Part 18

The mercury arc converted the alternating constant current into direct current required by the magnetite lamp.]

As the magnetite arc requires direct current for its operation, the obvious way to supply a direct constant current for series circuits is to rectify, by means of the mercury arc, the alternating current obtained from a constant current transformer. The terminals of the movable secondary coil of the constant current transformer are connected to the two arms of the rectifier tube. One end of the series circuit is connected to the center of the secondary coil. The other end of the series circuit is connected to a reactance which in turn is connected to the pool of mercury in the bottom of the rectifier tube. One-half of the cycle of the alternating current goes from the secondary coil to one arm of the rectifier tube through the mercury vapor, the mercury arc having already been started by a separate starting electrode. It then goes to the pool of mercury, through the reactance and through the series circuit. The other half cycle of alternating current goes to the other arm of the rectifier tube, through the mercury vapor, etc., and through the series circuit. Thus a pulsating direct current flows through the series circuit, the magnetic action of the reactance coil making the pulsations of current overlap each other, which prevents the mercury arc from going out.

INCANDESCENT LAMP DEVELOPMENTS, 1894–1904

With the development of a waterproof base in 1900, by the use of a waterproof cement instead of plaster of Paris to fasten the base to the bulb, porcelain at first and later glass being used to insulate the terminals of the base from each other, lamps could be exposed to the weather and give good results. Electric sign lighting therefore received a great stimulus, and lamps as low as 2 candlepower for 110 volts were designed for this purpose. Carbon lamps with concentrated filaments were also made for stereoptican and other focussing purposes. These lamps were made in sizes from 20 to 100 candlepower. The arc lamp was more desirable for larger units.

The dry battery was made in small units of 2, 3 and 5 cells, so that lamps of about ⅛ to 1 candlepower were made for 2½, 3½ and 6½ volts, for portable flashlights. It was not however until the tungsten filament was developed in 1907 that these flashlights became as popular as they now are. For ornamental lighting, lamps were supplied in round and tubular bulbs, usually frosted to soften the light.

This consisted of a tube about 1¾ inches in diameter and having a length up to 200 feet, in which air at about one thousandth part of atmospheric pressure was made to glow by a very high voltage alternating current.]

THE MOORE TUBE LIGHT

Geissler, a German, discovered sixty odd years ago, that a high voltage alternating current would cause a vacuum tube to glow. This light was similar to that obtained by Hawksbee over two hundred years ago. Geissler obtained his high voltage alternating current by a spark coil, which consisted of two coils of wire mounted on an iron core. Current from a primary battery passed through the primary coil, and this current was rapidly interrupted by a vibrator on the principle of an electric bell. This induced an alternating current of high voltage in the secondary coil as this coil had a great many more turns than the primary coil had. Scientists found that about 70 per cent of the electrical energy put into the Geissler tube was converted into the actual energy in the light given out.

In 1891 Mr. D. McFarlan Moore, an American, impressed with the fact that only one-half of one per cent of the electrical energy put into the carbon-incandescent lamp came out in the form of light, decided to investigate the possibilities of the vacuum tube. After several years of experiments and the making of many trial lamps, he finally, in 1904, made a lamp that was commercially used in considerable numbers.

As the carbon terminals inside the tube absorbed the very slight amount of gas in the tube, a feeder valve allowed gas to flow in the tube, regulating the pressure to within one ten thousandth part of an atmosphere above and below the normal extremely slight pressure required.]

The first installation of this form of lamp was in a hardware store in Newark, N. J. It consisted of a glass tube 1¾ inches in diameter and 180 feet long. Air, at a pressure of about one-thousand part of an atmosphere, was in the tube, from which was obtained a pale pink color. High voltage (about 16,000 volts) alternating current was supplied by a transformer to two carbon electrodes inside the ends of the tube. The air had to be maintained within one ten-thousandth part of atmospheric pressure above and below the normal of one-thousandth, and as the rarefied air in the tube combined chemically with the carbon electrodes, means had to be devised to maintain the air in the tube at this slight pressure as well as within the narrow limits required.

This was accomplished by a piece of carbon through which the air could seep, but if covered with mercury would make a tight seal. As the air pressure became low, an increased current would flow through the tube, the normal being about a tenth of an ampere. This accordingly increased the current flowing through the primary coil of the transformer. In series with the primary coil was an electro-magnet which lifted, as the current increased, a bundle of iron wires mounted in a glass tube which floated in mercury. The glass tube, rising, lowered the height of the mercury, uncovering a carbon rod cemented in a tube connecting the main tube with the open air. Thus air could seep through this carbon rod until the proper amount was fed into the main tube. When the current came back to normal the electro-magnet lowered the floating glass tube which raised the height of the mercury and covered the carbon rod, thus shutting off the further supply of air.

As there was a constant loss of about 400 watts in the transformer, and an additional loss of about 250 watts in the two electrodes, the total consumption of the 180-foot tube was about 2250 watts. Nitrogen gas gave a yellow light, which was more efficient and so was later used. On account of the fixed losses in the transformer and electrodes the longer tubes were more efficient, though they were made in various sizes of from 40 to 200 feet. The 200-foot tube, with nitrogen, had an efficiency of about 10 lumens per watt. Nitrogen gas was supplied to the tube by removing the oxygen from the air used. This was accomplished by passing the air over phosphorous which absorbed the oxygen.

