In the UK TV industry these are known as "Posistors" and, as has been stated, are used in the Degauss circuit.
When a Colour CRT TV is switched on, one of these is in series with the Degauss coil around the CRT face plate. A large AC current flows from the mains supply via the Posistor and into the coil and so Degausses the CRT shadow mask. This process prevents colour contamination of the image due to unwanted magnetism of the shadowmask. The posistor is essential to control the current passing into the coil or else there would be a nasty overheat in the coil ! These components often fail open circuit in CRT TV's and the user then complains of strange colour casts on part of the picture (mangetic contamination of the shadow mask). Magnetic contamination can occur naturally due to the earths magnetic field or man made sources such as Hi-Fi speakers next to the TV.