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View Full Version : Semi OT: Inch Standard

lazlo
10-11-2008, 01:02 PM
I commented in the Inch/Metric thread that the meter reference is pretty arbitrary -- 1⁄10,000,000 of the distance from the equator to the north pole through Paris, but because they came up with reference in the 18th century, they botched the survey data and the meter is actually a fifth of a millimeter short of the actual distance.

I was just reading the "Gauges and Gauging" series in the 1939 Model Engineer, and was fascinated by this quote about the inch reference:

"We have traveled a long way since the statute of Edward II in 1324, wherein it was laid down that one inch was to be equal to three round and dry barley-corns laid end to end. Experience showed them that barley-corns grew to a size that was almost invariably the same, so that any three could be drawn from a sack with certainty that the average combination would work our right enough."

doctor demo
10-11-2008, 01:13 PM
Way back when I was in grade school we were taught that the ''inch'' was originaly a unit of measure invented by (I don't remember exactly) some king some place and it was the length of his thumb when bent, from the tip to the first knuckle. Maybe it was the statute of Edward the first.

Steve

Allan Waterfall
10-11-2008, 03:24 PM
Must have had short thumbs in those days,I believe the average is approximately 1 and 1/2 inches nowadays.

Allan

SGW
10-11-2008, 03:32 PM
I've heard that the 3 barleycorns defined one shoe size, not an inch. I don't think 3 barleycorns comes close to being an inch. (Anybody got a sack of barley handy???)

chief
10-11-2008, 07:33 PM
Things that make you wonder WTF?

gnm109
10-11-2008, 08:56 PM
Things that make you wonder WTF?

Huh? And I always thought of the metric system as criminal...oh, sorry, I meant Whitworth...no....ahhhh, errr maybe it was BSF? Oh never mind... :)

10-11-2008, 09:09 PM
Standards have turely evolved. Whatever the meter was back when, It's what the Paris standard is now - but that's not close enough. The meter is now defined interms of somewavelength or other. Handy if you're looking to check your mike, huh?

Nowadays standatd have to be determined to on part in trillions if not finer. The GPS I pack around times satellite signals to nanoseconds. Fancy as it may be it pales beside the fancy gear.

The best I can do in my home shop is to keep my standards in good shape and one in a while when I can afford it, I send off my Jo blocks for calibration. Everything else is either self calibrating or seldom used to its limits. My Fluke 87 for example was calibrated in - um - 1996 I think and the tech told me he hardly had to tweak it. Kind of a surprize; it's taken a real beating over the last 25 years.

wierdscience
10-11-2008, 09:28 PM
An interesting take on the yard/inch-

http://www.sizes.com/units/yard.htm

In 1742, the Royal Society in London arranged an exchange of the standards with the Royal Academy of Sciences in Paris. To accomplish this, two identical brass bars were made and ruled lengthwise with three lines. The length of the yard, taken from a yard measure made in 1720 that was based on that of Elizabeth I, was marked off on one of the lines and labeled “E” (for English). The bars were then sent to the French, who marked off the length of half a toise on one of the other lines, labeled it “F,” kept one bar and sent the other back. The following year the Society decided to compare a number of the existing yard measures, and in the process of doing this the length of Elizabeth's yard was engraved on the third line and marked “EXCH” (for Exchequer). The E length turned out to be 0.0075 inch longer than the EXCH length. This bar is known as Royal Society bar No. 41.

About a decade later the government itself decided to look into the status of the country's weights and measures, and set up a 63-member committee under Lord Carysfort to do so. They rejected Elizabeth's standards as “very coarsely made...bent...very bad standards” but approved of the more modern design of Royal Society bar No. 41, and on the advice of experts decided to make a standard to an even more modern design, taking its length from the “E” line on the Royal Society bar. The standard was duly made in 1758 by a Mr. Bird, an instrument maker, and deposited with the Clerk of the House of Commons. None of the Carysfort Committee's recommendations were acted upon. At the request of a successor committee, Bird made a second, similar, yard standard in 1760.