Carbon dioxide gas (CO_{2}) gave a pure white light but at about half the efficiency of nitrogen. The gas was obtained by allowing hydrochloric acid to come in contact with lumps of marble (calcium carbonate) which set free carbon dioxide and water vapor. The latter was absorbed by passing the gas through lumps of calcium chloride. The carbon dioxide tube on account of its daylight color value, made an excellent light under which accurate color matching could be done. A short tube is made for this purpose and this is the only use which the Moore tube now has, owing to the more efficient and simpler tungsten filament incandescent lamp.

THE OSMIUM LAMP

Dr. Auer von Welsbach, the German scientist who had produced the Welsbach gas mantle, invented an incandescent electric lamp having a filament of the metal osmium. It was commercially introduced in Europe in 1905 and a few were sold, but it was never marketed in this country. It was generally made for 55 volts, two lamps to burn in series on 110-volt circuits, gave about 25 candlepower and had an initial efficiency of about 5½ lumens per watt. It had a very fair maintenance of candlepower during its life, having an average efficiency of about 5 l-p-w. Osmium is a very rare and expensive metal, usually found associated with platinum, and is therefore very difficult to obtain. Burnt out lamps were therefore bought back in order to obtain a supply of osmium. It is also a very brittle metal, so that the lamps were extremely fragile.

This incandescent lamp was used in Europe for a few years, but was impractical to manufacture in large quantities as osmium is rarer and more expensive than platinum.]

THE GEM LAMP

Dr. Willis R. Whitney, of Schenectady, N. Y., had invented an electrical resistance furnace. This consisted of a hollow carbon tube, packed in sand, through which a very heavy current could be passed. This heated the tube to a very high temperature, the sand preventing the tube from oxidizing, so that whatever was put inside the tube could be heated to a very high heat. Among his various experiments, he heated some carbon filaments and found that the high temperature changed their resistance “characteristic” from negative to positive. The ordinary carbon filament has a resistance when hot that is less than when it is cold, which was reversed after heating it to the high temperature Dr. Whitney was able to obtain. These filaments were made into lamps for 110-volt service and it was found that they could be operated at an efficiency of 4 lumens per watt. The lamps also blackened less than the regular carbon lamp throughout their life.

This incandescent lamp had a graphitized carbon filament obtained by the heat of an electric furnace, so that it could be operated at 25 per cent higher efficiency than the regular carbon lamp. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.]

This lamp was put on the market in 1905 and was called the Gem or metallized carbon filament lamp as such a carbon filament had a resistance characteristic similar to metals. At first it had two single hair pin filaments in series which in 1909 were changed to a single loop filament like the carbon lamp.

In 1905 the rating of incandescent lamps was changed from a candlepower to a wattage basis. The ordinary 16-candlepower carbon lamp consumed 50 watts and was so rated. The 50-watt Gem lamp gave 20 candlepower, both candlepower ratings being their mean candlepower in a horizontal direction. The Gem lamp was made for 110-volt circuits in sizes from 40 to 250 watts. The 50-watt size was the most popular, many millions of which were made before the lamp disappeared from use in 1918. The lamp was not quite as strong as the carbon lamp. Some Gem lamps for series circuits were also made, but these were soon superseded by the tungsten-filament lamp which appeared in 1907.

THE TANTALUM LAMP

The tantalum filament could be operated at 50 per cent greater efficiency than that of the regular carbon incandescent lamp. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.]

Dr. Werner von Bolton, a German physicist, made an investigation of various materials to see if any of them were more suitable than carbon for an incandescent-lamp filament. After experimenting with various metals, tantalum was tried. Tantalum had been known to science for about a hundred years. Von Bolton finally obtained some of the pure metal and found it to be ductile so that it could be drawn out into a wire. As it had a low specific resistance, the wire filament had to be much longer and thinner than the carbon filament. A great number of experimental lamps were made so that it was not until 1906 that the lamp was put on the market in this country. It had an initial efficiency of 5 lumens per watt and a good maintenance of candle power throughout its life, having an average efficiency of about 4¼ l-p-w. The usual sizes of lamps were 40 and 80 watts giving about 20 and 40 candlepower respectively. It was not quite as strong as the carbon lamp, and on alternating current the wire crystallized more rapidly, so that it broke more easily, giving a shorter life than on direct current. It disappeared from use in 1913.

INVENTION OF THE TUNGSTEN LAMP

Alexander Just and Franz Hanaman in 1902 were laboratory assistants to the Professor of Chemistry in the Technical High School in Vienna. Just was spending his spare time in another laboratory in Vienna, attempting to develop a boron incandescent lamp. In August of that year he engaged Hanaman to aid him in his work. They conceived the idea of making a lamp with a filament of tungsten and for two years worked on both lamps. The boron lamp turned out to be a failure. Their means were limited; Hanaman’s total income was $44 per month and Just’s was slightly more than this. In 1903 they took out a German patent on a tungsten filament, but the process they described was a failure because it produced a filament containing both carbon and tungsten. The carbon readily evaporated and quickly blackened the bulb when they attempted to operate the filament at an efficiency higher than that possible with the ordinary carbon filament. Finally in the latter part of the next year (1904) they were able to get rid of the carbon and produced a pure tungsten filament.

Tungsten had been known to chemists for many years by its compounds, its oxides and its alloys with steel, but the properties of the pure metal were practically unknown. It is an extremely hard and brittle metal and it was impossible at that time to draw it into a wire. Just and Hanaman’s process of making a pure tungsten filament consisted of taking tungsten oxide in the form of an extremely fine powder, reducing this to pure tungsten powder by heating it while hydrogen gas passed over it. The gas combined with the oxygen of the oxide, forming water vapor which was carried off, leaving the tungsten behind.