Page:Collected Physical Papers.djvu/364

From Wikisource
Jump to navigation Jump to search
This page has been validated.
344
SELF-RECORDING RADIOGRAPH

to be exceedingly feeble at 6 a.m. The rise in the intensity was rapid, attaining the maximum at 12 noon.

Fig. 107. Radiograph of variation of intensity of light from the sky during 12 hours in winter. The upper record shows the variation on a bright day, the maximum intensity being attained at 12 noon. The lower record exhibits irregular variation on a cloudy day. The practically horizontal record above the base line show that the electric resistance of selenium cell is practically unaffected by variation of temperature. Successive thin dots at 15 minutes’ interval, thick dots at intervals of an hour.

This will be designated as the light-noon. The intensity of light then declined at a rate slower than the rise; after 5 p.m. the fall of intensity was extremely rapid.

It was stated that there was a possibility of change of resistance induced by diurnal variation of temperature. In order to determine the extent of this variation, a spark record was also obtained before exposure to light. The dotted record near the base (fig. 107) shows that the resistance remained practically constant, inspite of the variation of the temperature.