Osglim Neon Festoon Indicator L.P.T.B.

This unsual small neon lamp is believed to have been employed in some capacity for the London Public Transportation Board, hence its marking L.P.T.B. on the glass. One other known example bears the UK Admiralty Pattern code AP 7922, which would imply that it was also used by the British Navy. The UK stock lists of NATO lamps for defence purposes also provide links to an equivalent lamp of the Air Ministry, type 5L/620, and of the War Office, type WB. 3911. It would therefore appear that this lamp had a rather wide application. The LPTB marking may imply that it was originally made as a custom design for London Transport, or simply that this customer was a large consumer which ordered lamps with its own marking to discourage pilferage.

The lamp has an asymmetric design and is intended for operation on a DC 220V supply - it contains an internal resistance to limit current. The London Underground and Trolleybus networks did operate on a DC supply but at 600 Volts. If this lamp would be operated at such potential, an additional external resistance would be required.

The cathode takes the form of an oval metal plate, while the anode is a small metal rod beside the plate. The lamp is filled with at least neon and also contains a considerable amount of mercury. That was commonly employed in high brightness neon lamps as a mechanism of increasing the volt drop and thereby the power dissipation, while also having a significant effect in extending lamp life under high operating currents.

Incidentally the name "Osglim" is frequently found on pre-1950's neon lamps manufactured by the GEC. The LPTB was also re-branded as the LTE in 1948, which allows the date of this lamp to be placed at the 1940s or earlier.

Manufacturer:	The General Electric Company of England PLC
Lamp Power:	0.5 Watts
Lamp Current:
Lamp Voltage:	220 Volts DC
Cap:	SV12
Bulb Finish:	Clear	Soda lime
Bulb Type:	T-26	T-8 (in eighths/inch)
Overall Length:	79 mm
Light Centre Length:
Electrodes:	Cathode: Plate	Anode: Rod
Atmosphere:	99% Neon, 1% Argon
Luminous Flux:	Not Published
Luminous Efficacy:	Not Published
Colour Temperature & CRI:	CCT:	CRI:
Chromaticity Co-ordinates:	CCx:	CCy:
Burning Position	Universal
Rated Life:	Not Published
Factory:	Hammersmith	England
Date of Manufacture:	Pre-1948
Original Value:	Unknown


References:	1) Private Communication, Ray Tye, 1996