In the temperature range from 5° to 25°C, (1) the current density vs. potential curves of anodic reaction of 0.02 M ascorbic acid developer on a silver electrode coated with gelatin and of cathodic reaction of silver bromide electrode coated with gelatin in the developer were measured (Fig. 3), (2) development current density vs. potential curves were graphically derived from above two curves (Fig. 5) and expressed approximately by the Butler-Volmer equations (eq.(7), Table 2), (3) the rates of development were estimated from the developed silver vs. development time curves (Fig. 6) calculated on the basis of above equation (eq.(7), (8), (9), (10)), (4) the activation energies were obtained by the Arrhenius plots of the rates and compared with those experimentally measured. The calculated activation energies decreased with an increase in the exposure level and were in good agreement with measured energies (Table 4). Thus the activation energy of development could be explained quantitatively.