Locate the anvil on the U-shaped frame, which is set on the inside of the frame and toward the top, and looks like a short cylindrical metal plate. Position the wire that requires measuring against the anvil.
Locate the screw gauge ratchet, which is on the end of the spindle cylinder and at the furthest distance from the U-frame. Begin to turn the ratchet in a clockwise direction to move the spindle toward the anvil. The spindle is a metal cylinder that moves from the other inside edge of the U-frame toward the anvil.
Continue to turn the ratchet so that the wire sandwiches between the spindle and the anvil. Listen for a click from the ratchet which automatically stops the spindle from moving any further.
Read the measurement on the horizontal scale that is in-line with the spindle. Check the number that is last to appear at the edge of the thimble and note it down on a piece of paper.
Read the measurement on the vertical scale located on the thimble. A horizontal line aligns to the vertical scale at the measurement mark you need. Note this number on the paper.
Add the numbers together as an expression of their respective subdivisions of scale on the screw gauge. Commonly for imperial scales this is a measurement to one thousandth of an inch. For example, if the horizontal scale is 3 plus 2 subdivisions, and the vertical scale is 4; the total diameter is 0.354 of an inch.