Originally Posted by
cafdfw
It's a vintage Japanese aircraft 'Turn and Bank' indicator Mike - Very cool!
The 'needle' at the top was gyroscopically driven, and indicated when the plane was in a turn.
The 'ball' worked on the same principle as a carpenter's level. It indicates if the plane is 'slipping or skidding' through the air. (flying sideways)
The old saying goes - "Step on the ball" meaning if the ball is to one side of center, apply more rudder by 'stepping' on the rudder petal to the side the ball is.