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The following picture, which I downloaded from Wikipedia Wu Experiment demonstrates Wu's experiment.

enter image description here

I think that the moment when Wu got a polarized direction of the emitting electrons from $\text{Co}^{60}$, they should already have concluded a parity violation, and they didn't need to flip the magnetic field. It is because when we put $\text{Co}^{60}$ in a magnetic field, if parity is conserved, then we would expect an emission of electrons on both directions. (If the parity if conserved, one couldn't identify the direction of the magnetic field, thus we expect both directions to have the same amount of electrons. Otherwise we can specify a direction.) Therefore, Wu would conclude a parity violation the first time when they saw the emission of the electrons. But why they needed to switch the magnetic field and do the prediction like in the above picture?

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A quick run through the experiment:

You want the greatest available $60$ Cobalt nuclei polarisation. But, even accepting the low temperature requirement, because the nuclei have tiny magnetic moments relative to the electrons, high magnetic fields were required.

Because you want to confirm the effect works in both directions (this is totally my guess, they must have been prepared for anything weird(er) at that stage), you reverse the system.

enter image description here

Image and extract source : [Doublexscience]

Cartoon of the Wu experiment: if parity was preserved, then the emission of electrons would change if you reversed the orientation of the experiment. Instead, she found that the electrons always shot out the same way relative to the cobalt atoms’ spin, showing that the weak force violates the law of parity. [Credit: Matthew Francis]

This:

  1. Makes the most use of all the time and effort you have gone through for relatively little effort.

  2. Makes sure the effect definitely is present and is not either an artifact of the system, or caused by an external source

  3. Also ensures that nothing else funny is going on, that you would be annoyed about if you had not done a simple task and someone else had discovered it.

  4. Madame Wu was a great experimental physicists, and probably got that way by being a perfectionist.

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  • $\begingroup$ It makes sense. You meant that she could have done it once, but for various reasons she had done it twice. $\endgroup$ – ZHANG Juenjie Oct 25 '16 at 23:48
  • $\begingroup$ Like you say, there should have been no anisotropy, so any indication of it, (as she got in the first run), should have been enough, but if you have it all set up why not double check? Repeatability is basic in experiments, so why not tweak it a bit when you are doing that. She knew what she was looking for, beforehand. If you can, I would also check the details of any other groups experiment, did they do the same, although that could be difficult to check. Disclaimer: I self study so I could be 100 percent wrong, but nothing else jumps out at me as regards this experiment. $\endgroup$ – user108787 Oct 26 '16 at 0:40

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