SIR – The recent letter by K H Chappell relating to the Avro Vulcan aircraft brought to mind another plane from the last war, the Lancaster Bomber and its electrical system, which I worked on.

I was called to military service in 1940 at the age of 19 and served as a wireless technician in the RAF, working on the South Coast receiving and miming German navigation beacon signals to counter their flights over Britain.

Radar superceded this method of wireless direction finding, and in 1941 I was sent on a course to RAF Yatesborough to study the latest navigation techniques (the notes I have retained). One system called H2S used the first 3cm resonant cavity magnetron, transmitting air to ground pulses, with the resultant echo being displayed on a CRT radial sweep timebase. The contours of this showed the target area for the Pathfinders over Germany.

The power supply for this equipment was a single-phase homopolar alternator characterised by magnetic teeth numbering 28 likepoles on the stator and 30 on the rotor, creating a rotating field with field to rotor speed ratio of 15 to one at a rotor speed of 4000 RPM, giving a frequency of 2000Hz at a regulated 80V AC.

A Lockett, Great Horton Road, Horton Bank Top