As its name suggests, this is a device for measuring the diameter of a sphere.


The instrument enables you to measure the height h shown in figure 1. d is the distance between a leg and the central screw.

Mathematics shows that:

diameter = (h2 + d2) / h


d can be measured or calculated. For this model, d2 is exactly 4/3, or 1.333.

To measure h, rest the legs of the instrument on the sphere and gently turn the screw until it just touches the sphere. Note the dial reading. Now place the instrument on a flat surface and gently turn the screw until it touches the surface, counting the number of revolutions as you do so. Note the dial reading and calculate the fraction of a revolution from the difference between the two readings. Call the total number of revolutions n.

Since Meccano screwed rods have 32 threads to the inch, h = n / 32. Knowing d, we can now work out the diameter using the above formula.

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