working at benches, and provided with steel molds. These are split in halves, being grooved according to the length and diameter of the carbon cylinders to be made. The molder weighs the flour in a scale, distributes it evenly over the surface of the mold, and places the steel cap upon it. The mold is then slowly heated in an oven, which causes the particles of combined pitch and carbon to become pasty. When the proper degree of heat is reached, the mold is taken from the oven, and placed under a hydraulic press, the pressure employed varying between 100 and 400 tons. From the press the molds are taken back to the benches, the cover and sides removed, and the "card" of carbons carefully lifted out. When they become cool they are separated from each other; and the little "fins" that have held them to their neighbors are scraped off each side, so that each carbon is left a fairly perfect cylinder. For lamps fed by a constant direct current, carbons are usually made by the molded process, to which they seem best adapted.

Forced Carbons. -Arc lamps for constant potential or alternating currents require a carbon whose particles are arranged differently from those in the molded process, and also in many instances a core of less dense material to insure steadiness of light. The flour for carbons to be made by this process is treated somewhat differently from that of the molded variety. It is usually shaped into cylindrical "plugs" about 6 inches in length, and from 2 to 6 inches in diameter. These are placed in front of the plunger of a hydraulic press whose action is horizontal, and are forced through its jaws, taking any desired cross-section from the outline of the die at the mouth. As fast as the carbons issue from the die, they are received upon a table, and cut to desired length. In order to make them "cored," a hole about

of an inch in diameter is left in the center of the carbon as it passes through the die, by the action of a "tongue," projecting into the orifice of the die from the inside. There are various combinations for the mixture that is used to fill the core, and the secret of its composition is usually guarded by manufacturers. This point in either process is called the "green carbon" stage. They appear shiny black in color, are quite heavy, break easily, and when held in the fingers, and tapped together, give only a dull sound. Both molded and forced carbons are next taken to

the furnace-room where the volatile matter contained in them is driven off. This is a process requiring great care. If they are baked too rapidly, they warp, and are hard to adjust in the lamps. If they are not baked sufficiently, they are too low in conductivity, and give a very poor light. In some cases the baking is performed in fire-clay pots, this being the process employed by many foreign manufacturers. In this country it is customary to lay the carbons in a large rectangular furnace, layer upon layer, separated by beds of sand, the entire mass protected by a covering of sand several inches in thickness, and subjected to heat until every carbon has reached a high temperature. The total time occupied is very considerable, being often one or two weeks from the time the charging begins until the process is completed. From the furnace the carbons are carried to the sorting-tables, where they are tested by rolling them on steel plates of true surface in order to separate the straight from the crooked ones. Some of the latter are sold as seconds, others are cut into short lengths, and the worst ones are rejected. Even in the best imported forced carbons there are often found from 2 to 5 per cent of badly warped carbons.

Molded carbons differ from forced carbons in many ways. They have a loose granular structure that runs lengthwise through the carbon, at right angles to the line of pressure. They also have the remnant of the web that holds a card of carbons together; and even if this is ground off, the surface is not perfectly cylindrical. Impurities are more likely to be found in molded than in forced carbons, and they are not as uniform as the forced article. They are used in series constant current lighting chiefly, where cheapness is the greatest consideration.

Copper plating these carbons is often resorted to, with the objects of increasing their conductivity, especially at the point of contact with the clamp, and prolonging their life. The copper sheathing protects the carbon near the arc from oxidizing so rapidly, and a 12′′ × g" coppered carbon in a 10-ampere lamp will burn about 14 hours, whereas the plain carbon of the same make will not last more than 12 hours.

The forced carbon is usually a higher grade of carbon than the molded, especially those imported from Germany and Austria. The texture is finer, and the material softer, than in the molded

form, while the grain runs transversely or at right angles to the line of pressure. Owing to the method of manufacture such carbons are more easily made to a given diameter, and are more uniform in diameter, structure, and straightness than the molded carbon. They have a comparatively high conductivity and are not copper plated. They are used for cored carbons particularly. The high grade and pure forced carbon more nearly resembles lampblack, and will make a mark on paper like a pencil, whereas the hard forced carbons will not. Where carbons are held by a small clamp far from the active end, and must fit closely but freely into an opening little larger than the carbon itself, too much stress cannot be laid on the necessity of securing straightness and uniformity. All carbons contain impurities, chiefly silica, iron, and smaller quantities of other substances. In the highest grade of imported carbons the silica is the chief impurity, with little else, but chemically pure carbons have not been produced by any manufacturer.

