Air Speed Indicator, Mk IVa, plane glass front showing movement

In this instrument, the pitot tube is connected with a metal pipe which branches inside the indicator and leads to two thin metal boxes; the inside of the casing is airtight and is connected to the static tube.

The inner face of the inner box and the outer face of the outer box are attached to an aluminium framework. The faces nearest one another are connected by pin joints to either side of a crank pivoted at its centre.

Increase of pressure in the boxes causes the crank to rotate, and so moves a lever arm from left to right. The outer end of this arm lies in a slot in a metal plate, at the other end of which is a toothed quadrant engaging a pinion on the spindle carrying the pointer.