Degaussing

This article was initially published in Today's Engineer on October 2011

The “clunk” you hear when you turn on a large CRT television or a computer monitor, and the electrical surge you can sometimes feel and hear near the screen, are modern reminders of an important World War II electrical technology which saved many lives. The sound is made by the degaussing coil, which — in television sets — is used to prevent the discoloration of the display which can occur if the beams of electrons triggering the different colored phosphorus cells are not correctly aimed by the magnetic field.

In 1919, the British developed a magnetic naval mine. Between the world wars, the Germans also developed a magnetic mine, whose more sensitive triggering sensor — calibrated in milligauss — could be adjusted. The gauss, a unit of magnetic density, is named after mathematician Carl Friedrich Gauss (1777-1855) who formulated a law of magnetism. The sensors in the magnetic mines detected the distortion in the earth’s magnetic field caused by the proximity of a large metal object (such as a ship). Dropped from aircraft by parachute into harbors and coastal waters, the magnetic mines rested on the bottom and were impossible to sweep using conventional minesweeping methods. They wreaked a terrible toll on British shipping in the English Channel during the first months of the Second World War.

On 23 November 1939, a fortunate aiming mistake by a Luftwaffe plane dropped a magnetic mine onto the mudflats of the Thames estuary near Shoeburyness, where the British were able to recover it at low tide, and disassemble it to understand how it worked. Charles Frederick Goodeve, later awarded an OBE, realized that magnetic coils running through a ship could be used to produce a bias field in the ship opposite to the ship’s existing bias. This opposite bias would cancel the magnetic distortion and prevent triggering the mines. Photographs of World War II warships, as well as high-value merchant ships such as troop transports, often show the degaussing coils running around the hulls. Building such coils into each ship was expensive, however, so Goodeve developed a process to “wipe” a ship by dragging a cable carrying a degaussing current over the hull and decks. This process had to be repeated every four to six months because objects moving through the Earth’s magnetic field will reacquire a magnetic bias of their own.

Because each ship’s bias differs according to size and also depending on where on the earth’s surface it was constructed (because of the earth’s magnetic lines of force), special ranges of magnetometers had to be constructed at various harbors to measure a ship’s existing bias so that it could be accurately opposed by the degaussing operation. The ship to be degaussed would travel over the range at a carefully calculated speed, and its bias read so that an opposite bias could be induced later by the cables.

While warships have degaussing coils built into them and use them as a matter of course, degaussing is not limited to warships. As late as the 1970s, merchant ships sailing in the English Channel, the Baltic Sea, and into German and Dutch ports, were “wiped” to protect them from magnetic mines left over from World War II.

Another, more current, use of degaussing is by libraries to prevent book theft. A magnetic strip hidden in the book is magnetized and demagnetized by a degausser. Although radio frequency detection devices have become a prevalent anti-theft technology in many libraries, degaussing is a legacy technology because of the large number of books which were acquired by libraries during prior years when the magnetic strip was the chosen technology.

Perhaps the most common modern use is the degaussers used to “wipe” magnetic media — especially computer hard drives — clean of data. (The word is the same as used for an entire ship.) Bulk degaussers for cleaning magnetic tape did a more complete job than the erase head if one wanted a clean tape for rerecording. Broadcast standard video tape, in particular, was expensive enough to be worth going to the trouble of wiping and reusing. In the case of computer memories, degaussing is a more secure method than merely deleting files because it changes the magnetic bias of the memory material. Thus, a technology that had its origin on a Thames estuary mudflat decades ago is still very much in use.