Altimeter, Tyco, 20,000FT, WWI to 1920's US Navy Japanese The knob was used to

The knob was used to set the flow rate

and then a small triangular index pointer which traverses the perimeter of the inner scale

Truth be told

AN/ARN-30 was used in versions of the following aircraft: UH-1

model 8DJ28AAN

Altimeter, Tyco, 20,000FT, WWI to 1920's US Navy Japanese The knob was used toHere is an Altimeter used in early military aircraft of the US Navy, very similar to the Type C altimeter used by the Army Air Service from WWI thru 1926. Crudely measured altitude from 0 20,000 ft, which was pretty high for aircraft of that era and requiring supplemental oxygen for the crew to exceed ~12,500 ft based upon modern standards. The knob rotates the face of the dial while the pointer remains fixed to set the present field elevation.