The National Engineer. Vol. VI, No. 8, Otto Luhr, Chicago, August 1, 1902, pp. 1 - 4.

Paper read before N.A.S.E. [National Association of Stationary Engineers], No. 15, Pittsburg, Pa.

Nernst Lamp.

By Murray C. Beebe.

It is now some twenty years since the incandescent lamp was first put into commercial service. Since that time nothing radical has been done in the way of improving the efficiency of this device. It is true that their price has been materially reduced through the use of improved machinery and methods of manufacture, and, too, their life has been somewhat lengthened. The arc lamp has also undergone changes and improvements which have adapted it to use on circuits more rational than those of the old series arc machines. With the term "enclosed arc" we associate an improvement in arc lighting, in that the life of the carbons has been wonderfully increased by the use of an enclosing globe, which prevents the free circulation of air around the arc. Yet it may be said that improvements in methods of electrical illumination have not hitherto fairly kept pace with the great advances made by the electrical industries along almost any other line. Although engineers and scientists have realized for some time that existing methods of illumination are frightfully inefficient, it is only within a very few years that much has really been accomplished toward improving this condition of affairs. Both the present day arc and the incandescent lamp leave much to be desired in the matter of illumination. How well that deficiency has been remedied by the Nernst Lamp may be left to you, as interested critics, to judge after you have seen them in operation.

Dr. Walter Nernst of Goettingen, Germany, was the first to utilize as an illuminant, the fact that certain of the refractory oxides of metals of the rare earths are conductors of electricity when hot. This important discovery was brought to the attention of Mr. Westinghouse, who was quick to recognize in it wonderful possibilities for improving existing methods of illumination. Since 1897, when Dr. Nernst first announced his discovery, the work of putting the new invention into commercial shape has been quietly but vigorously pushed to completion.

The nucleus or light-giving element of the lamp has been termed the "glower". It is made by expressing from a die, a dough made up of the oxides mentioned, mixed with a suitable binding material. the porcelain-like string thus made is cut into lengths, roasted, and suitable terminals are attached. The finished glower in the standard 220 volt lamp is about .025 inches in diameter and about one inch long. They operate in the open air, which stamps them immediately as something entirely different from the ordinary incandescent lamp filament you are used to. Further, they will withstand a much higher temperature than the carbon filament of the incandescent lamp, which accounts for the correspondingly greater efficiency, and the better color possessed by Nernst light, but of these I shall have more to say later on. Curiously enough, these glowers are insulators when cold, and hence require to be heated in some manner before they will conduct electricity sufficiently well to maintain themselves at a light-emitting temperature. The construction of a commercial and entirely automatic lamp then requires the addition of some heating device to provide for this initial heating. It was thought at first that the severe requirements imposed upon any heating device would effectually prevent the utilization of Nernst's discovery in a practical lamp, but this has not proven to be the case. "Heaters" for this purpose are made by winding fine platinum wire upon thin porcelain tubes. Platinum is quite capable of withstanding the intense heat in the vicinity of the glower and after all, its cost for this purpose is not great when it is considered that a large percentage of the platinum may be recovered from the worn out heaters.

The proximity of the glower to the heater soon results in the glower becoming a conductor, and the current then passes through it also. When this has reached a pre-determined amount, an electro-magnet or "cut-out" coil has become strong enough by virtue of the glower current passing through it to attract the contact piece or armature of the cut-out, which, heretofore has closed the heater circuit. This disrupts the heater current, leaving only the glowers in operation, until the next time the lamp is turned on. Ordinarily, about 30 seconds is the time required for starting. Opening the switch which controls the circuit allows the cut-out armature to drop into place again, thus connecting the heaters ready for operation. You already know that the glower becomes a better and better conductor as its temperature, or the current passing through it, is increased. This unstable condition of things would almost instantly result in broken glowers were not a steadying resistance placed in the circuit of each glower to choke down the current before it can reach an amount dangerous to that glower. This is accomplished by what has been termed a "ballast", the name signifying its function of holding down the current. Iron possesses exactly the opposite characteristic of the glower, in that its electrical conductivity diminishes with increase of temperature. This is true of all metals, but iron possesses it to a remarkable degree when at a dull red heat.

The size of the iron wire used in our standard ballast is exceedingly small, about .045 of a millimeter, or less than two one-thousandths of an inch, and this is somewhat smaller than a human hair. To prevent oxydation of this fine wire, it is enclosed in a small glass envelope, which is filled with an inert gas, the complete ballast resembling somewhat a miniature incandescent lamp.

Lamps have been developed in the following sizes for operation on alternating current circuits only.

Although the lamp is adaptable to operation in any position, the present line of manufacture is directed toward lamps to be hung in a downward vertical position, mainly because of the advantages possessed by a gravity cut-out. The use of gravity rather than of springs or other devices in the cut-out has resulted in making this the only moving mechanism in the lamp, a thoroughly reliable device.

Lamps of the same size have been developed for operation upon direct current circuits. These possess the same desirable characteristics in point of economy, color, etc., except the life of the glowers is somewhat less than when operating on alternating current circuits. There is already a considerable demand for such a lamp.

