In Fig. 20 is shown a view of a 12-inch sphere set up to measure the transmission of a diffusing glass. There is an opening of definite diameter in the top of the sphere, limited by a circular metal diaphragm, and the light from the lamp outside the sphere shines through this into the interior. Photometric measurement gives the value of this luminous flux. Then the diffusing glass is placed directly beneath the limiting diaphragm and another measurement is made which gives the amount of flux traversing the glass. In the case of diffuse reflectors the procedure is somewhat different. Fig. 23 shows the arrangement. The diffuse reflector which, as will be

P

Fig. 23-Measurement of coefficient of Fig. 24. Nutting's apparatus for measuring diffuse reflection, using an integrating coefficients of reflection. sphere.

seen, is placed at the center of the sphere with its reflecting side turned at an angle of 45 degrees to the light and away from the photometer. Thus no screen is needed in the sphere. The amount of flux admitted by the diaphragm being known, and the amount reflected from the diffusing surface being measured, the reflection coefficient at this angle of incidence can be computed. One method by which the amount of light admitted to the sphere can be checked up under similar conditions to those of the measurement of the reflected flux, is to place a mirror of known coefficient of reflection in the position occupied by the diffuse reflector. The amount of flux then measured divided by the known coefficient of reflection of the mirror, gives the amount of flux incident upon the diffuse reflector.

A singularly ingenious and elegant piece of apparatus for the measurement of coefficients of diffuse reflection has been devised

by Nutting.20 Thisinstrument is shown in plan in Fig. 24. The ring in the figure is covered on the upper surface by a dense milk glass. On the under surface it is covered by the diffuse reflector which is to be tested. A special photometric device is supplied whereby the brightness of the under surface of the milk glass may be compared with the brightness of the diffuse reflector. If a diffuse reflector had 100 per cent. reflecting power, its brightness would be the same as that of the milk glass. Any deficiency is due to its absorption. The photometer, which is a Martens-König polarization apparatus, gives a comparison between the two directly, and hence shows the reflecting power of the unknown surface. Direct and reverse readings must be taken in order to eliminate polarization

errors.

MEASUREMENTS OF PROJECTION APPARATUS

The elementary theory of a projector having a convex lens or a parabolic mirror and a nearly point source shows that when the source is placed at the principal focus of the mirror, the light rays leave the surface of the mirror with an angle of divergence which is equal to the angle subtended at that particular point of the mirror by the source of light. Therefore the illumination obtained from an apparatus of this kind diminishes with the distance, and if the distance is great enough, the exponent of the distance with which it diminishes is two. In other words, the illumination from the entire apparatus follows the inverse square law. With a small projecting apparatus accurately focused for "parallel" beam, it is not necessary to take any very great distance away in order to have the inverse square law apply. A goodly distance is, however, in all cases advisable, and in some cases imperative. For example, in the case of head-lamps which are focused so as to throw an imperfect image of the source of light a distance of 200 or 300 feet ahead, it is evident that the inverse square law could not be assumed without taking a distance considerably greater than 200 or 300 feet. Sometimes the focusing distance is shorter than this. In any case, in the photometric investigation of an apparatus which is to be used approximately at a certain distance, it is advisable to focus it for that distance and to make the measurement at that distance. These measurements can then be expressed as apparent candle-power of the

