A meter rod may not fit cross-wise inside a breaker (whatever that is).
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Assuming it is a rod, the difficulty lies in that it can't measure the diameter of the beaker. Unless you were to attempt to find out the Radius/Diameter, then work out the circumference from that.
Internal diameter.
An internal caliper gauge.
The radius of a circle is half the diameter.
Calipers are used to measure the dimension of anarticle. In measuring a section of pipe you would use an inside caliper to give the internal diameter, and an outside caliper to give the external diameter. The difference being the thickness of the pipe wall.