The electrical system in the Bristol Type 167, or Brabazon I Mark I, aircraft is one of the most ambitious installations so far fitted in any aircraft in Britain. The special requirements of this large aeroplane necessitated a power-supply system which, at its inception, was for many reasons unique, although its principles have since been applied in a number of other large aircraft.The object of the paper is to deal mainly with the development of the power-supply system, but some attention has been given to the problems associated with the design of aircraft electrical equipment in general, since the standard of development reached by this equipment merits consideration. The paper therefore deals briefly with the development of typical important items of aircraft electrical equipment, and shows how its specific weight has progressively decreased with its development. After a short review of the generating-speed problem and the ways in which it has been tackled, the performance characteristics of the present power-supply system are stated and its operating characteristics are finally discussed in the light of the results obtained by extensive instrumentation of a ground test-rig.The scope of the subject is large, and it is not possible to cover in the paper the design of the individual items of equipment or describe any of the specialized electrical controls which are now in common use in aircraft.