International projectionist (Jan 1941-Dec 1942)

quartz. Each end of the arc tube is shaped so that a chamber is formed in which a pool of mercury is retained. The electrodes project slightly beyond the surface of these mercury pools. In an experimental form one end of the lamp is fitted with a brass cap and the other end is supported in an ebonite mounting to insulate it from the water in which it is immersed. The hightension lead is taken to a terminal in this insulated mounting. The lamp is mounted in a water jacket consisting of two concentric glass cylinders supported between metal rings, one of which is earthed, and to which the water connection and one electrical connection are made. A rapid flow of water is passed through the jacket in order to cool the arc tube. Probably the most unusual feature of the water-cooled lamp is the very short run-up time, as full light output is available within three seconds after switching on. Also, the arc will re-strike immediately the supply is restored in the event of a failure of the supply. These delay times are so short owing to the small heat content of the lamp. This is a very great advantage over the other forms of high-pressure mercury vapor lamps. The life of the water-cooled lamp depends on the operating cycle, as frequent starting tends to shorten the life. The inside of the arc tube operates at a very high temperature and much attention has to be paid to the avoidance of devitrification of the quartz during operation. Although the internal pressure may exceed 100 atmospheres, even if a lamp should burst no damage results in this case, as the energy in the explosion is not great due to the small volume of the arc tube. The water jacket is sufficiently strong so that it will not be broken in the event of the inner lamp bursting. At the end of its life the lamp may be withdrawn and a new lamp inserted. Characteristics of Emission Perhaps the most useful feature of the water-cooled lamp is the very small amount of heat radiation. Spectral distribution comparison shows that a far greater proportion of the input is radioed in the infra-red region in a tungsten ilament lamp than in the case of the Type MD/H lamp. Out of the total radiation of 700 watts in the infra-red region from the Type MD/H lamp, 630 vatts is carried away by the cooling rater and the remaining 70 watts is radiated. Compared with a tungsten filament lamp of equal wattage which radiates nearly 700 watts of infra-red energy, the water-cooled lamp gives more than twice the light and less than half the total wattage radiated. This fact is strikingly borne out by available data on the rela FIGURE 2 Compact source lamp; left, just after starting; right, in fully run-up condition tive increase in temperature of the skin exposed to light from tungsten and the Type MD/H lamp. For equal increases in temperature, 4^2 times the illumination may be used in the case of the watercooled lamp. Operation of Type MD/H Lamp The Type MD/H lamp operates from an A.C. supply through a high-reactance transformer of special design, or from a high-voltage D.C. generator with a resistance ballast. The cooling water may be taken from the water supply mains, unless this supply is very "hard," although it may be preferable to operate the lamp from a self-contained pump which circulates distilled water through a fan-cooled radiator. The temperature rise in the cooling water is only a few degrees Centigrade. In order to safeguard against failure of the water supply with consequent damage to the lamp, a pressure switch is used to prevent the supply to the lamp from being applied, unless there is the required quantity of water flowing through the jacket. The water flow is about three quarts per minute for a 1,000-watt lamp. The water-cooled lamp at a loading of 840 watts per cm. compares very favorably with the carbon arc as regards color, except for the increase at the blue end of the spectrum, which may be readily corrected with a filter. This loading is too high to give lamps of Jong life, but practical tests have shown that a lamp with a loading of 800 watts per cm. and having a voltage gradient of 420 volts per cm. gives reasonably good color rendering when used in a motion picture projector. The Type MD/H water-cooled lamp of medium brightness is made in 500-, 1000-, and 2000-watt sizes, all of which operate at the same loading of 400 watts per cm. and have the same average brightness of 24,000 c.p.s.c. A 1000-watt lamp with a loading of 800 watts per cm., which has a shorter arc length and a higher average brightness of 48,000 c.p.s.c, has been made. The high brightness of this lamp makes it a possible source for motion picture projection. The mercury vapor pressure there in is very high, and, as mentioned previously, this lamp gives a reasonably good rendering when used for the projection of colored films. Owing to the higher loading, the life will necessarily be shorter than that of the normal 1000watt lamp, but it has nevertheless a useful value. In order to make a water-cooled lamp of higher wattage, without increasing the wattage per cm. of the arc tube, and thua reducing the life, it is necessary to increase the arc length proportionately to the wattage. The operating voltage must also be increased in the same proportion to keep the voltage gradient constant. Thus a high wattage lamp requires a very high operating voltage and has a long light source. The chief characteristics of Type MD/H lamps are as follows: (1) Small amount of heat output. (2) High brightness and a very efficient light source. (3) The light source is linear in shape, (4) The delay times for obtaining full light output and for re-striking are very short indeed. (5) The radiation has a high actinic value. (6) The light output may be readily controlled. These characteristics render the lamp suitable for a number of applications^ Among these may be mentioned film studio lighting where a very high level of illumination is required and the re* duction of the heat from the lamps has always been a problem. Similar considerations apply in lighting television studios; in this case it is also claimed that the spectral distribution of the water-cooled mercury vapor lamp is very siutable for using in conjunction with the Iconoscope. In conclusion, it is clear that both the air-cooled and water-cooled high-pressure mercury vapor lamps should find useful applications, particularly in the kinema. industry. While both lamps have certain disadvantages, these are outweighed in many cases by other characteristics. (See Editorial on Page 23) SLOT MACHINE MOVIES TO BE LICENSED IN N. Y. CITY A campaign to license all coin-in-the-slot motion picture machines in New York City is being launched by Paul Moss, license commissioner. Every such device, Commissioner Moss said, will be required to be licensed under the law, adding that any violation "will be dealt with." Manufacturers and distributors were aoS vised to arrange with the License Department to formulate plans to carry on their business under the provisions of the administrative code of the city which defines a motion picture as being "a display on a screen or other device of pictures or object3 in motion or rapidly changing scenery whether or not such display shall be accom. panied by lecture, recitation or music." i FEBRUARY 194 1 19