Birmingham Wire Gauge

The Stubs Iron Wire Gauge system (also known as the Birmingham Wire Gauge and not the same as, though similar to, the Stubs Steel Wire Gauge) is used to specify thickness or diameter of metal wire, strip, and tube products. The Stubs system was the first wire gauge recognized as a standard by any country when Great Britain adopted it in 1884; though nearly obsolete, it is the only wire gauge recognized in the United States through an Act of Congress.

The gauge starts at the lowest gauge number of 5Ø or 00000, corresponding to the largest size of 0.500" (12.7mm) to the highest gauge number of 36, corresponding to the smallest size of 0.004" (0.102mm). Size steps between gauges range from 0.001" between high gauge numbers to 0.046" between the two lowest gauge numbers and do not correspond to a particular mathematical pattern, although for the most part the steps get smaller with increasing gauge number. Concerning wire and fine tubing, the gauge number is used to specify the outside diameter of the product, whereas for larger mechanical tubing the gauge number specifies the wall thickness independent of the overall size of the tube.

In medicine, the Stubs system specifies the outside diameter of hypodermic needles, catheters, cannulae and suture wires. It was originally developed in early 19th-century England for use in wire manufacture, and it began appearing in a medical setting in the early 20th century.

Another common needle gauge system is the French catheter scale.

Needle wire gauge was derived from the Stubs Iron Wire Gauge.

...
Wikipedia