On many adapters, particularly the ones that power my home computer and laptop, there’s a “lump’’ in the cord close to the end that supplies power to the computer. What’s the lump’s purpose?
Your computer has all sorts of things inside it that switch currents on and off quickly. After all, the entire basis of your computer is lots of little switches that control all sorts of other little switches. Rapidly switching currents radiate radio waves, and those waves can cause interference on television sets, radios, cellphones, and other electronics.
You can demonstrate this easily if you tune an AM radio to between stations and rake a metal file over both terminals of a 9-volt battery to rapidly connect and disconnect them. You will hear a corresponding crackle on your radio. If you attach a long wire to the file to act as an antenna, you will get an even louder crackle.
Now think of your computer as a sophisticated version of the file and battery and you see the potential problem. Most of the computer is pretty well shielded against radio waves sneaking out, but the power cable is a potentially problematic antenna.
The solution is that lump. It’s made of a material called ferrite, which is one of a wide range of metal (mostly iron) oxides and goes around the wire. When a rapidly switching current tries to run down the wire, it produces a correspondingly changing magnetic field - an effect made stronger by the presence of ferrite, a magnetic material. It takes work to change the magnetic field, and that means the changing current is weakened, with most of its energy dissipated as a small amount of heat in the ferrite.
The steady current from the adapter is unaffected, since it does not change the magnetic field - the ferrite only gets in the way of currents that change quickly. The result is that the lump lets the steady current that runs your computer pass through unaffected, but stops the rapidly changing currents from your computer from using the power cable as an antenna.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
