On 6/23/13 10:48 PM, DaveH wrote:

the only problem is that you don't have a very accurate measurement of
the microwave oven frequency and the mode pattern isn't very "sharp".
So this might get you 1 sig fig. Granted, most folks only use 1 sig fig
3E8 m/sec, but that's just a happenstance since c happens to be close to
a round number.

And that gets back to another time-nuts kind of question..

How accurately can you measure length and time?  (in a science demo sort
of way.. without getting a Rb or GPSDO, etc.)  For most school age kids,
the sources of time available are fairly lengthy (e.g. 1 second ticks
from wwv by phone, stopwatches built into iphones, etc.)

Tape measures and meter sticks are readily available.

The time issue was effectively eliminated by the Michaelson-Morley
interferometer.  One used a monochromatic light and an array of mirrors
which split the light in opposite directions around the track.  The two
beams were recombined and an interference pattern resulted.  One counted
the number interference fringes passing by  while moving one mirror in
one path.

Knowing the number of fringes, wavelength of light and the mirror
movement, one could compute c.

http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

Actually used one of these in a physics lab in about 1962.  Produces a
quite reasonable estimate of c.  Other methods could be used to
accurately know the wavelength of light.

The other experiments to measure the gravitational constant G were
equally interesting.

The real reason for the interferometer was to try and detect the effect
of the "aether" which light was supposedly propagated in.  To everyone's
surprise, there was no detectable "aether".

Brian

On 6/24/2013 11:26, Jim Lux wrote:

No virus found in this message.
Checked by AVG - www.avg.com
Version: 2012.0.2242 / Virus Database: 3199/5936 - Release Date: 06/24/13

Using only "moderately" accurate equipment, like mechanical clocks and
meter sticks Albert Michelson has able to measure the speed of light and
determine it was a constant in all directions.  It was this work the
prompted Albert Einstein to think about what t means for C to be constant.

They were working at about the turn of the last century or before 1890 to
about 1905 and did not have lasers or HP universal contours.  They used
sunlight.

One experiment was done here where I live up at the Mt. Wilson Observatory.
They put light from a slit onto to a rotating mirror and then bounced it
off a fixed mirror back to the rotating one.  If the speed were infinate
the light would go right back up the slit.  But in reality the light misses
because the rotating mirror moves a little while the beam is in flight.

The advantage of this is that it is a direct measurement of the speed of
light that does not depend on many assumptions and can be done with
technology that was available in the late 1800's  One big limitation is
the atmosphere.  You need very stable air over the long path length

This was the experiment that got Einstein thinking.  He said he started at
age 16 to think about how the light from a moving lamp could be the same
speed as one from a stationary lamp.  It was total non-sense and impossible
at the time.  We have to remember that those experiments at the time were
are considered to be "Failed Experiments" because "C" could not be constant.

On Mon, Jun 24, 2013 at 4:26 AM, Jim Lux jimlux@earthlink.net wrote:

--

Chris Albertson
Redondo Beach, California

IR and acoustic distance sensors already made up with pulse i/o on
breakout boards are available at places like sparkfun (not an ad) or
robot store. In fact, a pair, one acoustic and one ir over the same path
might be very interesting indeed...hmmm
Don

Tom Van Baak

--
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
Skype: buffler2
www.lightningforensics.com
www.sixmilesystems.com

On 6/24/13 10:08 AM, Chris Albertson wrote:

Isn't that the Fizeau technique, which antedates Michelson's?

You need to know the rotation rate of the toothed cog or rotating
mirror, don't you?

You could get that by matching against something like a tuning fork, but
how do you measure the frequency of the tuning fork.

On Mon, Jun 24, 2013 at 1:55 PM, Jim Lux jimlux@earthlink.net wrote:

Michelson got the precision good enough that it finally put the question
to rest.  We used a miles-long baseline of very clear and still air.

I think you have to count the number of rotations over a larger time
interval.  And at the same time make sure the spin rate is constant.  They
used a large hexagon with mirrors on all the faces and spun it up using
some kind of clock work.

You can see if the wheel is speeding up or slowing down because the slit of
returned light should have constant offset.  So the experiment has a
built-in check on the rate remaining constant.  And then you count the
turns over some long interval using gears or what not.  They built a shed
to house this thing on the side or Mt. Wilson.  All that is left no is the
concrete foundation and concrete pier for the instrument.  This was a not
small scale lab experiment.  It must have been well funded to be able to
pour tons of concrete at a remote location like that.

I think the harder part is knowing what the long baseline is.  How to
measure 5 miles distance with the required accuracy in 1900?  Yes they have
survey equipment back them put how good was it?  I I don't think they
needed to know the exact length, just that it was a long constant length.
They were only trying to show the "C" was not constant.  But of course the
experiment "failed"

Chris Albertson
Redondo Beach, California

On 6/24/13 5:21 AM, Brian Alsop wrote:

There are easier ways to measure wavelength of an EM wave (Young's
double slit experiment, for instance), or measuring the voltage along a
transmission line carrying a reflected wave.

BUT, then, an interferometer (or melted marshmallows in the microwave
oven) tells you the wavelength of the EM radiation.  But you need to
know the frequency of that signal in order to calculate c from that
measurement.

and we're back to the "what can be done simply to measure time/frequency"

(I guess.. it's all really arbitrary.. a meter is so many wavelengths of
the light from a Krypton lamp)

On 6/24/13 2:26 PM, Chris Albertson wrote:

An Air turbine?, from the descriptions of "screaming" while in operation.

Measuring the 22 miles from Mt Wilson to Baldy was the work of the US
Geodetic survey. It would have been done by triangulation and reference
to a precisely measured baseline (which I believe was in Pasadena
somewhere).  1 ppm would have been achievable.

(Baseline street in the inland empire is based on San Bernardino peak,
and is the reference for the state plane grid in Southern California)

You need to know the wavelength.  You can get that from a diffraction
grating without any knowledge of the speed of light.

Brian

On 6/24/2013 21:56, Jim Lux wrote:

No virus found in this message.
Checked by AVG - www.avg.com
Version: 2012.0.2242 / Virus Database: 3199/5937 - Release Date: 06/24/13

Hi list !
Searching for "newton jupiter moons speed of light" I found this very
nice page:
http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html
A lot of the early measurements of c are described, with the accuracies
reached.
But most of the older methods are rather difficult to demonstrate to
kids, as they need long series of observations...
Best regards,
Jean-Louis

On 24/06/2013 21:26, Chris Albertson wrote:

--
Jean-Louis Oneto
OCA GeoAzur - 2130 Route de l'Observatoire
Caussols 06460 Saint-Vallier de Thiey - France
email: Jean-Louis.Oneto@obs-azur.fr
phone: (+33)[0]4.93.40.54.25