Question

The following passage is adapted from an essay on the history of atmospheric physics.

By the turn of the twentieth century, physicists were puzzled by a persistent phenomenon: electroscopes, which measure electric charge, slowly discharged over time even when kept in lead-shielded containers. This spontaneous discharge indicated that the air inside was somehow being ionized, or made conductive, by an external source of radiation. The prevailing scientific consensus attributed this ionization to gamma radiation emitting from radioactive materials, such as uranium and radium, embedded in the Earth’s crust. It was assumed that this terrestrial radiation would weaken rapidly as one moved away from the ground, eventually dissipating to zero at high altitudes.

In $1910$, the German physicist Theodor Wulf, a Jesuit priest and pioneer in electroscopy, sought to measure this expected decay. He designed a portable, highly sensitive electroscope and transported it to the top of the Eiffel Tower, approximately $300\text{ meters}$ above the ground. According to Wulf’s calculations, if the radiation originated solely from the Earth, the ionization rate at that height should have dropped to a small fraction of the ground-level value. Instead, his measurements showed that the radiation level at the tower’s summit remained at nearly $64\%$ of the ground value—far higher than could be explained by terrestrial emission alone, given that gamma rays should be heavily absorbed by the intervening air. While Wulf’s results suggested the existence of an additional source of radiation, his findings were largely discounted by contemporary physicists, who blamed instrumental defects or local atmospheric factors for the discrepancy.

Determined to resolve the mystery, the Austrian physicist Victor Hess designed a series of daring experiments. Hess reasoned that the only way to obtain definitive data was to take measurements at altitudes far exceeding the height of any man-made structure. He commissioned the construction of electroscopes capable of withstanding extreme temperature and pressure fluctuations and began a series of balloon ascents starting in $1911$.

Hess’s experimental breakthrough occurred on August 7, $1912$, during his seventh balloon flight. Accompanied by a pilot and a meteorological observer, Hess boarded the balloon *Böhmen* in Aussig, Austria (now in the Czech Republic). He carried three independent ionization chambers, which were hermetically sealed to prevent changes in air pressure from affecting the internal gas. The balloon drifted northwards, eventually reaching a peak altitude of approximately $5{}300\text{ meters}$.

As the balloon ascended, Hess meticulously recorded the ionization rates from his instruments. During the initial phase of the flight, up to an altitude of about $1{}000\text{ meters}$, the radiation levels decreased slightly, aligning with the theory of terrestrial decay. However, as the balloon climbed past $1{}000\text{ meters}$, Hess observed a striking reversal: the ionization rate began to climb steadily. By the time the *Böhmen* reached $5{}300\text{ meters}$, the rate of ionization was about four times greater than it had been at sea level. Because the air at such altitudes was too thin to contain significant quantities of radioactive crustal dust, Hess concluded that a highly penetrating radiation must be entering the atmosphere from above.

Crucially, Hess needed to determine whether this radiation was solar in origin. To test this, he had previously conducted a flight during a near-total solar eclipse on April 17, $1912$. Despite the Sun being almost completely obscured by the Moon, Hess recorded no drop in ionization levels. This critical observation led him to conclude that the source of the radiation was not the Sun, but rather deep space. Hess published his findings later that year, proposing the existence of an undocumented, extra-terrestrial source of radiation. Although his theory was initially met with skepticism, it was eventually confirmed by subsequent experiments, and in $1936$, Hess was awarded the Nobel Prize in Physics for his discovery of what Robert Millikan would later term "cosmic rays."

According to the passage, what did Theodor Wulf observe regarding the radiation level at the top of the Eiffel Tower during his $1910$ experiments?