You may notice how the indicator light in your mains sockets and switches dims with time and wonder why. These lights are usually small neon bulbs which glow when a small alternating current of 1 or 2mA passes through them. These bulbs are filled with neon gas which glows red when a current passes through it. There is no filament and no heating element. Neon is an inert gas which glows with an orangey red colour in these bulbs. If the current is too high or the electrodes are made of an unsuitable metal the metal evaporates from the surface of the electrodes and condenses on the inside of the glass bulb, making it opaque, so the light output dims as the glass gets covered.