The extreme end of the bulb is glass is pinched around the wires at DD so as to form a hermetical seal. If the filament is to be of a form to require it, an anchor F is introduced at the same time. The next step is the making of the bulb G, which is simply blown on the end of a glass tube HJ, in the ordinary way. then heated by a blow-pipe, attached to it at that point. the tube by melting the apart. and a small glass tube L M is The bulb is now disconnected from latter around at K and pulling the two The inside part A upon which the treated filament N has been mounted by means of pasted joints at E E F, is next introduced into the bulb G, and the two are united by fusing together the circular edges B B and K K. The partially completed lamp is now ready to be exhausted of air through the tube L M. The Objects of the Vacuum produced in the bulb of an incandescent lamp are: 1. To avoid the combustion of the carbon filament. 2. To reduce wear on the filament due to "air-washing. 3. To diminish the loss of heat from the filament. 4. To decrease the flow of current in the space around the filament. At various times, it has been attempted to attain the first of these objects by using an atmosphere of some gas or vapor, such as nitrogen or bromine, which it is expected will not combine with the carbon. But even a small quantity of any gas left in the bulb may tend to consume the carbon, partly by chemical combination, and partly by a mechanical action called air-washing that wears away the filament. The presence of any gas or vapor also causes a more rapid transfer of heat by conduction and by convection from the filament to the bulb. In a vacuum, on the other hand, the filament loses heat by radiation alone, so that a smaller quantity of energy is required to maintain it at a certain temperature and candle-power. Hence the efficiency is improved, being inversely proportional to the energy consumed, other things being equal. For the same reason the bulb of a vacuum lamp is cool enough to touch with the hand, even while burning, and will not ignite anything that may come in contact with it unless the heat is allowed to accumulate by leaving it for some time partly or completely surrounded by an inflammable material, such as cloth or wood. The bulb of a lamp containing some gas becomes considerably hotter, and is therefore less convenient to handle, as well as more likely to start a fire. The "air-washing" effect is not considered to be as important as formerly, the wearing out of the filament being due chiefly to projection of particles from its surface, and chemical action upon it if any active gas is present. The flow of current through the space around the filament is called the Edison effect, having been first observed by him. It is a loss of energy, since the pale bluish light that it produces adds little or nothing to the candle-power. This flow is greatly reduced when a nearly perfect vacuum is reached. In fact, lamps are tested to see if the vacuum is sufficiently high, by connecting them to an induction coil; those showing the pale glow throughout the bulb being rejected. On the other hand, the presence of any considerable quantity of gas would also stop the wasteful current, so that for this reason alone either a very high or a comparatively low vacuum is desirable. The flow of current by the Edison effect may take place without the blue glow, but Mr. J. W. Howell has shown that the two often go together. The blackening of the bulb which gradually occurs while the lamp is burning was found by Prof. W. A. Anthony † to be considerably less in lamps containing a slight atmosphere of bromine than in ordinary high vacuum lamps. The transfer of carbon from the filament to the bulb seems to occur as a sort of projection of particles along straight lines in a manner similar to the Crooke's effect. Hence it is quite natural that the presence of even a small quantity of vapor would interfere with the deposit by reducing the "mean free rath" of the particies. The blackening of the bulb by the deposit of carbon upon its inner surface is one of the important causes in the falling off in candle-power of lamps. This matter will be considered further under the head of "Relation between Candle-Power and Age" on page 416. The methods of exhausting bulbs used singly or in combination are as follows: 1. By means of mercury pumps. *Trans. Amer. Inst. Elec. Eng., vol. xiv. p. 27, Feb., 1897. Ibid., vol. xi. p. 132, March, 1894. 2. By means of mechanical pumps. 3. By the so-called chemical process. The first of these consists in connecting the tube LM (Fig. 352) to a Sprengel or other suitable form of mercury pump capable of producing the very high vacuum required. At first the quantity of bubbles in the tube of the mercury pump show that the air is being rapidly removed, but later the bubbles become fewer and smaller, until finally none are visible. This indicates. that no more air can be drawn out under the existing circumstances, but there is still considerable gas clinging to the glass, filament and leading-in wires. The lamp is now heated by passing current through the filament or by external heat in order to drive off these gases and allow them to be removed by the pump. When the vacuum is sufficiently high, the tube LM is softened close to the bulb by a blow-pipe flame and drawn out to form the tip 7 (Fig. 353), thus hermetically sealing the lamp. The second plan, employing a mechanical pump, is now capable of producing sufficiently perfect exhaustion for high vacuum lamps, being also used to save time and expense in removing the greater part of the air at first, when the final vacuum is obtained by a mercury pump or by the chemical process. In the chemical process the lamp is nearly exhausted by a mechanical or mercury pump, and some substance previously introduced into the bulb is then caused to combine with the small remaining quantity of gas. In most cases a small quantity of phosphorus is put in the tube LM, and ignited by heat applied to the outside. It combines with the residue of gas present to form solids or non-conducting gases which are practically equivalent to a perfect vacuum. It is found that red phosphorus, which is comparatively harmless, can be used instead of the yellow form, that would be injurious to the employees who handled it. Bases and Sockets. The sealed lamp is now ready to receive the base which supports it, and at the same time makes the necessary electrical connections that supply it with current. Many forms of base have been used, the most prominent being the Edison standard type shown in Fig. 354. This consists of a brass shell formed into a screw-thread, to which one leading-in wire is soldered at P, and a brass button to which the other leading-in wire is soldered at R. To hold the parts together and insulate them from each other, the spaces between are filled in with soft plaster of paris as indicated by dots in Fig. 353. This is allowed to harden and is then dried, otherwise the moisture would short-circuit the terminals. At present porcelain pieces are generally used instead of plaster. The corresponding Edison socket, which is the same as that used with plug cut-outs (Fig. 336), is made with a screw-thread and contact point to receive the base of the lamp and make electrical connections to it. Simplicity and cheapness are the chief advantages of this form; and it is gradually displacing other types in this country. The Sawyer-Man or Westinghouse lamp base, illustrated in Fig. 355, is provided with electrical contacts similar to those of the Edison type; but the lamp is held in place by inserting it in a socket consisting of spring clips or fingers which grasp it on all sides. The Thomson-Houston base has a central hole in which a thread is cut, so that it may be screwed down upon a projecting screw in the socket. The Swan, and similar bases represented in Figs. 358-360, are of the bayonet type, having small pins on the sides which fit into slots in the socket, being inserted and then turned slightly in order to lock them. All of these may be classified under the three heads of the screw, clip, and bayonet |