There followed a succession of committees making various recommendations and suggesting various standards, culminating in the act establishing imperial measure from May 1, 1825. This act declared the yard Bird made in 1760 to be the prototype of the imperial yard. All length measures were to be based on it. Less than ten years later, in October 1834, the Houses of Parliament burned down, destroying both of Bird's yards.

The government than appointed a committee (1838) to oversee the construction of new standards of a yet more modern nature. The task of constructing the new yard was given to Francis Baily (1744–1844), who conducted extensive research on the best choice of alloy for the bar, and designed its form before dying. The task passed to the Reverend Sheepshanks.

By this time, it was possible to measure a standard's length with a precision of one part in ten million. To give some idea of the care taken in constructing the new yard standards: to account for thermal expansion of the metal, new thermometers were constructed accurate to a hundredth of a degree. To eliminate the effect of bending, the standards were measured floating in a pool of mercury.

Sheepshanks began by constructing some standards which he compared with existing yard standards that had been compared with the Bird yard. He concluded that his bar no. 2 was 36.00025 inches of the average value of those yards, and hence of the true yard, and the commission agreed. Forty new standards were then made to Baily's design and meticulously compared both with bar no. 2 and to one another, to find the one that would be 3600000/3600025ths of bar no. 2. This task took years.

Finally one bar was selected to be the prototype. The four next best became “Parliamentary standards,” and the remaining 35 were distributed to various cities and friendly powers. In 1855 the selected bar was made the legal standard (18 & 19 Victoria, c 72 s 2). Sheepshanks had died the day before.

For the next development, see international yard.

lazlo
10-11-2008, 10:02 PM
The task of constructing the new yard was given to Francis Baily (1744–1844), who conducted extensive research on the best choice of alloy for the bar, and designed its form before dying.

Ah, that's interesting Wierd. The Model Engineer article mentions "Baily Metal" which was a bronze alloy that has a very low coefficient of expansion.

After a bit of Googling, I found the recipe for Baily's "Gun-Metal No. 4":

Copper: 16 Parts
Tin: 2 1/2 Parts
Zinc: 1 Part

The ME article describes the same issues with the Elizabethan standards, and also discusses the controversies over using the ends of the standards for the reference measurement, versus scribed lines, which the final version used.

They also added gold plugs at the airy points, so they could measure the reference without floating it on mercury.

wierdscience
10-11-2008, 10:08 PM
Ah, that's interesting Wierd. The Model Engineer article mentions "Baily Metal" which was a bronze alloy that has a very low coefficient of expansion.

After a bit of Googling, I found the recipe for Baily's "Gun-Metal No. 4":

Copper: 16 Parts
Tin: 2 1/2 Parts
Zinc: 1 Part

The ME article describes the same issues with the Elizabethan standards, and also discusses the controversies over using the ends of the standards for the reference measurement, versus scribed lines, which the final version used.

They also added gold plugs at the airy points, so they could measure the reference without floating it on mercury.

The paragraph I find amazing is this one-

By this time, it was possible to measure a standard's length with a precision of one part in ten million. To give some idea of the care taken in constructing the new yard standards: to account for thermal expansion of the metal, new thermometers were constructed accurate to a hundredth of a degree. To eliminate the effect of bending, the standards were measured floating in a pool of mercury"

One part in ten million,not bad for 1838:)

lazlo
10-12-2008, 09:39 AM
By this time, it was possible to measure a standard's length with a precision of one part in ten million.

One part in ten million,not bad for 1838:)

Good point :D So... how did they measure to 1 part in 10 million in 1838? :)

Evan
10-12-2008, 11:16 AM
Same as now, interferometry. Newton published his book OPTIKS in 1704 and all the principles of interferometry were known by about 1800.

One part in ten million is the ratio of the wavelength of blue light to an inch.

oldtractors
10-13-2008, 08:30 PM
I've heard that the 3 barleycorns defined one shoe size, not an inch. I don't think 3 barleycorns comes close to being an inch. (Anybody got a sack of barley handy???)

My newest engineering book "Roper's Engineers Handy Book" (1892) defines an inch as being 3 barleycorns"

Jim