coils, since a better degaussing coil takes up more space. A degauss causes a magnetic field inside the tube to oscillate rapidly, with decreasing amplitude. This leaves the shadow mask with a field opposing any external field (such as that of the Earth), removing the discoloration. Many televisions and monitors automatically degauss their picture tube when switched on, before an image is displayed. The high current surge that takes place during this automatic degauss is the cause of an audible "thunk", a loud hum or some clicking noises, which can be heard (and felt) when televisions and CRT computer monitors are switched on, due to the capacitors discharging and injecting current into the coil. Visually, this causes the image to shake dramatically for a short period of time. A degauss option is also usually available for manual selection in the operations menu in such appliances. In most commercial equipment the current surge to the degaussing coil is regulated by a simple positive temperature coefficient (PTC) thermistor device, which initially has a low resistance but quickly changes to a high resistance due to the heating effect of the current flow. Such devices are designed for a one-off transition from cold to hot at power up; "experimenting" with the degauss effect by repeatedly switching the device on and off may cause this component to fail. The effect will also be weaker, since the PTC will not have had time to cool off. Magnetic data storage media Data is stored in the magnetic media, such as hard drives, floppy disks, and magnetic tape, by making very small areas called magnetic domains, change their magnetic alignment to be in the direction of an applied magnetic field. This phenomenon occurs in much the same way a compass needle points in the direction of the Earth's magnetic field. Degaussing, commonly called erasure, leaves the domains in random patterns with no preference to orientation, thereby rendering previous data unrecoverable. There are some domains whose magnetic alignment is not randomized after degaussing. The information these domains represent is commonly called magnetic remanence or remanent magnetization. Proper degaussing will ensure there is insufficient magnetic remanence to reconstruct the data. Erasure via degaussing may be accomplished in two ways: in AC erasure, the medium is degaussed by applying an alternating field that is reduced in amplitude over time from an initial high value (i.e., AC powered); in DC erasure, the