SO/H Detatchable Low Pressure Sodium

Updated
04-VI-2015
Developed by Philips in early 1932, the SO/H design was the first commercial sodium vapour lamp to become popular and enter mass production. The discharge tube is U-shaped and fabricated from 2-ply sodium-resistant glass, and to allow this to attain the necessary wall temperature of 260C for maximum efficacy, it is operated inside a dewar vacuum jacket to provide thermal insulation. At end of life it was customary to replace only the inner tube, the same dewar being used up to about six times, before its vacuum would tend to go soft and no longer provide the necessary insulation.

Although the SO/H lamp was technically superseded by the superior SOI/H 'Integral' design after 1955, it remained in production until about 1965, for replacement purposes. Illustrated here is a selection of SO/H lamps from a variety of different manufacturers, exhibiting some of the different construction techniques employed.

It is worth noting here that the rate at which the glasses used in some of these lamps cleans up argon gas can be rather high. In particular, if the lamp has not been lit for more than twenty years or so, the argon gas needed for starting may be adsorped quite rapidly after ignition. If the lamp is then turned off, the loss of this gas makes re-igniton extremely difficult. However if it is left burning for some five hours or so after its first start, this will be sufficient to re-release much of the argon and it will then be safe to turn off the lamp and know that it should strike again.

Philips Lamps

Philora

60W

SO/H lamp of Dutch 2nd generation
1938

Philips

45W

SO/H lamp of British design
1952

Philips

140W

SO/H lamp of British design
1951

Philips

60W

SO/H replacement inner of British design
1954

Philips

45W

SO/H replacement Bamboo discharge tube
1957

Philips

60W

SO/H lamp with Bamboo discharge tube
1958

Philips

85W

SO/H lamp with Bamboo discharge tube
1955

Philips

140W

SO/H lamp with Dimpled discharge tube
1958
   

Osram-GEC Lamps

Osram-GEC

140W

SO/H lamp of early production style
1949

Osram-GEC

45W

SO/H discharge tube of last production style
1965
   

BTH-Mazda and AEI-Mazda Lamps

Mazda

60W

SO/H lamp with hard glass discharge tube
1950

Mazda

140W

SO/H lamp with hard glass discharge tube
1953

Mazda

85W

SO/H discharge tube of hard glass design
1957

Mazda

60W

SO/H discharge tube of soft glass design
1965
       

Soviet Lamps

MELZ

140W

DNAO-140 Soviet Style low pressure sodium
1966