On this page
In 1912 most physicists thought the faint electricity in the air came from radioactive rocks in the ground. If that were true, the higher you went, the weaker it should get. Victor Hess took the question up in a balloon basket, and found the opposite. This post covers who he was, how his instruments worked, his own numbers worked through, and what a student can learn from the way he ran the experiment.
Who was Victor Hess?
His Nobel Prize facts page gives his birth as 24 June 1883 near Peggau, Austria, and his death on 17 December 1964 in Mount Vernon, New York. His Nobel biography says he studied at Graz and from 1910 to 1920 was an assistant at the Institute of Radium Research of the Vienna Academy of Sciences. In 1931 he moved to Innsbruck and set up a cosmic-ray station on the Hafelekar mountain at 2,300 m. He shared the 1936 Nobel Prize in Physics with Carl Anderson, who found the positron; Hess's half was "for his discovery of cosmic radiation".
What problem was he trying to solve?
A charged electroscope slowly loses its charge, even when sealed, because something keeps making ions in the air inside it. The 1936 presentation speech describes the puzzle: the effect could not be removed, however thick the lead shielding. Measurements on the Eiffel Tower and in earlier balloon flights showed some fall with height, but not as fast as expected if all the radiation came from the ground. The speech notes that Hess first measured how strongly air absorbs gamma rays and studied the errors in his instruments before he flew.
What did the 1912 flights show?
Hess reported his seven 1912 ascents in Physikalische Zeitschrift. An English translation by De Angelis and Arcaro Schultz lets you read the paper itself. He carried two sealed Wulf electrometers with 3 mm walls and a third, thin-walled instrument, and took readings mostly at night so he could hold the balloon at a steady height for hours.
His table of means for instrument 1, in ions per cubic centimetre per second, shows the pattern:
At ground level: 16.3
200 to 500 m: 15.5
1,000 to 2,000 m: 15.9
3,000 to 4,000 m: 19.8
4,000 to 5,200 m: 34.4
The small dip near the ground was the absorption of gamma rays from the Earth's crust, worth about 3 ions per cubic centimetre per second. Above that, the value climbed. On the seventh flight, from Aussig on the Elbe, the balloon reached 5,350 m; the translators' notes date it to 7 August 1912. He concluded that "radiation of very high penetrating power enters from above into our atmosphere". On an April 1912 flight during a partial solar eclipse he saw no drop, and he found no clear day-night difference, so the Sun was unlikely to be the source.
Worked example: from a falling fibre to ions per second
Hess's paper gives enough data to check his calibration. Instrument 1 had a volume of 2,039 cm³ and a capacitance of 1.597 cm (an old CGS unit; 1 cm is about 1.113 pF). He wrote that a charge loss of 1 volt per hour meant 1.56 ions per cm³ per second.
Capacitance in SI. C = 1.597 × 1.113 × 10⁻¹² F = 1.777 × 10⁻¹² F.
Charge lost per hour. For ΔV = 1 V, Q = CΔV = 1.777 × 10⁻¹² C.
Number of ions. Each ion pair carries one elementary charge to the electrode. With his value e = 4.65 × 10⁻¹⁰ esu, which is 1.551 × 10⁻¹⁹ C, N = Q/e = 1.146 × 10⁷ per hour.
Per cm³ per second. 1.146 × 10⁷ / (2,039 × 3,600) = 1.56. That is his number.
With today's e = 1.602 × 10⁻¹⁹ C you get 1.51, slightly lower. So a ground reading of 16.3 ions per cm³ per second meant the electrometer lost about 16.3 / 1.56 ≈ 10.4 volts every hour.
The rise is also easy to state as a ratio: 34.4 / 16.3 ≈ 2.1, which is why the presentation speech says the radiation at 5,000 m was "twice as intensive" as at the surface. If you want to revise Q = CV, our post on capacitors in DC steady state covers it.
What mistakes do students make with this story?
"Cosmic rays are just very strong light." Hess first thought of very powerful gamma rays, the presentation speech says. But his own Nobel lecture already describes "showers" of particles in cosmic rays, and Anderson's positron found in them.
"Hess saw cosmic rays directly." He measured ionisation, the effect of the radiation, not the particles. Cloud chambers and photographic plates came later; see our posts on C. T. R. Wilson's cloud chamber and on Bibha Chowdhuri's photographic plates.
"The Sun makes them." Hess's eclipse and night readings argued against the Sun as the main source.
Syllabus. Cosmic rays are not a JEE (Main) topic, but the capacitance (Q = CV) behind his calibration is, and so are exponential laws such as radioactive decay.
If ionisation and absorption are hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can a student take from how he worked?
Test your instrument before you trust it. He checked insulation leaks, refocused the eyepiece and studied absorption in air before any high flight.
Hold one variable steady. Long night flights at constant height let him separate real changes from reading errors, which he estimated at a few per cent.
Use more than one instrument. He counted a fluctuation as real only when all instruments showed it at once.
Write your conclusion carefully. He wrote that his results "seem most likely to be explained by the assumption" of radiation from above, and listed the other explanations he had ruled out.
Next step: redo the calibration above for his instrument 2 (2,970 cm³, 1.097 cm) and check that you get about 0.736, his 0.7355.
