Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 130,443 pages of information and 207,227 images on early companies, their products and the people who designed and built them.
of Spring Road, Letchworth, Herts. Telephone: Letchworth 446. Telegraphic Address: "Sigma, Letchworth". (1937)
of Letchworth, Herts. Telephone: Letchworth 1045. Cables: "Sigma, Letchworth". (1947)
1937 Listed Exhibitor - British Industries Fair. Gas Calorimeters. Gas and Air Flow Meters. Steam Flow Meters. Pressure Recorders. Automatic Gas Mixing. Relay Operated Exhauster Regulators and Retort House Governors. CO2 Recorders. Distant Recorders and Distant Indicators. (Stand Nos. Ca. 813 and Ca.712) 
1947 Listed Exhibitor - British Industries Fair. Manufacturers of Fine Measuring Instruments including Mechanical, Electrical, Optical and Pneumatic Comparators; Automatic Inspection Equipment. Gas Testing Equipment including Recording and Indicating Calorimeters, Specific Gravity and Pressure Recorders, etc. Automatic Control Equipment. (Olympia, Ground Floor, Stand No. A.1074)