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  #1  
Old 05-28-2001, 11:03 PM
galyean galyean is offline
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While reflecting on high school experiences this evening, I recalled a project given to us by our physical science teacher. His instructions were to build a device to measure time excluding sundials.

Most students brought candles, but the teacher demonstrated their fallibility by turning on his desk fan which caused them to burn faster. A few students poked small holes in buckets or bottles and measured the time it took for the contents to empty. He appeared less than satisfied with our creativity, yet he didn't clue us in on what he thought to be a satisfactory solution.

What homemade time measuring devices can the teeming millions come up with? How reliable are they? Is there a chemical reaction that takes a given amount of time consistently? How long can the device reliably measure time without human intervention?
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  #2  
Old 05-28-2001, 11:12 PM
Finagle Finagle is online now
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Well, the hour glass and it's variations work pretty well. Somewhat sensitive to vibration and maybe temperature.

Pendulums work quite well, except that you have to either count very accurately or build an indicator to keep track of the swings. And then you might as well build in some counter-weights to swing the pendulum and call it a clock.

You could, if the climate cooperates, build a floral clock in which the flowers blossom at various hours of the day. Not particularly accurate, but if that's the level of technology you've got, no one is going to be on time anyway, so who cares?
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Old 05-29-2001, 06:22 AM
partly_warmer partly_warmer is offline
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This is one of those subjects that can get as complicated as you like. Just let me say first that your teacher cheated by turning on a fan. Just because it's possible to mechanically disturb a clock doesn't mean it's a poor clock in all conditions! Put the candle in a jar!

How accurate you can get at home depends on how much you want to spend. One of the factors that wasn't mentioned is that any system that can be reproduced can get more accurate time simply by putting several together, and averaging the result.

Part of the choice would depend on what time interval you were measuring over, too. 1 second, 1 hour, or 1 year?

The water clock is not a bad idea, especially if there is a way to keep the top at the same level (using an overflow). That way the pressure is constant on the other flow, which is the one that would be used for timing. Just don't let your professor put a Bunsen burner under the water.
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Old 05-29-2001, 06:58 AM
Chas.E Chas.E is offline
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Quote:
Originally posted by partly_warmer
Put the candle in a jar!
umm.. a candle will go out in a sealed jar.
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Old 05-29-2001, 08:17 AM
DrMatrix DrMatrix is offline
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Quote:
Originally posted by Chas.E
umm.. a candle will go out in a sealed jar.
Umm. . . So, don't seal the jar.
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  #6  
Old 05-29-2001, 08:24 AM
KneadToKnow KneadToKnow is offline
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You could make a crude hourglass out of sand and two plastic soft-drink bottles (I'd use 3-liter ones myself), though it wouldn't be very stable until most of the sand fell through.

As partly_warmer pointed out (BTW, welcome to the SDMB), it depends on the amount of time you want to measure. If you're trying to time an egg, that's one thing. If you're cooking a turkey, that could be something else.

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  #7  
Old 05-29-2001, 08:34 AM
Some Guy Some Guy is offline
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Quote:
Originally posted by galyean


Most students brought candles, but the teacher demonstrated their fallibility by turning on his desk fan which caused them to burn faster.
Was it an oscillatingfan; i.e., did it move back and forth? If so, you've got the basis for a pretty good timing device right there - all you would have to do is standardize one position to measure it from, then time the interval in which it moves away from that point and cycles back. (It's not necessarily simpler to use one of the endpoints, since fans tend to pause at both extremes of their motion range).

You should have just nabbed the fan...
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  #8  
Old 05-29-2001, 09:15 AM
erislover erislover is offline
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Less than satisfied with your creativity? Hmm.

I would wonder what sort of time he expected to be able to measure. That is, are we making a replacement clock? Or are we merely creating a device that could time something?

Anything with regular motion can be a clock. We used a rotating wheel (powered by a motor) in physics class to make velocity and acceleration measurements (it had a string and small ball attached to it which would make marks on paper as it moved through).
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  #9  
Old 05-29-2001, 09:53 AM
Patty O'Furniture Patty O'Furniture is offline
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Are you allowed to build a proper clock? If you have the skills, this seems to comform with the rules. The easiest clock to build is a simple counter that counts pulses from the power line. In the US, they occur at the rate of 60 every second, which can easily be scaled down for seconds, minutes & hours.

