# What is the function of this cable surrounding the CRT of old televisions

I have opened old televisions, and each time, I'm wondering what is the function of this cable, roughly insulated with some tape.

Note: I know what are the risks of opening televisions.

It's part of the degaussing system. "Degaussing" refers to removing any residual permanent magnetic field. This is important because electron beams are deflected by magnetic fields. Residual magnetism in parts of the unit can cause strange abberations and color fringes in the picture.

To degauss the unit, a strong AC current that gradually diminishes is run thru a large coil. What you see is part of that coil system. The AC current causes a varying magnetic field. This pulls and pushes on anything that might retain a small permanent residual field. These pulls and pushes are gradually made weaker so that in the end the residual field in anything that might retain one has converged to zero.

The degaussing "control system" was often just a positive temperature coefficient thermistor in series with the coil, and that connected directly to the AC line. When first turned on, the thermistor is cold and conducts a lot of current. This warms it up, so that it gradually conducts less current thru the coil over a number of AC line cycles.

You can often hear a short hum from these TVs right after being switched on.

• There is another wire visible on the extreme right of the photo. This is a ground strap that is vital in ensuring that the aquadag coating on the CRT is at ground potential. If you remove it, you can get a devastating shock from high voltage. Be very careful to discriminate degauss coil from this grounding strap. Nov 22, 2016 at 17:14
• @glen: Good point. It looks like the two wires the OP circled are insulated, which makes them part of the degaussing coil. As you say, grounding the outside of the picture tube is important not only to avoid shock and sparks inside the unit, but the outside is part of the capacitor that holds the high voltage. The glass of the tube with conductors on both the inside and outside are used as the high voltage capacitor to hold up the high voltage supply between flyback pulses. It's impressive how everything in a TV does two or three things. Nov 22, 2016 at 17:19

The "residual magnetic field" mentioned in the other answer is the sum of the magnetic elements in the device and the magnetic field of the Earth. The deguassing system minimizes the net magnetic field, by allowing the magnetic field of the CRT to align nose to tail with the external (Earth) magnetic field.

The CRT should be degaussed whenever it is moved, turned upside down, pointed in another direction, or had other magnets (speakers, fridge magnets, whatever). Originally this was done by a technician with a degaussing wand: later, by pushing the 'degaussing' button, later, every time you powered on a CRT with automatic degaussing.