The carbide alone is a good conductor, and gives a very satisfactory light,[636] but electrodes made from this compound without additions have several disadvantages. The life is short, and the arc soon becomes flickering and unsteady. A deposit of the badly conducting dioxide gradually accumulates on the anode, and once the current has been interrupted, this deposit renders it very difficult to strike the arc again. These disadvantages are largely overcome by a series of improvements recently patented in Germany by the Allgemeine Elektrizitäts Gesellschaft of Berlin. Addition of small quantities--4·5 per cent.--of chromium carbide increases the length of life;[637] the unsteadiness and flickering are greatly diminished by incorporation of powdered coke, cryolite and fluorspar,[638] or better, of the titanofluoride of calcium or cerium,[639] whilst the addition of finely divided sulphur (or selenium or tellurium)[640] greatly reduces the disadvantage due to the throwing off of incandescent particles. The British Thomson-Houston Company patents a similar electrode,[641] in which a carbon-mixture is used instead of coke, and the electrode is manufactured with a carbon shell. For this purpose, the paste prepared from the powdered mixture may be filled into a hollow carbon rod, or the lightly baked pencil may be coated with pitch and heated to a high temperature. The use of a mixture of cerium fluoride and tungstate, with carbon and cryolite, is also said to prevent flickering.[642] [636] Weedon, _Trans. Amer. El. chem. Soc._ 1911, ~16~, 217. [637] _D. R. P._ 231231, February, 1911. [638] _Ibid._ 233125, March, 1911. [639] _Ibid._ 251837, October, 1912. [640] _Ibid._ 234466, May, 1911. [641] _E._ 6500, 1912. [642] Guay, _U. S. P._ 1039522, September, 1912. In arc lamps in which pencils containing titanium compounds are used, the anode is generally made of copper, and is placed below the cathode, the reverse being the case where carbon electrodes are employed. The copper is inactive, and contributes nothing to the light; if the anode be of suitable dimensions, it wears away very slowly, whereas the cathode, containing the titanium compound, is rapidly consumed. In lamps in which carbon electrodes are used, the light is emitted chiefly from the extremities of the electrodes, the path of the arc being comparatively non-luminous; the light has the familiar reddish-yellow colour characteristic of the earlier forms of arc lamps. Where titanium pencils are employed, however, the light is emitted almost entirely from the arc itself, the electrodes contributing very little, and is of a pure white colour, very different from that of the carbon lamp.