Some recent developments of high-current arcs, with special reference to searchlights

With ordinary carbons (i.e. carbons free from any special salts or mixtures) an improved efficiency in terms of the flux of light per watt expended is obtained by running at higher current density. (2) It is not possible to run an ordinary carbon arc with any degree of smoothness, steadiness or silence at a greater current density than 0.15 ampere per sq. mm. of the cross-section of the positive carbon, but if the arc stream be acted upon by a system of external magnetic forces tending always to keep it central, a current density of 0.31 ampere per sq. mm. can be attained with smooth and silent operation. (3) A device for producing the necessary external forces is described. (4) The use of this ?control? renders the soft ?core? generally to be found in positive carbons unnecessary and even disadvantageous. (5) Ordinary solid carbons of greater diameter than 20 mm. must be copper coated in order to carry current the rating of at the 0.31 ampere per sq. mm. without becoming red-hot along their entire length. (6) For arcs at from 80 to 300 amperes a cored copper-coated negative of such diameter that the current density is about 1 ampere per sq. mm. of its cross-section has been found most suitable. (7) Whilst the impregnation of carbons with salts or other mixtures does make for smoother running in some cases and lowers the potential difference required for a given current and arc length in all cases so far tried, yet the total flux of light received in searchlight beams from such arcs at working ranges is much less than that from a plain carbon arc at the same current and current density, both absolutely and per watt expended. (8) In view of the fact that the National Physical Laboratory tests with impregnated carbons showed that they were only very slightly inferior in intrinsic brilliance to ordinary carbons, the absolute importance of outdoor photometric tests over working ranges is emphasized. The methods adopted are described. (9) The potential difference per mm. of arc length is lower with these high-current arcs than with the comparatively low-current arcs. (10) The potential difference between the carbon holders for constant-current ?controlled? arcs increases with the arc length up to a certain length, when it suddenly falls and then steadily increases again. Investigations of this have been done principally with 200-ampere arcs at lengths of from 20 mm. to 84 mm. The drop in pressure occurs at a shorter length with impregnated carbons than with plain carbons.