Carbons may be cut to any desired length by nicking them all around and breaking them as one would a glass tube. In inclosed arcs and in alternating arcs both carbons are the same size, whereas in focussing high-tension lamps and in open arc lowtension lamps, the upper carbon is usually about " larger in diameter than the lower. The only carbons that are copper-plated are those used in high-tension, series, constant current lamps.

Globes. The glassware used to inclose the arc has received little scientific attention heretofore, and it is not at all unusual to attempt a five or even one per cent saving in generating the current, and allow a 30 per cent loss in its utilization to go neglected. Globes are made of three materials, clear glass, opal (or opaline or opalescent) glass, and a combination of the two called alabaster. Arc lamp globes are either blown or molded. If the former they will vary in regularity, some being thicker, more or less curved, etc., than others. Often the mark of the tool used by the glassblower to shape the globe as he turns it will produce streaks. Molded globes are quite regular, but frequently show the joints running vertically down the side of the mold. Clear glass globes when clean, thin, and of good quality transmit 90 to 95 per cent of the light. When dusty, thick, or of poor glass the loss is easily doubled. The most common defects in these globes are bubbles,

ribs, and other inequalities which cast shadows, and render the illumination very uneven.

Opal globes are made of a glass into which some substance, frequently iron, has been introduced, making it translucent, but partly destroying its transparency. This is done to diffuse the light, and to cut off certain undesirable colors. The globes will vary from one through which outlines can be readily distinguished, to those whose appearance resembles a china plate. The denser globes effect a greater diffusion, but frequently cut off the greater part of the light. A light opal globe may cut off 20 to 35 per cent and a heavier one from 35 to 50 per cent.

Alabaster globes are made of two layers of glass, one clear and the other translucent. They are usually very dense, and cut off 40 to 60 per cent of the light. In order to combine the high transmissive power of a clear globe with the diffusion of an opal one, it is customary to grind clear globes, dividing the surface into equal portions. The dividing line may be either vertical or horizontal. The former is usually employed where the light is to be thrown in a direction away from the spectator, as, for instance, into a show window from a lamp hung in front. The horizontally half-ground globes are often used for the illumination of large interiors where an intense light is to be thrown on the ceiling and upper walls for diffusion and a less glaring light to the floor below. The effect of the grinding, which is usually done by a sand-blast applied to the outer surface of the globe, is to diffuse the direct rays of the arc, and form a brilliant scintillating surface. When the roughening is caused by acid the effect is less marked. When applied to both outside and inside of a globe the diffusion approaches that of light opal. A very successful effect for interiors is produced by grinding the lower two-thirds of the surface of the globes, thus cutting off the direct arc rays, even to those standing some distance away. The general shape of arc lamp globes is such as to protect the arc from side winds, with less attention to inclosure from rain or With open arcs the shape should be such that the globe is easily cleaned without removal from the lamp, not apt to crack from changes in temperature, and of such curvature that dust, insects, etc., fall into the cup usually placed beneath.

snow.

With inclosed arcs a shape which will not show a long, dark portion in the shadow of the negative carbon should be chosen.

The distribution of the light as well as its intensity is greatly effected by the globe used, largely owing to internal reflection. The curves in Figs. 271 and 272 show approximately how the vertical distribution varies with different globes.

Recently a new form of globe, the holophane (wholly luminous), made of clear glass after the designs of Blondel and Psarovdaki, has come into use. This globe diffuses the light perfectly, so that every part of the globe sparkles equally brightly, redistributes the light so as to throw downward many of those rays that would go upward or otherwise be lost, and is capable of varying the distribution to suit the purpose to which it is to be applied.

The distribution of the light is effected by prisms, whose section is somewhat like the teeth of a circular saw, molded in horizontal rings on the outside of the globe. The vertical section of a holophane, Fig. 273, shows that each tooth differs somewhat from its neighbor. The prisms on the uppermost portion of the globe are so designed that the rays striking them will be totally