Photo-conductance phenomena in the silver halides and the latent photographic image.- Part II

Although the experiments which have been described do not permit definite conclusions to be drawn regarding the nature of the photo-current, it is very probable that it is to a large extent, if not entirely, electronic, as predicted in a preliminary note to ‘Nature’ (May 4, 1929). As, however, the method of attacking the problem has been by a comparison with the dark current, the mechanism of which is known from the work of previous investigators, it is advisable first to refer briefly to the question of electrical conduction through unilluminated fused salts. The work of many previous investigators has definitely shown that in the dark at room temperature the conduction process is purely electrolytic, there being no evidence of any electronic current in salts of the silver chloride-silver bromide type. Tubandt and his co-workers have shown that in silver bromide all the dark current is carried by the silver ion only. The experiments of Phipps, Lansing and Cooke, of Phipps and Leslie, and of Phipps and Part­ridge, show that the transport of electricity by means of the kation only is a common characteristic of many solid salts. These authors found that from room temperatures Up to the melting points the kation is the only carrier of electricity in the case of the chlorides and bromides of silver, lead and thallium, but that in the case of sodium and potassium chlorides, the anion also conducts at the higher temperatures. In all these cases the relation between the con­ductance (σ) and the absolute temperature (T) was found to be log σ = A - B/t, where A and B are constants, which is the same equation as we have found to hold in our experiments over the entirely different range, from room tempera­tures to -180°C.