Go further · Chapter 2 · 2c · People story
How did a cigar help discover spin?
The short answer (tap to open)
When Otto Stern and Walther Gerlach first checked their glass plate in 1922, the silver layer was too thin to see. Stern later told how the deposit only appeared when he breathed on it. His cheap cigars, he said, filled his breath with sulphur, which turned the invisible silver into black silver sulphide. The two spots appeared.
How did a cigar make spin visible?
Card 1 · Frankfurt, 1921
Otto Stern, a theorist turned experimenter, and Walther Gerlach, a skilled experimenter, set out to test a prediction about atoms in magnetic fields. Their beam of silver atoms is so faint that a night's run leaves a deposit only a few atoms thick.
Card 2 · Nothing there
Gerlach takes the glass plate out and sees nothing. He hands it to Stern.
Card 3 · The cigar
Stern, on a small salary in the inflation years, smokes cheap cigars full of sulphur. As he later tells it, while he peers at the plate his breath reacts with the silver and turns it into silver sulphide, which is jet black. Slowly, a pattern appears.
Card 4 · Two lips, not a blur
In February 1922 a sharper run shows it clearly: two separate marks, like a pair of lips. Gerlach sends Niels Bohr a postcard with a photo of the split and congratulations on what they took as confirmation of Bohr's theory.
Card 5 · The wrong reason, the right result
The split really showed electron spin, unknown until 1925. The experiment was right before the theory was.
Card 6 · After
Stern, who was Jewish, left Germany in 1933 and won the 1943 Nobel Prize in Physics in the US. Gerlach stayed and worked on Germany's wartime nuclear programme. Decades later, in 2003, two physicists recreated the moment. Breath alone did nothing, but cigar smoke blackened the silver.
Check
One quick one
Collect
Your aha card
A cheap cigar made spin visible.
Sometimes the experiment is right before anyone understands it.