Materials and processes improved over time, such as the removal of the feldspar that contains conductive potassium ions, and the ingredients became kaolinite, talc (steatite) and alkali earth fluxes such as barium or calcium carbonate. Steatite has good mechanical and dielectric properties and relatively low power losses at higher frequencies; so steatite-based porcelain (with 10% kaolin) is well suited for electrical components. It is used for applications such as variable capacitors, coil formers and structural insulators (bushings, spacers and supports). However it is difficult to fire and it is vulnerable to heat shock. Cordierite (magnesium aluminium silicate) is very resistant to thermal shock, so it is more suitable for applications such as electric heater plates and earlier electric fires that had the heating wire wound around a ceramic tube.

Ceramic components can be glazed to minimise surface effects such as moisture ingress into pores that can increase electrical conductivity. They are glazed with a feldspathic glaze or a self-glazing ceramic can be used.

Porcelain was also the material originally used for spark plugs, as mentioned earlier, but the feldspar caused electrical leakage and the quartz caused weak thermal shock resistance. The need for progressively higher performance sparkplug insulation for automotive and aircraft applications during World War II provided the motivation to improve the body material that led to a higher quality 95% alumina body, which started the development of molecularly tailored ceramics. Alumina has a high mechanical strength, relatively high thermal conductivity and technology existed for good metal/ceramic bonding. The sparkplug insulator could withstand an electrical stress in excess of 1 kV/mm at 900 ºC and a pressure of 1600 psi. Alumina is also used to improve the performance of bushings and power resistor cores.

Ceramic spark plugs: Pognom Constructeur
(aeroplane), Nationale eyquem, Red Head
Vitristone and Mosler Versuvious Vitite
(car), a replacement porcelain and Joly
French (aeroplane)

15.18 Electric Lighting

As mentioned earlier, a major driving force for artificial light in the 19^th century was to extend the day for agricultural work. Initially arc lights using carbon electrodes were used with portable electric generators.

From the middle of the 19^th century there was great competition to produce the first commercial incandescent filament electric light in a glass bulb. Initially developers used a fine carbonised bamboo filament and platinum wires to feed through the glass envelope, as its expansion properties matched that of the glass.

Carbon filament lamps, 220/100 volts
- source Wikipedia via ulfbastel

Further development in Russia, USA and UK led to the carbon filament being replaced by metals, and finally the double-coiled tungsten filament used today. The two major proponents, Edison (USA) and Swan (UK) eventually joined together as Ediswan. The companies that were created from their work on electric bulbs included GE of America, Thorn of UK and AEG of Germany. The filament lamp became universally popular for over 100 years and is only now being slowly phased out.

In 1898 Nernst developed a lamp based on a rod of the ceramic cerium oxide, and he called it a “glower”. This was a solid electrolyte that radiated bright light when conducting electricity, but it only became conducting at high temperature, so it had an electric heater as a spiral around it. Amusingly, such a lamp used by Nernst in demonstrations was “switched on” by heating with a match above its threshold for conduction and went out by blowing on it (so cooling it), just like a candle! Unfortunately it took minutes to warm up and was not suitable for domestic applications.