Excerpt from On the Penetrating Radiation at the Surface of the Earth

Strong from his measurements drew the conclusion that the atmosphere contributed by far the major portion of the penetrating radiation. He found the intensity of the radiation greater in summer than in winter, and he too noted a double diurnal period in the ionisa tion in his electroscope. Precipitation of rain or snow always produced a drop in the intensity of the radiation. But all changes in intensity were eliminated when he surrounded his electroscope with thick lead and iron screens. Strong's measurements are rather remarkable for the extremely wide variations which they indicated and it is difficult to account for them. On Jan. 30, 1907, for example, he observed ionisations at various times of the day represented by 12, 82, 100, 77 on an arbitrary scale, and again on Sept. 3, 1909, ionisations were observed represented by 25, 18, 50 and 155, 150, 42, 10, 15 on the same scale. Variations so extensive as these do not appear to_ have been observed by any other investigators and they seem to point to some very special and exceptional local conditions.

Some experiments were also made recently on this phenomenon by D. Pacini at Sestola, in Italy.' This investigator used an alumin ium-leaf electroscope and studied the ionisation in air enclosed in large zinc receivers. He too found daily maxima and minima values in the ionisation. His minimum observations ranged from 8, 9, 10 to 12 ions per cc. Per second while his maximum observations extended in some cases to as high as 30 ions per cc. Per second. His ionisation values present a double daily period with two maxima at two to three o'clock and nine to ten o'clock, and two minima from seven to eight o'clock and from twelve to one o'clock.

Wulf, ' too, who devised a new type of electrometer the movable system of which consists of a double conducting quartz thread, has applied this instrument also to the investigation of the penetrating radiation. His experiments were carried out at Valkenburg in Holland, both on the surface of the earth and in the chalk quarries in the neighbourhood of that municipality. He too found a parallelism between the intensity of the penetrating radiation and that of the atmospheric potential, maximum values being obtained for both phenomena in the summer about eight, or nine in the morning and evening, and minima about noon and midnight. In winter the midday minimum was only slightly marked.

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