The Cosmos short path valves SP-18 were introduced in 1925 all had thoriated tungsten filaments and thus were dull emitters. The red spot being designated as an AF and output valve and the green spot being designated as a general purpose valve.
Our exhibit is clearly marked with a blue spot and the etched markings say SP-18 Blue this variant was introduced in about 1926. The BBC stamp is also visible on the right of the picture.
The short path construction can be seen from the picture. All electrodes run vertically within the envelope. The flattened cylinder of the anode contains the grid with fixings at one end of the flattened spiral only. Within the other rounded end of the grid is threaded the filament. The distance for the electrons to travel was thereby considerably reduced compared with the ORA type of modified R construction.
The wide glass tube envelope is 25 mm in diameter, and excluding the B4 base pins, is 85 mm tall.
References: 1043. Type SP-18 Blue was first introduced in 1926. See also1926 adverts.