Carbon Filament with Mk.2 Vitrite Cap
Following the introduction of the first lamp caps around 1885, it took more than a decade before a suitable dielectric material was developed which could perform the role of mechanically fixing the electrical contracts in place and maintaining the necessary electrical insulation.

Wooden caps having brass terminals were employed for the earliest caps from 1880, these being affixed to the bulb with plaster of Paris. By 1881 the wooden parts were dispensed with to save costs, and the cap was moulded around the bulb neck entirely in plaster. This material was not ideal because the length of time required for drying was very long, and it tended to re-absorb moisture and lose its insulating properties when lamps were used in humid atmospheres. Nevertheless it remained in service until 1900, when moulded porcelain parts were introduced. This solved the moisture issue but was relatively expensive. Owing to porcelain's requirement for high firing temperatures, the contacts had to be added afterwards by crimping around the insulator, which proved to be a labour intensive operation.

An adequate solution was found in 1901 when Alfred Swan, the younger brother of Sir Joseph Swan, successfully developed a special black glass called Vitrite, having suitable dielectric properties and which could be easily moulded around the brass contacts. From that year both Ediswan and Robertson introduced bayonet caps moulded entirely in Vitrite. It was found that sometimes the vitrite would crack, a problem which was solved in this 2nd generation lamp which features a brass reinforcing ring around the end. By 1902 a further improvement was made by reverting to the original full brass collar, with vitrite at the end of the cap.
Manufacturer: Robertson Lamps
Lamp Power: Approx 40 Watts
Lamp Voltage: 220 Volts
Lamp Current: 0.46 Amperes
Cap Type: B22d Vitrite with Brass Ring
Bulb Type: Pt-60 Pt-19 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Filament Type: Carbon Squirted Cellulose
Atmosphere: Vacuum
Luminous Flux: 100 lumens 8 Candle Power
Luminous Efficacy: 2.5 lm/W
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: CRI:
Chromaticity Co-ordinates: CCx: CCy:
Rated Lifetime: Approx. 800h
Burning Position: Universal
Overall Length: 127 mm 6 inches
Light Centre Length: 83 mm 3ΒΌ inches
Factory: Hammersmith England
Date of Manufacture: 1901-1902
Original / Present Value: Unknown
 
References: 1) The History of the Incandescent Lamp, J. Howell & H. Schroeder, The Maqua Press, 1927.
2) Private Communication, Ray Tye, 1998.