prolonged use, and may, indeed, appear either very soon after installation or not at all. They are in all probability due, therefore, to impurities or variations in the composition of the terminals or of the glass tubing.
It seems certain, therefore, that the formation of one or more of these oxides of nitrogen is the cause of the reduced pressure of the tube. Apparently, Mr. Moore has come to the same conclusion - it may be by investigation - for he has often spoken of his light as one which "burns air."
Now, numerous investigations on the fixation of, atmospheric nitrogen have demonstrated that the formation of oxides of nitrogen are purely thermal effects. This being the case oxides of nitrogen will be formed in the tubes wherever there is sufficient heat development, and that is throughout the tube, since 85% of the electrical energy put into the tube is converted into heat. Throughout its entire extent it becomes uniformly hot.
The heat development of the terminals is not appreciably greater than in the body of a tube, though bolometric measurements may show some slight difference. But this is certainly not considerable; so that in view of the far greater reacting volume of gas in the body of the tube it appears that the formation of oxides of nitrogen takes place in the Moore tube throughout its entire extent where it is accompanied by the emission of light.
The chemical actions here involved may not necessarily be direct ones. Ozone or other unstable but active molecular complexes may be concerned in it, and the reactions may be reversible and have low reaction velocities, but the sum total result of electrical activity in the Moore tube is, apparently, the formation of a stable oxide of nitrogen accompanied by and intimately connected with the continuous emission of light rays.
This conclusion is important in that its verification will furnish a practical substantiation of the theory of the chemical nature of gaseous luminosity advanced by Prof. Armstrong and already cited in this article. And verification is not difficult, it includes two steps: First, identification of the substances formed in a tube and an investigation whether the concerned reaction velocities are greatly influenced by heat. Second, an observation of the effect of temperature variations in the conducting gases on the candlepower of the light which they emit, these temperature variations to be controlled by means external to the tube. If, for example, NO2 is formed, and if the reactions producing it give rise to luminous effects, then cooling the tube to a very low temperature during its activity, e. g. by liquid air, would lower the candle power, for the velocity of N02 formation is greatly retarded by reduced temperature (see Guye, Foerster and Nernst, loc. cit).
The probable result of such an experiment is indicated by one cited by Prof. Armstrong. He states that Prof. Dewar, in an experiment before the Royal Institute, cooled a phosphorescent Crookes tube with liquid air and that its discharge at once ceased. Prof. Armstrong attributes this result to cata1 lytic rather than thermal effects. If cooling to a low temperature produces similar effects in the Moore tube, as from the close analogy it may be expected it will, the dependence of the luminosity of a mixture of conducting gases on the velocities of gaseous chemical reactions will have been proven. This could not before have been proven, because only a vacuum tube as extensive as the Moore tube could give reaction products in amounts which could be identified.
Experiments such as suggested, if they verify the theory outlined above, will also teach several things about the Moore tube and vacuum-tube lamps in general. The first of these is, that in order to secure high-efficiency gaseous light-emitting reactions must be employed which are exothermic or whose reaction velocities possess a relatively low temperature coefficient. That is, it must not be necessary to change much of the electrical energy into heat in order to maintain the high temperature for a rapid reaction velocity, as is necessary in the formation of oxides of nitrogen. Or if suitable reactions of this nature cannot be found the necessary acceleration of reaction velocity must be secured by the use of catalytic agents.
The second conclusion to be drawn from the above theory - an inference, indeed, which is palpably evident from what is already known concerning the Moore tube - is that there is a limit to its life. Air is constantly admitted to the tube, enters into chemical reaction therein, and the reaction products remain in the tube. If gaseous they will gradually accumulate therefore, and ultimately extinguish the light by displacement of the active gases; even if they were solid they would coat the interior of the tube and make the light very inefficient. It is to be expected that the Moore tube will occasionally need repair, therefore, though the contrary seems to have been stated.
Advantages and Defects of the Moore Vacuum Tube Light
Mr. Moore claims for his light a high efficiency, good actinic value, low intrinsic brilliancy, safety, perfect illumination without shadows and a very long life, indeed, that it will last for years without repair. The efficiency of the tube has recently been the subject of careful photometric tests by the New York Electrical Testing Laboratories, which showed that the average luces per unit energy were 20.0 for the Moore, 11.2 for the Nernst, and 3.6 for the carbon-filament incandescent lights, which is equal to .65, 1.1 and 3.5 watts per candle power, respectively. This efficiency is for the rose-red light only. In tubes emitting white light the efficiency is stated to be only about half the above, that is, 1.5 watts per candle power. This is not as high an efficiency, therefore, as is obtained with the metallic-filament incandescent lamps now being introduced.
Mr. Moore at the present time rarely installs tubes giving a white light, probably because of their lower efficiency and unreliability; it is necessary to introduce other substances, and the tube conditions favorable to a long life or even satisfactory service are not yet thoroughly understood. The white light only is suitable for general use, of course, but the rose-red tint is satisfactory for exterior lighting and in rooms where proper color values are not essential.
The principal advantage of the vacuum tube is its Ion-intrinsic brilliancy. Its light is but 12-candle power per linear foot. It has no extremely bright portion common to nearly every other form of artificial light, which strain the eyes whenever they are in the angle of vision. This evil is now being lessened somewhat by the general use of frosted globes or shades, but even these do not bring these other forms down to as low an intrinsic brilliancy as that of the Moore tube.
The perfect illumination without shadows, that is, from every angle of the room, which has been claimed as an exclusive advantage of the Moore tube is equally well secured by the use of small units like low candle-power incandescent lamps suitably placed entirely around the sides of the room or on the ceiling at intervals of a foot or so. This method of installation produces the freedom from shadows quite as well as does the Moore tube.
The tubes are difficult to repair, and leave the room entirely without lighting facilities for a long time in case of a failure or accident to any part of the system. This fact, the high initial cost, and its want of what may be termed flexibility, that is, the impossibility of turning off or on at will a part of the tube illuminating any desired portion of the room, will militate against the rapid adoption of the Moore lamp for general illuminating purposes.
By C.J. THATCHER, P.H.D
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