The 4102D is a telephone repeater triode. The original Western Electric 102D design dates from 1925 and the 4 prefix denotes that manufacture was in the UK by STC.
Originally the spherical envelope was fitted with a metal skirted base. This exhibit is of much later manufacture but follows the original electrode construction.
See also the experimental M25 for similar construction that was also believed to be derived from Western Electric.
In 1946 STC produced the 31/142A as a replacement for the 4102D.
The British STC company (Standard Telephones and Cables) was, before the 1925 take over of the parent Western Electric company by International Telephone & Telegraph (ITT), a subsidiary office of Western Electric. For years after the separation STC relied on Western Electric technology.
The ribbon filament has grid and anode electrodes on both sides. The assembly is held rigid by a glass rod attached to the pinch.
The filament forms a single inverted V. The anode is formed by two bright flat plates.
The anode plates occupy an area smaller than the grid.
The base pins are short and of solid construction, they fit a bayonet socket.
The grid wires are attached to a support frame and composed of many separate wires.
The filament ribbon is held in a hook attached to spring located in the top of the glass support.
The filament ribbon is oxide coated. The geometry of the system can clearly be seen. The distance between the anode plates is 12.5 mm or half an inch.
The lower end of the filament is attached to stout wire supports that have been folded over and crimped.
The balloon envelope is 59 mm in diameter, and excluding the base pins is 95 mm tall.
Reference: Datasheet. Type 4102D was first introduced in 1925.