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How, a century ago, could Stern and/or Gerlach KNOW that they had created single silver atoms?

How were they moved, or accelerated?

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Atomic spectroscopy was very advanced 100 years ago (1920s) and we must appreciate their intelligence. If a metal like silver is being heated to the extent of boiling in high vacuum, all you get is silver atoms. Just like when liquid water is strongly heated, one would get gaseous water molecules- each molecule is separate.

Stern and Gerlach write in Der experimentelle Nachweis des magnetischen Moments des Silberatoms. (The experimental proof of magnetic moments of silver atoms), Zeitschrift fur Physik 8.1 (1922): 110-111.

Ein Silberatomstrahl von $1 / 20 \mathrm{~mm}$ Durchmesser geht in hohem Vakuum $(10^{-4}$ bis $10^{-5} \mathrm{~mm} \mathrm{Hg})$ hart an der Kante des schneidenförmigen Polschuhs eines Elektromagneten [Halbringelektromagnet nach du Bois2] vorbei. Der Strahl kommt aus einem kleinen $(1 / 2 \mathrm{~cm}^{3}$ Inhalt$)$, elektrisch geheizten, stählernen Öfchen durch eine im Deckel befindliche, $1 \mathrm{~mm}^{2}$ große, kreisförmige Offnung. Der Ofen ist von einem wassergekühlten Mantel umgeben. Etwa $1 \mathrm{~cm}$ vom Ofenloch entfernt passiert er die erste kreisförmige Blende $(1 / 20 \mathrm{~mm}$ Durchmesser$)$ in einem Platinblech. $3 \mathrm{~cm}$ hinter dieser passiert er eine zweite, ebensolche Blende, die sich am vorderen Ende des Schneidenpols des Elektromagneten befindet.

A silver atom beam of $1 / 20 \mathrm{~mm}$ diameter passes in high vacuum $(10^{-4}$ to $10^{-5} \mathrm{~mm} \mathrm{ Hg})$ very close by the edge of the blade-shaped pole shoe of an electromagnet (half-ring electromagnet according to du Bois). The stream comes out of a small $(1 / 2 \mathrm{~cm}^{3}$ capacity$)$, electrically heated, steel furnace through a $1 \mathrm{~mm}^{2}$ sized circular opening located in the lid. The furnace is surrounded by a water-cooled jacket.

Translated with www.DeepL.com/Translator (free version) with some editing by @Hobbamok (native German speaker).

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    $\begingroup$ "hart" here means "very close" or "extremely close": The beam of silver atoms passes extremely closely by the edge of the blade-shaped pole shoe of an electromagnet. $\endgroup$
    – njuffa
    Aug 31 at 4:33
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    $\begingroup$ @njuffa, I took the liberty of editing it myself, native German speaker as well and its a better translation (though now it may be a bit weird english lol) $\endgroup$
    – Hobbamok
    Aug 31 at 10:49
  • $\begingroup$ Yes, the word "hart" was being mistranslated or perhaps the usage is old. It is still amazing that machine translations are so accurate now. $\endgroup$
    – M. Farooq
    Aug 31 at 15:13
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    $\begingroup$ @Hobbamok It should be "passes very closely" as "close" modifies "passes" which means it is used as an adverb. $\endgroup$
    – njuffa
    Aug 31 at 16:05
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    $\begingroup$ @Joshua, "hart" was quoted from the German text. which means hard. It was not an English word. $\endgroup$
    – M. Farooq
    Aug 31 at 18:10
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For the second part of the question: they weren't accelerated in any way other than by heating, they formed what is known as a thermal beam. Atoms in the gaseous phase are always moving, with an average speed determined by their temperature. At higher pressures the atoms all collide with each other about every 100 nanometres, but under high vacuum they can travel long distances in straight paths. If you heat a sample (under vacuum) to the point that it produces a significant amount of vapour, and then let the vapour pass through two slits or apertures (called collimators) to select only the atoms going in a certain direction, you form a (low intensity) beam as shown in this sketch of the apparatus (with Cs instead of Ag), from here:

Stern-Gerlach apparatus showing two apertures

As long as the distances aren't too large and the temperature of the beam isn't extremely low, the effect of gravity on the trajectory of the beam is negligible and it can be treated as travelling in a straight line.

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