Wikipedia:Reference desk/Archives/Science/2018 December 16

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 16

Galileo pulse meter

According to this book, Galileo invented a pulse meter based on a swinging pendulum. By adjusting the length of the string of the pendulum, the user could alter its speed to match their heart rate. That particular speed would correlate with a particular length of string, and there was a scale measuring how long the string was at a particular time, thus telling you your heart rate.

My question is; how did the user insure that the pendulum maintained a constant momentum so that variations in speed could only come from variations in the length of the string? As I picture this in my mind, I imagine that the user would have just a few seconds to measure the patient’s pulse and adjust the pendulum accordingly, before the measurements became inaccurate. Puzzledvegetable (talk) 00:04, 16 December 2018 (UTC)

I don't know what your source exactly says but a pendulum has a nearly constant period and not speed. When the motion slows down due to friction and air drag, the period doesn't. Pendulum says: "The period depends on the length of the pendulum and also to a slight degree on the amplitude, the width of the pendulum's swing." If the source said speed then it probably meant period. PrimeHunter (talk) 00:30, 16 December 2018 (UTC)

@PrimeHunter: Thanks for explaining. I found a very helpful article online that clarifies how a pendulum works. According to explainthatstuff.com: "If there were no friction or drag (air resistance), a pendulum would keep on moving forever. In reality, each swing sees friction and drag steal a bit more energy from the pendulum and it gradually comes to a halt. But even as it slows down, it keeps time. It doesn't climb as far, but it covers the shorter distance more slowly—so it actually takes exactly the same time to swing. This handy ability (technically called isochronism, which just means "equal amounts of time") is what makes a pendulum so useful for timekeeping." Puzzledvegetable (talk) 01:29, 16 December 2018 (UTC)

Note that a metre is (almost exactly) the length of a pendulum beating seconds. 2A00:23C0:7903:B200:6D35:1B0:E2BB:692D (talk) 10:14, 16 December 2018 (UTC)

From 1789^{Citation requested} to 1889, the definition of the metre was one ten-millionth of the distance from the pole to the equator.

That's for the historical reason that on 8 May 1790 – The French National Assembly decided that the length of the new metre would be equal to the length of a pendulum with a half-period of one second. These days the second is defined as exactly the time for light to travel 299792458 metres in vacuum. DroneB (talk) 15:13, 16 December 2018 (UTC)

Huh? The why does our Metere arcticle say

"The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole"?

Also see History of the metre. which says

"As a base unit of length, many scientist initially favored the "seconds pendulum" (a pendulum with a half-period of one second), but this was rejected when it was discovered that it varied from place to place with local gravity. A new unit of length, the metre was introduced - defined as one ten-millionth of the distance from the North Pole to the equator."

--Guy Macon (talk) 15:38, 16 December 2018 (UTC)

There is no contradiction here, as reading our Metre article, specifically Metre#Timeline will make clear. DroneB (talk) 19:55, 16 December 2018 (UTC)

I don't think that comment, with its useful links, needed to be in small type. However, it does not really address the issue Guy raises. More below. --76.69.46.228 (talk) 07:27, 17 December 2018 (UTC)

My choice of tiny type was for politeness. Specifically I posted a timely correction to my learned colleague Guy after he reacted to my post with an exhalation "Huh?" coupled with a misspelled link to a non-existing article that, had it only been found and read, would have revealed the "Huh?" to be a non-question. Regardless what "issue" IP User 76.69.46.228 thinks remains unaddressed here Guy Macon's [posted claim that the meridional definition of metre dates from 1789] can be challenged. I deduce that it is an error of false precision generated when Guy copied the globe image from History of the metre article with its underlying text hastily paraphrased from that article's lede summary, neglecting the detailed article content that supports 1791. DroneB (talk) 17:30, 17 December 2018 (UTC)

A meter today is the distance the light travels during 1s/299792458. And the speed of light is 299792458m/s Doroletho (talk) 23:24, 16 December 2018 (UTC)

Guy states above the reason why the "seconds pendulum" definition was rejected. According to The Measure of All Things by Ken Alder, these objections were already known in 1790 when the National Assembly was working on the law, so Talleyrand had wording put into it that said the length of the pendulum should be determined at 45° "or whatever other latitude might be preferred", with the French Academy of Sciences to establish a commission to execute the plan. Therefore the law of 1790 was more a plan for a definition than an actual definition, and I think this is why the 1791 definition is usually considered the first one.

