WHEN two carbons are attached to an electrical supply of 100 volts direct current and are made to approach each other, nothing happens until the distance between them is less than that of a very thin sheet of paper. Ordinary air is a very excellent non-conductor. If the carbons be touched together, the current begins to flow and an enormous amount of heat and a temperature of approximately 3,500° C. is generated at the point of contact, for it is at this point that most of the resistance of the circuit is located. If the carbons are now separated slightly, e.g., half an inch, and if the air between the carbons were in the same condition as before, the flow of current would cease immediately; but the air between the carbons is not in the same condition. The entire region is filled with ionized gases and has now become a good conductor. The reason for the changed condition lies in the property of white-hot bodies to give off positive and negative ions and electrons in huge quantities. These ions carry the electric current. The negatively charged electrons given off at the cathode (negative electrode) bombard the anode, producing an intensely hot spot called the “crater.” Most of the light from the pure solid carbon arc comes from this positive crater, i.e., it is pure temperature radiation produced as a result of the high temperature of the crater. The ionized vapors produce very little light, that which they do emit being largely confined to the so-called “cyanogen” band located in the long ultra-violet and violet portions of the spectrum. It is not surprising that the pure carbon arc emits practically no radiations corresponding to the short ultra-violet of sunlight, for the glowing carbons do not attain a sufficiently high temperature to emit the short wave lengths of radiation which the sun, at a temperature of 6,000° C., can produce. Impregnation of the carbons with salts or metals produces the flame arc which is characterized by greatly increased light production in the ionized vapors and lowered temperatures, and therefore lowered light production, at the positive crater. Such an arc is vastly more efficient as a producer of light, for more of the energy is converted into light, and less into heat, than in the pure carbon arc. The kind of light emitted and the efficiency are dependent on the chemical nature of the impregnating materials. It can be understood that as the electrode temperatures decrease, and more and more of the electrical energy is consumed in the ionized vapors, the efficiency and light production increase rapidly. The electric arc reaches its peak of attainment in the vacuum quartz mercury arc. Here the electrodes are two pools of mercury maintained at a temperature between only 300 to 400° C., i.e., comparatively cool. These give off some low temperature infrared and heat, but no visible or ultra-violet light.