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1948-49 Theatre Catalog, 7th Edition, Page 404 (391)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 404
Page 404

1948-49 Theatre Catalog, 7th Edition, Page 404

of an inch, and it might be assumed that at the instant contact is broken, the physical separation is of this magnitude, which would give a voltage gradient of five billion volts per inch.

It naturally follows that such a tremendous voltage gradient, acting in combination with the free electrons around the white-hot electrode tips, causes enough of them to liow across the gap as the carbons are pulled apart so that current continues to flow through the hot gases, and an arc is established. In turn, the high concentration of power within the narrow confines of the arc produces light, after the following fashion:

To begin with, there is incandescent carbon at its volatilization temperature of over 6500 F (3600 G). Since a temperature of only 2600-2900 F (1425-1595 C) is enough to produce a nwhite heat", it is apparent that incandescent carbon alone is responsible for a good share of the brightness of the carbon-arc crater; for all of it, as a matter of fact, in the low-intensity carbon arc, and from a fifth to a half of the total brightness in common high-intensity trims?

The increased brightness of the highintensity are is the result of the combination of a high current density, i. e., a high concentration of electrons in the are stream, and an atmosphere in the positive crater region rich in ttflame materialsll volatilized from the special coring of the positive electrode. These flame materials are in most cases compounds of the cerium group of rareearth metals, combined in a mixture with carbon in the core. As the carbon shell burns away to form a crater, as indicated by Fig. 2, the core is exposed to the extreme are temperature and is vaporized into the crater enclosure. Here, the rare-earth particles are bombarded by electrons to produce very intense light. It is perhaps helpful here to picture a maelstrom in the positive crater, with many billions of rareeearth atoms continually colliding with as many electrons. As the result of each collision, a rare-earth atom absorbs energy from an electron, and is transformed into an "excited, state. In other words, the excited atom possesses an amount of energy in excess of the normal stable value. Moreover, as defined by quantum theory, this excess energy may have any one of a number of discrete values, depending upon the number and arrangement of the electrons circulating around the atomic nucleus. The rare-earth atoms have many electrons (cerium has 58, circulating in 14 different orbits) so that the likelihood of scoring a hit on such a large, well-populated target is correspondingly increased. At the same time, there are a great many excited states possible, so that the likelihood is excellent that a hit will produce excitation. Since the rare-earth atoms are not stable in these excited states, they immediately give up their excess energy. This they do in the form of pulses of radiation, each having a particular wavelength associated with the excited State; or an excited atom may return to a normal energy level in a series of discrete steps, emitting radiant-energy


FIG. 2. Diagram showing the mixture of rare-earth atoms and electrons in the positive crater of the

high-intensity carbon arc.

pulses of as many different wavelengths on the way. It is characteristic of the rare-earth atoms that these energy pulses are of wavelengths to which the human eye is sensitive, and that they are distributed in such great numbers over the range of visual sensitivity that an essentially equal energy spectrum or a ttwhite" light is produced. In this way, the brightness of the high-intensity carbon-arc crater is increased manyfold over that of the plain carbon arc (to over ten times, in the laboratory).

Fig. 3, which shows a picture of a typical high-intensity carbon arc, can now be Viewed with a new understanding of what is going on. From the incandescent tip of the negative carbon underneath, countless numbers of electrons are being drawn out into the arc stream and accelerated like bullets toward the positive electrode by the voltage gradient along the arc stream. To make enough electrons available, i. e., 63, followed by 17 zeros, electrons per second for each ampere, the negative tip must be heated to a very high temperature, hence the bright tip and red heatback of this electrode. These electrons rush across the arc stream, meeting nothing much except air atoms until they approach the region of the positive carbon, 3. bluish light resulting from collisions with the air atoms in the arc stream. At the crater, and particularly inside it, the electron stream encounters the rare-earth atoms, with a resultant

production of brilliant white light. Under the influence of convection currents established by the hot gases, a bright stream of excited rare-earth atoms emerges from the crater and drifts upward into the tail Hame.

FIG. 3. The inclined trim high-intensity carbon arc. (18.6-mm positive carbon at 150 umperes with the negative directed upward at an angle of 53 degrees from the horizontal.)
1948-49 Theatre Catalog, 7th Edition, Page 404