Screw Gauge

A screw gauge is an instrument used to accurately measure the thickness, diameter, and width of very small objects, such as a strand of hair, a coin, or a sheet of paper. Invented in the seventeenth century by William Gascoigne, it measures tiny dimensions that a standard ruler cannot measure accurately.

Parts of the instrument

• U-frame, which forms the outer body of the tool.
• Anvil and spindle, which hold the measured object between them.
• Barrel, which carries the horizontal pitch scale.
• Thimble and ratchet stop, which are used to turn the internal screw.

How rotation turns into linear motion

A screw gauge operates by converting rotational motion into linear motion. Turning a screw causes it to travel inward in a straight line. The distance moved in one complete turn equals the distance from one thread to the next, which is called the pitch. In this instrument, the screw is designed so that one complete rotation moves it inward by exactly 1 millimeter.

Scales and least count

• The standard horizontal ruler on the instrument is called the pitch scale.
• A circular cylinder around the screw has one hundred divisions and is called the head scale.
• Dividing the 1 millimeter pitch by one hundred equal divisions gives 0.01 millimeters.
• This value of 0.01 millimeters is called the least count, which represents the smallest accurate measurement the tool can record.

Measuring an object

A normal scale can show that an object like a toothpick lies between 4 millimeters and 5 millimeters, but it cannot show the exact reading in between. A screw gauge solves this by combining the pitch scale and the head scale. For an object resting past 4 millimeters, the pitch scale reading is taken as 4 millimeters. If the head scale reads 58, that number is multiplied by the least count of 0.01 millimeters to give 0.58 millimeters. Adding 4 millimeters and 0.58 millimeters gives the final accurate measurement of 4.58 millimeters.