Negative Type.—Fig. 3 exhibits the effect on the negative type as exemplified by Sn and Pb coated with Br. It appears that in these, warmth is favourable to quick recovery. For in India with Pb coated with Br the self-recovery was obtained with the greatest ease. Here a more careful adjustment was necessary. At first the flash of radiation produces a sudden increase of resistance, from which there is no immediate
recovery, but an inspection of the galvanometer spot at once makes it evident that some internal struggle has been going on; the spot trembles, and then after awhile the internal friction is overcome, and the substance suddenly recovers. But soon after this preliminary stage the recovery becomes perfectly automatic and instantaneous. Each flash of radiation produces a responsive galvanometer twitch, immediately followed by recovery.
Some of the stages are well seen in the curves given. In (a) the recovery takes place in 50 seconds; in (d) in only 6 seconds. (e) and (f) show recovery in lead coated with bromine. The recovery gradually became very quick, from 4 seconds to 3 seconds; after that it was too quick for record.
In all the above cases it will be noted that the curve of recovery is convex to the abscissa; that is to say, it is at first very rapid, but in the later stage it becomes slower.
