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In 1900 many physicists thought radio could never cross an ocean. Radio waves, like light, travel in straight lines, and the Earth curves away beneath them. Guglielmo Marconi tested the idea instead of accepting it. This post covers his life, the geometry of the curvature problem worked out step by step, what happened in December 1901, and what students can learn from a result that came before its explanation.
Who was Guglielmo Marconi?
Marconi was born in Bologna on 25 April 1874. His Nobel biography says he was educated privately and as a boy studied the work of Maxwell, Hertz, Righi and Lodge. In 1895, on his father's estate at Pontecchio, he sent wireless signals over about one and a half miles. In 1896 he took his apparatus to England and was granted the world's first patent for a system of wireless telegraphy. By 1899 he had signalled across the English Channel, and in 1900 he took out patent No. 7777 for "tuned or syntonic telegraphy", which let a receiver respond to one transmitter and not others.
His work built on Heinrich Hertz's demonstration that electromagnetic waves exist; see our post on Hertz and radio waves.
Why did people think Earth's curvature would stop radio?
In his 1909 Nobel lecture, Marconi recalls that "a somewhat general opinion prevailed that the curvature of the Earth would be an insurmountable obstacle to long distance transmission", just as it is for signalling with flashes of light.
His own tests suggested otherwise. In January 1901 he signalled between St. Catherine's Point on the Isle of Wight and the Lizard in Cornwall, 186 miles apart. He notes that neither station was more than 100 metres above sea level, "whereas to clear the curvature of the Earth a height of more than 1,600 meters at each end would have been necessary." You can check that number yourself.
Worked example: how tall would the masts need to be?
Picture a mast of height h on a sphere of radius R. The line of sight from its top just grazes the surface at the horizon, at a distance d. The radius to that point meets the line of sight at a right angle, so by Pythagoras:
(R + h)² = R² + d², which gives d² = 2Rh + h².
Because h is tiny compared with R, drop h² to get d² ≈ 2Rh, or h ≈ d²/(2R).
Now apply it to Marconi's test, taking R = 6.37 × 10⁶ m:
Distance: 186 miles × 1.609 km per mile ≈ 299 km.
Two masts of equal height share the distance, so each must see 149.7 km to a common horizon point midway.
h ≈ (1.497 × 10⁵ m)² / (2 × 6.37 × 10⁶ m) ≈ 2.24 × 10¹⁰ / 1.274 × 10⁷ ≈ 1,760 m.
That agrees with Marconi's "more than 1,600 meters". Turn it around: with 100 m masts, each sees √(2 × 6.37 × 10⁶ × 100) ≈ 35.7 km, so the straight-line limit is about 71 km. Yet signals travelled about 299 km.
Common mistakes:
Using the full distance for each mast. That gives a height four times too large, about 7 km.
Mixing kilometres and metres. Put everything in metres before squaring.
Keeping h² without need. It changes the answer by well under 1 per cent here, and it complicates the algebra.
What happened on 12 December 1901?
The transmitter at Poldhu in Cornwall used generating plant of about 25 kilowatts. Its antenna was a fan of wires hung between masts only 48 metres high. A planned receiving station at Cape Cod was wrecked by a storm, so Marconi took two assistants to Newfoundland at the end of November 1901 and set up a temporary station at St. John's. On 12 December, he writes, signals from England were "clearly and distinctly received."
In February 1902, on the liner Philadelphia, readable messages arrived up to 1,551 miles from Poldhu and test letters as far as 2,099 miles. On that voyage Marconi also noticed that range at night was "usually more than double" the range by day. He could not explain it.
The 1909 Nobel Prize in Physics was shared by Marconi and Ferdinand Braun "in recognition of their contributions to the development of wireless telegraphy". Marconi died in Rome on 20 July 1937.
So why did the waves follow the curve?
Marconi suggested the earth connections at both stations mattered. The fuller answer came later. The Nobel summary for Edward Appleton explains that physicists assumed radio waves were reflected by a layer in the atmosphere where the Sun's ultraviolet light had freed electrons from atoms. By studying interference between waves that took different paths, Appleton established the existence of this layer, the ionosphere, in 1924, and won the 1947 Nobel Prize. Because the layer is produced by sunlight, it is less surprising that Marconi found day and night ranges so different. For where radio sits among other waves, see our guide to the electromagnetic spectrum.
What can a student take from Marconi's way of working?
Marconi treated an expert prediction as something to test, and he scaled up in steps: 1.5 miles, the Channel, 186 miles, then the Atlantic. He also recorded the daylight effect even though he had no theory for it.
Three habits to copy:
Estimate first. Before a problem, guess the order of magnitude, then check it, as you just did with the 1,600 m figure.
Scale up in stages. Master the simple version of a problem type before the hardest one.
Log the unexplained. Keep a page for results that surprised you; they are often the next thing to learn.
If wave reflection and interference are hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
Work out tonight how far a 30 m tall TV tower can see to the horizon, then check it with d ≈ √(2Rh).
