The principle of the servo altimeter, introduced by Smiths Industries in 1958, was that aneroid capsules were relieved of all but the lightest mechanical work involved in the electrical detection of their position. The operation of the instrument was performed by an electrically powered servo.

This resulted in significant gains in accuracy, the possibility of extension of the range to 100,000ft, and the use of five-digit counter presentation making misreading virtually impossible. This conformed to the conclusions reached by the UK Altimeter Committee. In spite of these advantages, and apparent complexity, the servo altimeter achieved a consistent reliability well above that of mechanical altimeters.