Does anyone now what kind of experiment they used to determine that the electron is a dipole?
It's the Stern-Gerlach experiment performed in Frankfurt am Main in 1923 by Stern and Gerlach with a fundamental result showing the quantum nature of the electron's magnetic dipole moment (related to its spin). So one should move this thread to the quantum forum.
In case you mean electric dipole:
That it's not a dipole: Hinds and Ytterbium
The idea that the electron has spin and consequently a magnetic moment was initially proposed to explain details of atomic spectra. According to Wikipedia,
"The idea of a spin angular momentum was first proposed in a 1925 publication by George Uhlenbeck and Samuel Goudsmit to explain hyperfine splitting in atomic spectra.[n 3] In 1928, Paul Dirac provided a rigorous theoretical foundation for the concept in the Dirac equation for the wavefunction of the electron.[n 4]"
Now, there are many techniques that are used, some involving spectra and some involving isolated electrons, like this link.
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