Example of interference patterns in a shopping cart: The honeycomb moulding is the same dimension on each side of the cart, but optically they're different because the far side of the cart appears a bit smaller. A sideways movement of the camera of only a half inch makes the interference centers appear to move from one end of the cart to the other. The cart was 3 feet long - a spatial displacement multiplication of 64:1. Read on for how this applies to industry...

Eyeball accuracy isn't good enough for precision parts:
There was a time when measurement accuracy was pretty much limited to the resolution of the human eye (finest practical scale markings = 1/100 inch.) Somewhat finer measurements could be made using a magnifying glass, but the method was unreliable for various reasons. This limited the precision of scientific instruments and prevented the blessings of an industrial economy from coming to pass. Some inexpensive, reliable method was needed of making measurements ten times finer than could be done by eyeball accuracy.

A French scientist named Pierre Vernier (1584-1638) noticed a common phenomenon - interference patterns - and adapted it to the creation of an astoundingly useful measuring device. Interference patterns occur often in nature, such as waves of different frequency passing one another on the surface on a lake. Vernier realized an artificial interference pattern could be used in a novel way to make accurate measurements.

Few people are aware of inventions that underlie other inventions - or that underlie our entire industrial/technical culture. The Vernier scale is one such invention. Vernier's concept leveraged eyeball-accuracy to a much greater precision. It allowed the manufacture of precision scientific instruments, accurately sized standard parts, and land surveying to unheard levels of exactitude. The scientific enlightenment and the industrial revolution that followed both profited from the use of this invention.

The Vernier scale is a displacement multiplier. Essentially it works by interference in two slightly dissimilar scales. As one scale moves along the other, the point of exact alignment between them moves very much faster than the scales themselves. You can use a crude Vernier device to make a better one, ratcheting up the precision in a reversal of the usual degradation of scale-copying to almost the limit of the material and design (about 1/1,000 inch). Vernier made this level of measuring precision feasible in the 1700's.

Vernier made accurate measurement simple, then another inventor's creation made it cheap. Joseph Rogers Brown devised an extremely accurate linear dividing engine. By 1850, Brown's machine made Vernier scale instruments inexpensive enough for every machinist and mechanic to have them. (The same machine was later used to produce accurate slide rules, which greatly assisted the art of engineering computation.)

Eventually other precision measuring methods were invented (their prototypes developed and certified using the Vernier device] but none so inexpensive and common as the Vernier. Machinists everywhere became expert at reading the Vernier scale. It became simple and quite expedient for all manufactured goods to be made of standard parts - a boon to industrial culture.

Modern ultra-high-precision methods of measurement use the same principle. The curvature of a telescope mirror is measured by the interference of direct versus reflected light of a given wavelength. Simply imagine a Vernier caliper with scribed lines a few angstroms apart to get the idea - and the resulting degree of precision is expressed in wavelength: "This mirror is ground accurate to 1/4 wave."

Such precision makes possible the construction of atomic clocks that enable GPS navigation, with its enormous (and still growing) economic effect. Semiconductor device manufacture would be impossible without ultra-precise optical frequency scale-interference measurement. Vernier came upon a principle in measurement that is still in use today. It is safe to say he would have been astounded at the ways his work is applied.

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