||The transmission of morse signals over short distances can be achieved simply by the rapid switching of a small incandescent lamp. As ranges increase, signalling speeds must decreased owing to a limitation of incandescent lamp technology. More powerful lamps are naturally required, and these contain larger filaments. The greater mass of metal takes longer to bring up to incandescent temperatures and longer to cool down, which causes pulses to blur together.
During the second world war there was an increased requirement for long-range signalling and faster lamps were required. A satisfactory solution was eventually achieved with this so-called rapid nigrescence incandescent lamp, developed for installation on the masthead of a ship. Rather than using one large filament, fourteen small filaments are employed. Due to the smaller wire diameter, their thermal inertia is low and the light-up time is fast. In order to decrease nigrescence (cooling) time, the bulb is filled with a helium-nitrogen mixture. On account of helium's high thermal conductivity the lamp is inefficient and short-lived.
The filaments are mounted in a series of vertical rings on a complex wire frame, to form a relatively compact source. This is located at the centre of a spherical bulb, one side of which is mirroed so as to project a beam of high luminous intensity. It is equipped with with a pre-focus cap for accurate alignment in the optical system. Interestingly, the hand-made glass stem assembly employs the old-fashioned Jaeger construction, in which the exhaust tube pierces one side of the stem rather being integrated into the pinch-seal. Today these lamps have been superseded by mechanical shutters to interrupt the beam of a standard lamp.