If use of any kind of technology is out, then the pendulum is my choice. You'd have to hang it from a high enough place (at least three stories, I think) so that the earth's movement will drive it. The easiest place to do this is at the north pole, where the pendulum's direction of travel will change 1/24th of a circle every hour (about 15). As you move further south, you have to take into account that the direction of swing will change more slowly. In Washington DC, where the Foucault Pendulum was swinging in the (I think) museum of natural history, the change was only 9 per hour. So the markers (pegs or whatever) that the pendulum's bob knocked down would have to be spaced at the correct intervals depending on your location between the pole & equator. BTW, at the equator there would be no change in the pendulum's direction of swing, and in the southern hemisphere it will swing in the opposite direction.

The trick is in keeping the oscillation from damping out without affecting its natural direction of swing. According to a site on the Foucault Pendulum, they say:

Quote:
Like all pendulums this one loses a bit of energy with each swing due to friction from air currents and vibrations in the cable and other factors. Thus, left to itself the pendulum would swing in shorter and shorter arcs until after a few hours it will decrease almost to zero. To keep the Foucault Pendulum going, one must replace the energy lost with each swing. This can be done by giving the pendulum a little "kick" with each swing.

To do this, two iron collars are attached to the cable near the top. There is a doughnut-shaped electromagnet built into the ceiling, and the iron collar swings back and forth inside the hole of the doughnut. When the pendulum cable reaches a particular point in its swing, it is detected by an electronic device and the magnet is turned on at just the right time to give the collar (and thus the cable and the bob) a little "kick" in the exact direction of its natural swing. This restores the energy lost during the swing and keeps the pendulum from stopping. It has no effect on the direction of the swing, and thus does not interfere with the demonstration that the earth is rotating.
Building a pulse counter sounds 10 times easier to me.
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  #10  
Old 05-29-2001, 01:46 PM
astro astro is online now
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What did this teacher expect you to do? Start forging main springs and assembling escapements?


If you can built a device out of Radio Shack parts and a used 10' satellite dish to measure the rotation period of rapidly spinning neutron stars that's a pretty accurate time standard.


Now where did I put that cesium...
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  #11  
Old 05-29-2001, 10:47 PM
Chas.E Chas.E is offline
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Quote:
Originally posted by DrMatrix
Quote:
Originally posted by Chas.E
umm.. a candle will go out in a sealed jar.
Umm. . . So, don't seal the jar.
If the jar wasn't sealed, the teacher could disturb the flame with the fan, which was the whole reason for using the jar in the first place.

If this was me, I'd make a water clock that operated more like an hourglass. Back when I was a chemistry student, I used to do some glassblowing, I used to make my own pipette tips that were finer than anything you could buy. The outlet was as thin as a capillary tube, and I used them to get lab results down to 6 decimal points of precision while the other students were lucky to get 2 decimal points. You can get an extremely well controlled flow of water with a handmade pipette tip, and it would not be subject to much disturbance from temperature since the flow rate is termined by surface tension. If that isn't good enough, I'd build a water clock with an osmotic membrane.
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  #12  
Old 05-29-2001, 11:57 PM
Mikahw Mikahw is offline
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Well, you could attach a weight a spring. If the weight and spring are properly matched up, It'll work for a long time. If not, either the weight won't move the spring, or the spring won't bring the weight back up.

The period of the weight/spring combo doesn't change. As long as it's going even a little bit, it will keep time somewhat accurately.

I have no idea what your teacher would say to that though. He might just stop the spring with his hand and say "nyah" in a really demeaning voice..
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  #13  
Old 05-30-2001, 12:47 AM
DrMatrix DrMatrix is offline
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Chas.E

A tall enough jar would function as a chimney and at least make it difficult to affect with a fan.

