The “rule of thumb” column just ran in my local paper, the Pacific Sun. I wanted to comment that Cecil’s summary is in substantial agreement with a fairly lengthy article on the same subject by a professor named Henry Ansgar Kelly. A point Kelly raised, but Cecil didn’t mention (as I recall), was that the connection between the phrase “rule of thumb” and the supposed wife-beating rule apparently first arose in a 1977 article by a journalist named Terry Davidson.
I’ve posted an extract from the Kelly article at http://www.monitor.net/~jberger/ruleofthumb.txt in case anyone wants to check it out. The full article is in the September, 1994 edition of the Journal of Legal Education, page 341; most good-sized law libraries should have it.
I’m not disagreeing with Cecil, but it is interesting to note that a physics instructor claimed that the term “rule of thumb” came from slightly different origins. This was many years ago and he gave a number of examples that I can no longer remember, however he said that there were many “rules of thumb”. Most related to science and building trades. Some of these rules are now called right hand rules. Here are a few that I can remember:
Your right thumb points in the direction of torque when your fingers point in the direction of the first vector and are curled towards the second vector.
When your right thumb points in the positive X direction in a cartesian coordinate system, your index will point towards positive Y and your middle finger will point towards positive Z (when these three fingers are held at right angles to each other).
When your right thumb points in the direction of current in a wire, the curl of your fingers shows the direction of the magnetic field.
Using your right thumb to determine the direction, your fingers point in the direction you have to turn a screw to make it move in the direction of your thumb. (not as easy to remember as “righty-tighty, lefty-loosey”, but hey, these were the dark ages).
He also mentioned the use of the thumb to measure things in several ways, not just as a substitute for an inch. Very tall things could be measured by comparing the length of the first thumb joint with the ‘apparent’ height of a distant object of known height and then using this to determine the the height of an object of unknown height at the same distance.
There were several more… sorry I can’t remember.
So, is it possible that Cecil’s answer is incomplete and there might not be simply A RULE of thumb, but MANY RULES of thumb?
In electrical terms you mean Flemings Left-hand rule and Right-hand rule.
Imagine a conductor that is placed lengthways in a static magnetic field.
The index points along the direction of current flow, the forefinger gives the direction of magnetic flux and the thumb gives the direction of movement or force.The current flow is of course in the direction as the conductor(eg piece of wire)
Using the right hand predicts the directions for a motor and the left hand for a generator.
Maxwells rule is similar but what you do is to grasp the(insulated )conductor with all fingers ensuring that the thumb points toward the lower potential. The way your fingers wrap around gives the direction of magnetic field.
I dunno, it seems to me that the physics (and engineering) Right-Hand Rules are a bit specialized… I can’t quite see physics jargon becoming as widespread as the expression “rule of thumb”. Besides, in all my years of physics (ok, so that’s not THAT many, but still…), I’ve enver once heard of any of the right hand rules being refered to as a rule of thumb.
I think you’re probably right, Chronos. It makes more sense that a commonly used phrase like “rule of thumb” would come from a “blue collar” trade. I’m not an expert, so don’t bar-b-que me for saying this, but it seems that most of our cliches and catch phrases originate with the common man, as it were. And generally, many of them have fairly ancient origins. I would guess there weren’t as many engineers and physicists in, say, 2500 B.C., as there were carpenters, farmers, ditch diggers, donkey wranglers…
Well, I’m not going to defend this point to the death, but it seems to me that no one has really said anything to make me think that my old physics teacher was full of hooey.
casdave writes:
Actually, I think it was the Maxwell rule I was remembering.
Geet writes:
Actually, this is a key point. The distinction between these trades used to be a lot fuzzier than they are today.
Cecil skirted the big issue here, alluding to it only in his comment “I know it looks like I’m on some sort of rabid antifeminist crusade here. But at least we’ll keep the etymologies straight.”
Cecil has had three recent columns debunking some of the myths coming out of the radical feminist movement: 1) violence against women on Super Bowl Sunday, 2) numbers of children living with lesbian moms, and 3) the “rule of thumb” nonsense. While Cecil slays nonsense wherever he finds it, the pickings seem to be especially easy in the “women’s studies” arena.
A web site calling itself “The Debunker’s Domain” (http://www.debunker.com/) by Robert Shaeffer has several pages devoted to debunking this junk. He says:
What about “light-year” and “quantum leap”? They originated
in physics and are now part of everyday vocabulary
(even though they are used incorrectly by most)
So, the term “rule of thumb” Couldn’t possibly have come from those “ancient laws”? Many of our other phrases have come from British common-law, so why does he insist it’s American?
I think the point is that there were no ancient laws, or that at least there is no evidence of any such laws; rather a form of this UL was already in circulation in the 19th century-we are not the first generation to pass along great stories. In this earlier form, however, no connection was made between this mythical law and the phrase “rule of thumb”; that is a modern addition, the evalution of which can be traced.
I agree with Manda Jo. You can find citesfor the term in American court cases in the 1800’s, but not in British law cases cited in Blackstone(late 1700’s). Of course it could be a British expression. It probably was. It just had nothing to do with the law. Probably used by British carpenters. Or 17th Century electricians:D
Actually, I think right-hand rule and left-hand rule came from mathematics, from vector multiplication (“cross product”). However, that has nothing to do with “thumb” per se, it’s the right hand rule. You use your thumb, your index finguer, and your middle finger. Why would that expression give rise to “rule of thumb” rather than “rule of index finger” or “rule of middle…” er, well, drop that last one.
I, of course, agree with the Master’s detailed explanation of the phrase “rule of thumb”. I wish, however, he’d expanded on it’s origin as a measurement. Actually, the disance from the distal knuckle of the average male’s thumb (especially in the days of yore) approximates an inch; from whence that “standard” of measurement arose.
Yes, Dex, they do come from the vector cross product, but in all of the math and physics that I’ve seen, cross products are used a lot more commonly in physics. The area vector is really the only purely mathematical use I know of for it (and even that’s used a lot in physics), but in physics, you’ve got torque, all the magnetic phenomena, Coriolis force, etc. To boot, unlike the dot product (which can be defined for any vector space), the cross product is strictly an artifact of three dimensional space.
I’m not questioning it’s utility, Chronos, I agree with you entirely that the vector cross-product is more useful in physics than in mathematics. I’m questioning whether the origin is physics (incorporated into mathematics) or whether the origin is mathematics (utilized by physics).
[sarcasm] Another point: just because there was no such ancient law, is no reason that the term can’t be derived from that non-existent ancient law. Which is to be master, that’s the question, as Humpty Dumpty so aptly put it. [/sarcasm]
Back in my college days we learned the various right-hand rules for cross product, magnetic fields, and so on, but we reserved a special “left-hand rule” for use when entering the room to take a tough final exam. Holding the left thumb, index finger, and middle finger in the familiar orthogonal fashion and placing them to the side of the head, we’d say “Head up, eyes straight, take it in the ear.”
I noticed during my all-too-brief stint as a carpenter’s helper that the width of my thumb (not the distance between knuckles) was about an inch. While I acknowledge that hand/digit sizes differ, it seemed to me at the time that in the absence of a more exact measuring stick, a guy could just lay his thumb against something, mark it off and be pretty sure that it was roughly an inch. I know this is inexact, but in the case of some rough cuts or estimates to be done in small increments, the “rule of thumb” would be good enough. For smaller measurements, on the other hand, you would always want to have a red c-hair handy.
)conductor with all fingers ensuring that the thumb points toward the lower potential. The way your fingers wrap around gives the direction of magnetic field.