Between 1911 and 1913, Hess undertook the work that won him the Nobel Prize in Physics in 1936. For many years, Scientists had been puzzled by the levels of ionizing radiation measured in the atmosphere. The assumption at the time was that the radiation would decrease as the distance from the earth, the source of the radiation, increased. The electroscopes previously used gave an approximate measurement of the radiation, but indicated that at greater altitude in the atmosphere the level of radiation might actually be higher than that on the ground. Hess approached this mystery first by greatly increasing the precision of the measuring equipment, and then by personally taking the equipment aloft in a balloon. He systematically measured the radiation at altitudes up to 5.3 km during 1911-12. The daring flights were made both at day and during the night, at significant risk to himself.