The main features are alike in all lamps. The various sizes are made by assembling one, two, three or six standard glowers with their requisite ballasts, etc. The replacement of perishable parts has been simplified by the use of aluminum plug contacts on glowers, heaters and ballasts, doing away with corroding contacts and troublesome screws. Convenience of making renewals is still further facilitated by having glowers and heaters mounted upon an easily removable piece called the "holder", which may be plugged into place very much as you are accustomed to replacing an incandescent lamp in its socket. In fact, the housing of the lamp which contains the ballasts and cut-out may be considered as a special Nernst lamp socket, and with a few spare holders in stock, the lamp inspector has only to pass from lamp to lamp, changing holders where necessary and then repairing the old ones at odd times, and where proper facilities are at hand. The lamps contain no combustible material whatever and all parts are assembled in a substantial manner. The six-glower 500 watt lamp is the largest unit, and is furnished with either sandblasted ball, eight inches in diameter, or a dome shade. The former is neat in appearance and is much used where a general or artistic illumination is required. For many purposes the dome shade is desirable for the reason that the light is directed downward, where it is needed for special illumination. In case the multiple glower lamp is used with a dome shade, an opalescent "heater case" is provided also, the purpose of which is to diffuse the light as well as to stagnate the heat, thus facilitating more rapid starting. The three-glower 250 watt lamp is a more suitable size in many cases. Especially where ceilings are low, more units of less power are much to be preferred on account of the more even distribution of the illumination thus possible.

In general the 250 watt lamps spaced about fifteen feet apart give an effective general illumination. When ceilings are high enough, say over fifteen feet, 500 watt lamps spaced 20 to 22 feet between lamps is adequate for almost any purpose. For illuminating storage rooms and passageways and the like, of course, fewer lamps will suffice.

Those of you who chanced to see the exhibit of 100 or more six-glower Nernst lamps in the electrical building of the Pan-American Exposition must have been struck with the beauty of that illumination. They operated during the entire season with no trouble whatsoever beyond the occasional replacing of glowers and heaters. At the Westinghouse shops at East Pittsburg there are in operation 500 lamps. The majority of these are operating upon 3,000 alternation (25 cycle) circuits. The average life of the glowers on 7,200 alternations (60 cycles) is about 800 hours; on 3,000 alternations the life is about 400 hours. Operation upon 3,000 alternations is a field into which the arc lamp has no access, and the use of incandescent lamps, especially of low candle power, on this frequency is particularly fatiguing to the eye, because of the effort which the retina makes in following the fluctuations of the light intensity with the alternating current wave. The fact that Nernst lamps are operative on 3,000 alternation circuits assumes considerable importance because no unnecessary complications are introduced into the power plant, where it is desirable to use this frequency for both lighting and power transmission. Alternating current circuits of 220 volts (nominal) are best for operating Nernst lamps.

The Nernst lamp is so much more satisfactory for operation on alternating current circuits that all energies are largely directed toward the supplying of the demand in that direction.

As to economy, the power required by the Nernst lamp is about one-half of that used for the incandescent lamp, for equal illumination. Compared with the arc, the Nernst efficiency is slightly less, when considering the light emitted in all directions, yet for equal consumption of power, the superior distribution of the Nernst, combined with its absolute steadiness and pleasing color, give the impression of decided superiority in favor of the latter. After all, in many industries, this question of the economy of the lamp alone, while of considerable importance, is quite overshadowed by the economy in manufacture which may be effected by proper illumination of machines and work in progress. Avoidence of dense shadows is of utmost importance.

Our criterion of illumination is, of course, diffused daylight. When dependent upon artificial illumination, we wish to retain as far as possible the same characteristics as possessed by such daylight. The Nernst glowers operate at a higher temperature than the carbon filaments of the incandescent lamps. The result is not only a great increase in the efficiency but in improved color, since a greater proportion of the rays emitted occupy a higher position in the solar spectrum. In the arc, violet and blue rays predominate, while with the incandescent lamp we find less of the blue, with orange predominant. The Nernst light occupies a position between the two, closely resembling diffused daylight, and is particularly adapted to the accurate discrimination of colors. Shades which differ slightly by daylight, lack tone and individuality under the incandescent lamp, but are nicely brought out when viewed by the Nernst lamp. This is well demonstrated by a piece of apparatus in which one-half of a band, made up of strips of various colors and shades, is illuminated by Nernst and the other half by incandescent light.

After seeing this I am certain you will agree that the superiority in this respect lies overwhelmingly in favor of the Nernst. As to comparison with the arc lamp in this matter of color discrimination, it is related that a grocer of a New England town exposed for sale some cranberries, and because they were illuminated by an enclosed arc they were admired as the largest blueberries of the season.

As to care and maintenance, the average life of the glowers is 800 lamp hours and from 2,000 to 3,000 lamp hours for the heaters. The life of the ballasts may be considered indefinite under ordinary conditions. A 5 per cent rise in line voltage is entirely permissible. Continued running on 10 per cent over voltage may somewhat impair their life.

To those of you who may not have followed the development of the Nernst lamp at all, the impression may linger that it is still in the experimental stage. This is erroneous. There are now in operation May 1, under widely varying commercial conditions, in the neighborhood of 500,000 candle power in Nernst lamps. These are used in stores, residences, factories, office and public buildings, as well as for street lighting.

Much interest has been awakened by the entrance of the Nernst lamp into the field of commercial lighting and occupying, as it does, a position between the incandescent and the arc lamp, nothing but the greatest success is predicted for it.

[5 Figures omitted: (1) Holders for six, three, and one glower lamps. (2) How to grip holders without disturbing glowers. (3) Six glower lamps with housing removed. (4) Outdoor type, six glower lamp. (5) Indoor type, six glower lamp.]

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