The CV1731 neon voltage stabiliser is on a B4 base and is a design progression from the Cossor S130 that dates from the mid 1930's. The valve was specified by the GPO and thus also known as the POSV2.
The striking voltage is 160 V and the burning current is from 6 to 75 mA. Within the current range the regulation is 5 volts.
The lower end of the cylinder electrode. In CV1731 the leads are shielded by ceramic insulators.
Within the outer cylinder are two supports for the central electrode. In this design the top of the cylinder is closed and the lower end is open.
A close-up of the inner electrode. It is a flat mesh passing up the centre of the outer cylinder.
The balloon envelope is 50 mm in diameter and, excluding the B4 base pins, is 106 mm tall.