Mikahw
The teacher couldn't stop the spring with his hand if you put the spring/weight assembly into a sealed jar.
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  #14  
Old 05-30-2001, 01:52 AM
Chas.E Chas.E is offline
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You know, I was just thinking about some old Amish toys I recall seeing around my area. They're complex racks of ramps, you put marbles or ball bearings into the hopper, and they run down the ramps one at a time, through little mechanisms, tripping little levers and moving little cogs and etc. When the marble gets to the end of the run, it trips a little lever and one more marble is released. And then it occurred to me, that's similar to Galileo's original gravitational experiments with timing the rate of balls rolling down ramps. You could make a nice little clock that uses rolling marbles down ramps as the timing element. Gravity is a constant so the rate the marbles move can't be affected by anything local unless it has the mass of a small planet.
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Old 05-30-2001, 02:49 AM
Little Nemo Little Nemo is offline
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Just bring in a sealed jar of air and tell him that statistically speaking, there's an atom of Cesium 133 in the jar, vibrating at exactly 9,192,631,770 each second.
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  #16  
Old 05-30-2001, 05:30 AM
Autumn Wind Chick Autumn Wind Chick is offline
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A little off-topic, but I think your teacher had a particular idea in mind. Any other ideas are not on the One True Path to Enlightenment and must be rejected. Sorry to slam your teacher, but I think he had this common failing.

And whatever device he constructed, I could slow down with a hammer.

Faced with this problem today, I think the thing to do is to run some internet searches for canned lesson plans. (I doubt that your teacher was using his own material.)

A quick google search found the following:

"2.6 Measuring Time. STC. (Developed by National Science Resources Center, Washington, D.C.) Burlington, N.C.: Carolina Biological Supply Co., 1994.

Grade: 6 In Measuring Time, students explore timekeeping first by observing the natural cycles of the sun and moon and then by building and investigating mechanical devices designed to measure time. . . ."

Once you've figured out which lesson plan your teacher is using, you simply look up the device, and figure out what your teacher is driving at.

Yes this is cheating in some sense, but keep in mind that otherwise, the game is hopelessly rigged against you.

Autumn
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Old 05-30-2001, 08:19 AM
KneadToKnow KneadToKnow is offline
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Quote:
Yes this is cheating in some sense, but keep in mind that otherwise, the game is hopelessly rigged against you.
How'd you do on the Kobayashi Maru test, Autumn Wind Chick?
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  #18  
Old 05-30-2001, 09:54 PM
Sam Stone Sam Stone is offline
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We don't have enough parameters anyway to make a reasonably informed decision. How long must the clock run? How accurate must it be? What equipment can you use? Does it have to create a record of the time, or is it enough to just observe it?

All of these are going to vastly change the answer. For example, if you only have to time something for a minute or so, then you can use lots of things like a spring and a weight, or a pendulum, or a homemade hourglass, etc. If the time scale is on the order of days, then a lot of these won't work unless you can either build with extremely high precision or to a massive scale.

The pendulum is probably the simplest. The period of a pendulum can be determined accurately given its length. Make it long enough and heavy enough, and the amplitude won't matter either. 2*pi*sqrt[L/g] where L is the length, and g is the force of gravity acting on the pendulum. Notice that amplitude (how far back you pull the pendulum) and the weight of the bob on the end aren't factors in the equation - that means it's easy to build one that will do what you want, and it also means that its period won't change much as it slowly comes to a halt. As long as its moving, it's recording accurate time.
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Old 05-31-2001, 02:22 AM
roryaxis roryaxis is offline
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I would have just gotten up in front of the class and started counting...my 'one one thousand, two one thousand..." is probably as accurate as a clock in a jar or a bucket with a hole in it, and I'm not a sundial. I can perceive the flow of time and can relate that information to another person in a somewhat accurate fashion, therefore, as far as 'thinking outside the box' type creativity, I think it's a good solution.
Of course, I didn't actually build myself, although one could argue many metaphysical ideas about that one.
Perhaps you could have gotten and a male and female classmate together and in nine months plus a few years you could have something that could count the numbers, a sort of clock, and I think that making a baby is far easier than building a three story pendelum on the north pole, and it fulfills the assignment.
That's pretty creative, in my humble opinion.
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  #20  
Old 05-31-2001, 07:14 AM
grimpixie grimpixie is offline
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Quote:
Originally posted by Chas.E
Quote:
Originally posted by DrMatrix
Quote:
Originally posted by Chas.E
umm.. a candle will go out in a sealed jar.
Umm. . . So, don't seal the jar.
If the jar wasn't sealed, the teacher could disturb the flame with the fan, which was the whole reason for using the jar in the first place.
Why not seal the fan???

pG
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