This picture is taken from the Problem Pictures Numbers resource of photographs with activities for mathematics teaching.
If you purchase this resource the photographs are available in higher resolution and there are additional pages containing hints and solutions.
This instrument measures curvature. The screw in the middle has been turned so that it is just touching the ball. The scale on the left shows the height of the screw. It is about 4.5 mm above the three feet.
If you used a larger ball, would this height be more or less?
How could you calculate the diameter of the wooden ball? The legs of the instrument are 35 mm apart.
How could you use a similar principle to measure the diameter of the earth?
The spherometer was kindly loaned by Adrian Oldknow.