176 RADIATION BIOLOGY 



mercury pressure increases, ionized mercury takes over the conduction 

 of the current. Modern lamps start automatically without mechanical 

 tilting. The medium-high-pressure mercury arcs do not restart immedi- 

 ately after being extinguished; the lamp must cool sufficiently so that 

 the vapor pressure is low enough for restarting. This requires up to 

 10 min, depending on the ambient temperature and the type of lamp. 

 Restarting time for the capillary arc is only a few seconds owing to the 

 rapid dissipation of heat by the air blast or cooling water. 



Application. Because of the very high efficiency of production of 

 ultraviolet energy at 253.7 m^i, the low-pressure mercury arc is used as a 

 source of germicidal energy and for the excitation of phosphors in the 

 fluorescent lamp. The medium-pressure arcs are especially useful as 

 intense sources of monochromatic energy when used with filters. At 

 medium pressure the background of continuous radiation is not excessive, 

 and nearly all the energy is in the lines. They have been used to supple- 

 ment the radiation of the incandescent lamp, which is deficient in the 

 blue. They have not proved very satisfactory for the growing of plants, 

 principally because all mercury arcs are relatively deficient in red energy. 

 Comparative tests have shown that the mercury arc is considerably less 

 efficient than the incandescent or fluorescent lamp for plant growth 

 (Withrow and Withrow, 1947). 



The high-pressure arcs are especially useful as optical sources of high 

 intensity. The capillary arc is very efficient with a monochromator 

 because of its linear dimensions and very high intensity. The thermal- 

 radiation continua of the capillary- and the short-arc lamps are suf- 

 ficiently great so that monochromatic flux of high intensity can be iso- 

 lated from any region of the visible and ultraviolet. 



AMALGAM ARC 



Although the mercury arc is a source of high brightness and luminous 

 efficiency, it is limited for many applications because of its deficiency in 

 the red. Operation at high pressures partially supplements the red 

 deficiency by a continuum, but the red flux is still weak in proportion 

 to the shorter wave lengths. Efforts to supplement the red by the use 

 of mercury amalgams of metals with strong red emission lines have proved 

 partially successful. Cadmium with strong visible lines at 468, 480, 509, 

 and 644 m^u and zinc with lines at 472 and 636 m^u amalgamate readily 

 with mercury, and their emission spectra appear along with the spectrum 

 of mercury. 



A capillary arc at 100 atm with an amalgam of 13 per cent cadmium 

 and 17 per cent zinc produces a source that closely approximates sunlight 

 in color (Elenbaas and Riemens, 1950; Bourne, 1948). The principal 

 limitation of such sources is relatively low power efficiency. This is of 

 little consequence in experimental work, and such sources deserve more 

 attention for photochemical investigations than they have received. 