20 Nutting, Trans. I. E. S., Vol. 8, page 412, 1912.

apparatus at that distance, and in so doing the inverse square law is not assumed. It is evident that measurements of this character must perforce be made at night and that the portable photometer is practically the only apparatus that can be used for the purpose. It is advantageous to set the projector on a stand which can be rotated about a vertical axis and which has a divided scale whereby the angle at which the measurement is taken may be read off. By taking a series of measurements covering a few degrees on either side of the axis of the beam, data may be gathered whereby a candlepower distribution curve may be plotted. In view of the narrowness of such a beam a plot in polar coördinates is as a general thing of little use. It is good practice to plot these measurements in rectangular coördinates, putting angles in the axis of X and apparent candle-power in the axis of Y. If this distribution curve is carried far enough, it may be integrated according to the Rousseau method and the total flux of light emitted by the apparatus thereby determined. This, compared with the total flux of the lamp alone, gives the loss of light in the apparatus. A plot so made enables the exact position of the maximum candle-power, which should be the beam candle-power, to be determined. In the case of lamps for flood lighting the efficiency of the apparatus is of great importance and hence a minimum loss of light in it should be striven for, more perhaps than is the case with projectors. The value of the lost light may in this case most readily be determined by the use of the integrating sphere.

RECENT DEVELOPMENTS IN ELECTRIC LAMPS

BY G. H. STICKNEY

INTRODUCTION

3

Lectures by Drs. Steinmetz,1 Hyde2 and Whitney, in the 1910 Course, treated of the physical and chemical principles of light production, and described the electric illuminants from the scientific standpoint.

On this foundation it is the purpose of this lecture to trace the more important of the recent developments and describe briefly the principal lamps now in common use.

From the great mass of available data, an attempt is made to present such information as will be of most practical value in selecting and applying electric lamps.

Since arc lamps are usually furnished as complete units they are so treated. Incandescent lamps, however, are equipped with a great variety of reflectors and other accessories, which are furnished separately. It has, therefore, been found most expedient to provide a separate lecture on such accessories and give but slight reference to them in this lecture.

LAMP DEVELOPMENT

The basis of all artificial lighting is the means for converting electrical or other energy into light. Advances in the lighting art have followed in the wake of the improved, practical light source, and it is here that the greatest possibility for future advance lies. The most efficient illuminants are still very far below the ideals of efficiency, while many of them offer much opportunity for improvements, as regards reliability, convenience and maintenance.

Few, if any of the recent improvements in light producers have come by chance. They have rather been the result of arduous and expensive research by trained physicists, chemists and engineers in well organized laboratories. Even when an improved principle of light production has been discovered, practical devices have had to be designed, machinery for manufacturing economically and in quantity developed, sizes and other characteristics determined upon, in order that the improvement could be utilized to advantage.

While all these items cannot be perfected in advance of the practical application of the appliance, it is remarkable how few changes are necessary. It is a tribute to Thomas A. Edison that so many of his standards still hold.

PROGRESS SINCE 1910

In general, the progress since 1910 may be summed up in (a) improved efficiency, (b) reliability and safety, (c) economy of maintenance, (d) adaptability, (e) simplicity and convenience.

Accompanying these improvements there has been a sponding increase in intrinsic brilliancy of light sources, which, while advantageous for certain applications, has in general been undesirable. Fortunately, however, diffusing devices can be readily applied, giving an over-all result much in favor of the improved illuminants.

TENDENCY AS TO TYPES

Among the incandescent units the tungsten filament or "Mazda" lamp has assumed predominence. The tantalum and Nernst lamps have practically disappeared from manufacture, while the use of metallized-carbon filament or "Gem" and carbon lamps has decreased very rapidly in the last four years.

The actual percentages reported by the National Electric Light Association show that approximately 80 per cent. of all incandescent lamps sold during 1915 in this country were of the tungsten filament type. Incandescent lamps, as a whole, have increased in importance, encroaching on fields of lighting formerly assigned to other illuminants.

While many enclosed carbon arc lamps are still in use, especially in street lighting, their manufacture has dwindled to a very small number, giving way to more efficient illuminants. The flaming arc has been changed from an open to an enclosed lamp, and has been applied to street, industrial and photographic lighting whereas formerly its principal application was spectacular lighting.

The "luminous," "magnetite" or "metallic flame" arc lamp has become one of the leading street illuminants, especially since the ornamental types became available, while the multiple lamp used in industrial lighting is not now exploited.

CLASSIFICATION

Steinmetz' classified electric illuminants as (a) solid conductor, (b) gaseous conductor, (c) arc conductor, and (d) vacuum arc.

For