The Tape Measure

Measurement is where every trade begins. A drawing has dimensions; the tape turns them into marks on material.

Units

Early units came from the body: the cubit (forearm), the foot, the hand. Standardisation followed trade and law. In 1620 the English mathematician Edmund Gunter introduced a surveying chain 66 feet long with 100 links; 80 chains make a mile and 10 square chains an acre, which is why so many American property lines and road widths are multiples of 66 feet. The metric system, from revolutionary France, gave the world a decimal alternative, now used in most countries and in science.

The spring tape

Cloth and steel tapes wound on reels existed earlier. In 1868 Alvin J. Fellows of New Haven, Connecticut, patented a spring-return tape measure. Improvements in the twentieth century added a concave blade that stays stiff when extended, a blade lock and printed stud marks.

Reading a tape

A standard imperial tape divides the inch into sixteenths. Stud marks every 16 inches and diamond marks every 19.2 inches (five per eight feet) help framers lay out without arithmetic. The end hook moves: pulled, it measures from its inside face; pushed against a wall, from its outside face. The movement equals the hook's thickness.

Beyond the tape

Laser distance meters measure by timing reflected light. Surveyors measure with total stations and satellite receivers. Machinists measure in thousandths of an inch with calipers and micrometers (see From Drawing to Part). Each trade has its own precision; the lesson is the same: measure twice, cut once.