A Magnetic Shield for Hunting the Electron's Asymmetry express
The magnetic field in this setup must be as smooth as glass: the slightest ripple would blur the signal. The engineers built two coils that cancel each other's field externally and packed them in three layers of ferromagnetic plates. Inside, they created a volume of one meter by four centimeters where the field varies by less than one billionth of a tesla, like sea level not fluctuating more than the thickness of a hair.
The mu-metal plates, an alloy that draws magnetic field lines into itself like a sponge soaks up water, are made to be disassembled. The largest weighs 19 kilograms and can be carried easily by two people. The key was not to bend the plates during assembly: a bend leaves stress in the metal, and it starts to "remember" the old field. So the mounts are designed to prevent any warping.
When the field inside the coils is reversed to cancel out errors, the external field barely leaks out. Thanks to this, the shields do not become magnetized and do not need demagnetization after each switch. The residual field stays below one billionth of a tesla for up to 17 hours, even if the field is switched every 30 seconds.
The result: compared to the previous setup, magnetic-field-related errors have been reduced by a factor of about 40. At the same time, the working zone is five times longer, and the number of shield layers is three instead of five. Now we can search for the electron's "warp" with much greater confidence.
Mu-metal is so sensitive to deformation that even a light tap with a hammer can ruin its magnetic properties, so the plates are transported in special containers and never dropped.