During a closed-wrap magnetic treatment, heavy-gauge copper cables encircle the hull and superstructure of the vessel, and high electrical currents (up to 4000 amperes) are pulsed through the cables. This has the effect of "resetting" the ship's magnetic signature to the ambient level after flashing its hull with electricity. It is also possible to assign a specific signature that is best suited to the particular area of the world in which the ship will operate. In drive-in magnetic silencing facilities, all cables are either hung above, below and on the sides, or concealed within the structural elements of facilities. Deperming is "permanent". It is only done once unless major repairs or structural modifications are done to the ship. Early experiments With the introduction of iron ships, the negative effect of the metal hull on steering compasses was noted. It was also observed that lightning strikes had a significant effect on compass deviation, identified in some extreme cases as being caused by the reversal of the ship's magnetic signature. In 1866, Evan Hopkins of London registered a patent for a process "to depolarise iron vessels and leave them thenceforth free from any compass-disturbing influence whatever". The technique was described as follows: "For this purpose he employed a number of Grove's batteries and electromagnets. The latter were to be passed along the plates till the desired end had been obtained... the process must not be overdone for fear of re-polarising in the opposite direction." The invention was, however, reported to be "incapable of being carried to a successful issue", and "quickly died a natural death". Color cathode ray tubes Color CRT displays, the technology underlying many television and computer monitors before the early 2010s, use degaussing. Many CRT displays use a metal plate near the front of the tube to ensure that each electron beam hits the corresponding phosphors of the correct color. If this plate becomes magnetized (e.g. if someone sweeps a magnet on the screen or places loudspeakers nearby), it imparts an undesired deflection to the electron beams and the displayed image becomes distorted and discolored. To minimize this, CRTs have a copper, or often in the case of cheaper appliances, aluminum, coil wrapped around the front of the display, known as the degaussing coil. Monitors without an internal coil can be degaussed using an external handheld version. Internal degaussing coils in CRTs are generally much weaker than external degaussing