Reciprocity failure of photographic materials at short exposure times

Abstract

Recent work in which photographic materials were exposed at low temperatures (Berg and Mendelssohn 1938; Webb and Evans 1938; Berg 1939 a ; Evans and Hirschlaff 1939) has given results which can be explained by the assumption that an electronic and an ionic process occur in the formation of the latent image (Trivelli 1928; Gurney and Mott 1938). At low enough temperatures these two processes are separated because the ionic mobility is reduced to zero and the ionic process is postponed until the emulsion is warmed up prior to development, whereas the electronic mobility is but little affected. It should be possible to separate these two processes even at room temperature, because the electrons are much more mobile than the ions, and thus for sufficiently brief exposures the electronic process should be over before the heavier ions have moved appreciably. If this were correct, no difference would be found between exposures sufficiently short to make certain of this separation, and shorter exposures still. In other words, reciprocity failure, the occurrence of which has been accounted for by the electronic and ionic processes taking place side by side (Gurney and Mott 1938), should disappear. The shape of the reciprocity curve is well represented by a catenary over the range hitherto investigated (Webb 1935). According to the present argument the catenary-like curve would be ex­pected to bend over and become horizontal at its short exposure end. The longest exposure on the horizontal part of the curve should then bear some relation to the mobility and concentration of movable silver ions, and, in so far as these properties depend on temperature, the position of the “bend-over point” should depend on temperature also. Experiments on these lines are reported below.