How Everything Works
How Everything Works How Everything Works

Question 942

Is there an effective shield for the EMF generated from mercury vapor ballasts? — CS, Washington, DC
An electric field can always been shielded by encasing its source in a grounded conducting shell. Electrically charged particles in the shell will naturally rearrange themselves in such a way as to cancel the electric fields outside the shell. But magnetic fields are harder to shield, particularly if they don't change very rapidly with time. The difficulty with shielding magnetic fields comes from the apparent absence of isolated magnetic poles in our universe—there is no equivalent of electrically charged particles in the case of magnetism. As a result, the only way to shield magnetic fields is to take advantage of the connections between electric and magnetic fields.

Because changing magnetic fields are always accompanied by electric fields, the two can be reflected as a pair by highly conducting surfaces or absorbed by poorly conducting surfaces. In these cases, the electric fields push and pull on electric charges in the surfaces and it is through these electric fields that the magnetic fields are reflected or absorbed. However, this effect works much better at high frequencies than at low frequencies, where very thick materials are required. Appliances that operate from the AC power line have magnetic fields that change rather slowly with time (only 120 reversals per second or 60 full cycles of reversal each second) and that are extremely hard to shield with conducting material. Instead, their magnetic fields have to be trapped in special magnetic materials that draw in magnetic flux lines and keep them from emerging into the surrounding space. One of the most effective magnetic shield materials is called "mu metal", a nickel alloy that's like a sponge for magnetic flux lines. Since it also conducts electricity pretty well, it is an effective shield for electric fields. So if you wrap your mercury vapor ballasts in mu metal, there would be almost no electric or magnetic fields detectable outside of the mu metal surface.


Copyright 1997-2017 © Louis A. Bloomfield, All Rights Reserved
Privacy Policy