Hi Neville, Welcome to the world of extreme accuracy... Not to discourage you; but to challenge you -- see answers below.
> My work with a pendulum seems to point to the need to know the exact > phase and amplitude of the pendulum as being a sticking point. Right, but calculate this to the microsecond in time or to the micron in displacement. It's a really hard problem. There is white noise. There is drift. It will affect the stability of the pendulum. There is no "exact" measurement; even if it's time or if it's distance. > The pendulum can be mounted firmly, with no yielding or flexing of > brackets on a masonry mount bedded well into the ground. Sorry, there is no such thing as "firm" and at some level everything flexes or bends. Do the math. A highway I-beam looks super super strong. Until you see one transported on a truck and you notice it bends maybe an inch over a span of 80 feet. That means it flexes by 0.1%! That's a micron per millimeter. Can't have that in a pendulum. Or consider... A pendulum with a 10 pound bob on a ten pound frame will cause the frame to move as much as the bob. Clearly, this is bad. So, hey, use a 100 pound frame. Now the support moves only 1/10th as much. Still bad. Use 1000 pounds; No, use a house foundation that's 10,000 pounds, that's 5 tons. You've still only attenuated the movement of the frame to 1/1000th the lateral movement of the bob. Do you see the problem? The bob moves 1 cm but your frame moves 0.01 mm. Yikes. Do you need 50 tons? Or 500? When dealing with precision pendulums you cannot say something has "no effect". You have to calculate what the effect actually is. It might be small but it is never zero. It will affect your accuracy. > The pendulum can be made mechanically stable with accurately fitted > joints between shaft and bob, and shaft and suspension. There's several hundreds years of history of attempts to achieve this. Very hard problem. > The pendulum can be excited with a minute magnet and an air cored > coil to drive it with a few nanowatts. Careful with the magnet. The earth is a magnet also and the local field changes over time; both over the long-term, and over the short-term (e.g., depending on solar activity). At the level of a precision pendulum, electromagnets are also highly voltage, current, and temperature sensitive. Do the math to see what your lower bound is. There is also drift and perhaps hysteresis in magnetic systems. Nanowatts of magnitude are fine. But can you hold it constant to picowatts over the duration of the experiment? > The pendulum can be shielded from air currents and vibrations. Shielding is relative. Most places on planet earth experience vibrations. Air currents are usually solved only by putting the pendulum in a vacuum. However, that might introduce other challenges. Note that Shortt did not operate in a high vacuum. > However accurate signals for phase and amplitude measurement with > accuracy of microseconds for phase and microns for amplitude seem to > be the challenges that must be met, this information is necessary to > generate the pendulum drive so that the pendulum is the only > frequency determining element. Correct. However, the danger here is that many modern methods to determine precise and accurate pendulum phase and amplitude depend, directly, or indirectly on clever digital electronics, precise timing, or microprocessors; electronics whose very accuracy is driven by, you guessed it, quartz oscillators. When you say "information" you imply a measurement system that "knows" the behavior of the pendulum. So what is the "real" clock then: the mechanical pendulum or the electronic measurement system that provides feedback to the pendulum? That's a hard question. You have to be very careful that your pendulum is still free and not benefiting, in any way, from the precise measurement and feedback afforded by modern quartz timekeeping. Otherwise what you a building an "almost-free" pendulum clock that is simply governed, or stabilized, by a quartz oscillator. > Compensation for temperature and barometric pressure are do-able, > although the legendary clocks were in a vacuum (more or less) and in > underground clock vaults kept at constant temperature. Nothing is constant. Being underground just means that your temperature doesn't vary by degrees, it varies by tenths of degrees. Again, do the math; determine the actual tempco of your system. > I still have hopes of getting great performance from a room > temperature clock at ambient pressure. I applaud your efforts. The proof will be in the data you get from your clock. It might be possible. It certainly would be very interesting. Expect to collect a year of data to be sure. Keep us, or the members of NAWCC HSN 161 updated. Several HSN members, I think, are trying to match the performance of the 1920's Shortt pendulum clock. If you want to write more, perhaps time-nuts isn't the best forum; I think we're mostly modern electronics timekeepers here. /tvb > See > http://ph.groups.yahoo.com/group/synchronome/photos/view/c1ba?b=1 > > cheers, Neville Michie > Can't see that URL from here. Can you email it to me, offline? _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
