The blackening of photographic plates by positive ions of the alkali metals

A short review is included of previous work on the blackening of photographic plates by positive rays and rays of an analogous nature.The blackening of Eastman x-ray plates, by positive ions, has been measured as a function of the energy of the ions of Li, Na, K, Rb, and Cs. The energy of the ions required to produce a photographic density of D = 0.3 with a one minute exposure at a current density of 1.32 × 10^?8 amperes per cm.² ranged from 1420 electron-volts in the case of cæsium to 860 electron-volts in the case of Li⁷. Approximately 10⁵ ions are required at these energies to render developable one silver-halide grain. For a threshold density, D = 0.04, under the same conditions the energy ranged from approximately 920 to 460 electron-volts for Cs and Li⁷ respectively.Sensitivity comparisons were made between x-ray plates and process and schumannized process plates. Potassium ions with an energy as low as 137 electron-volts were recorded on Schumann plates, and it is possible that positive ions of one-third this energy can be recorded.     

On electrets

(I) Evidence has been given that the so-called permanent electrets of Eguchi are not permanent, but that their polarization decays with the time. At room temperatures this decay is too slow to be observed directly; but it is the decay of the polarization that, at least in part, accounts for the presence of the free charges that are always observed. At higher temperatures the results that have been obtained are consistent with the view that the rate of decay of the polarization increases very rapidly with the temperature; so much so that the polarization may be almost completely destroyed at a temperature which is not high enough to cause any appreciable softening of the waxes that form the substance of the electrets.(2) As is to be expected from Lord Kelvin's hypothesis as to the cause of pyro-electricity, these electrets have been shown to possess the pyro-electric property.(3) The piezo-electric effect, which always accompanies the pyro-electric property, has been shown to be exhibited by these electrets.