Note incidentally that the "seconds pendulum" definition was rejected because it would be tied to a particular location and therefore might not be accepted worldwide, but the "quarter-meridian" definition that replaced it was also tied to a particular location, as it was specifically based on the meridian passing through Paris. But as Alder points out, the commission's chairman, Jean-Charles de Borda, provided a list of good reasons for this. To actually determine the length of the meridian, they would have to survey a sizable chunk of it, through country that was already well surveyed at the time, with both ends of the chunk at sea level, and passing through the latitude of 45°. Which meant a meridian through France from the English Channel to the Mediterranean. And when they actually sent a team to survey it (which is what the book is mainly about), the result was internationally accepted. --76.69.46.228 (talk) 07:27, 17 December 2018 (UTC)

I would put it a different way. It seems to me the key issue is whether there is any connection between the seconds pendulum definition and the later meridional definition. From what I can tell, neither the metre article nor the history of the metre article say there was say there was. The fact that some people came up with the seconds pendulum definition (or whatever you want to call it) is irrelevant if there was no concern about trying to approximate that definition with the new definition, and nothing in our articles suggest there was. While the fact these are similar may seem awfully coincidental, the fact that their definition was "one ten-millionth of the distance from the North Pole to the equator" and their decimalisation outlook and considerations of what would make a useful length makes it a lot less clear it was anything other than a happy accident. In other words, when the meridional definition was being designed, the designers could have said, 'we don't give a damn what definition people came up with before, we just want to choose something which works well'. By comparison, since then the redefinitions have generally attempted to keep the same length. I'm sure you're right that this is mostly because it was never an official definition but a working plan that was abandoned. So they had no real reason to try and approximate the older plan/definition, but later they actually had metre that people had been using for a while so changing the actual length in any significant way would have been insane. And all this means Guy Macon was right to question the claim being made. Nil Einne (talk) 15:51, 19 December 2018 (UTC)

What happens to an electrical device when it gets the wrong power?

Too much, and you can burn a fuse (or whatever). But what if you get too little, like connecting a 220V device in a 110V power source or one source with too much variation? Or what happen by the wrong Hz? I imagine my microwave oven, which sets its clock through the power, won't show the right time, but what else? What's the worst that can happen in terms of damage?--Doroletho (talk) 23:19, 16 December 2018 (UTC)

• It's complicated. There are many sorts of device, there are many failure modes. In general, "inefficient" operation leads to excess heat dissipated in the machine, and that can cause overheating and an insulation failure.

A 60Hz motor or transformer will have sheet iron laminations in its armature. The losses here increase with lower frequencies, so although many 50Hz machines of similar power work just fine throughout Europe on 50 Hz, an imported US motor designed badly for 60 Hz with no margin beyond that, will overheat. This is a common failure in imported workshop machinery.

A powerful 12V motor, such as a car engine starter motor, won't have enough power to turn the engine fast enough to start if its run under-voltage. This - mostly the tendency to run the starter for too long - is a classic example where a motor (normally only designed for brief use) will easily overheat.

There can also be problems with induction motors, where 'slip' becomes excessive. If the motor is adequately powerful to run at the correct speed, then it works fine. But under-voltage it may lack power and so run too slowly. This has a secondary effect where the 'slip', a difference between the physical speed of the rotor and the speed of the rotating field, becomes excessive. This increases the induced current in the rotor (induction motors transmit power to their rotors by induction, not through brushgear) and so the rotor may burn out. Normally a small reduction in speed acts as a feedback mechanism, so that the increased slip increases torque and causes the motor to safely speed up to its designed speed - however too much of this drops out of the controllable region and becomes a failure. Andy Dingley (talk) 23:31, 16 December 2018 (UTC)

Your microwave clock is probably controlled by a crystal, not the mains, unless you have an analog clock on it! I run my immersion heater off a 110V transformer in my 240V off grid house to reduce the peak load on the batteries, but it would take four times as long to come up to temperature. Greglocock (talk) 23:40, 16 December 2018 (UTC)

I had exactly that problem at a performance last week. My amplification for my upright bass, which had been very reliable previously, failed utterly; I was getting no sound. Had to play unamplified, not as planned. Turns out the problem is I had two wall warts in my system, one rated 12v 500ma, the other rated 9v, 1000ma. Of course, they had identical connectors. The device that wouldn't work -- a little tube preamp -- wanted the 9v 1000ma. Nothing died; but no sound. I hate wall warts. You should too. --jpgordon^{𝄢𝄆 𝄐𝄇} 05:10, 17 December 2018 (UTC)