The Metre

The metre
In the discussions leading up to the French adoption of the metric system in 1791, the leading candidate for the definition of the new unit of length, the metre, was the seconds pendulum at 45° North latitude. It was advocated by a group led by French politician Talleyrand and mathematician Antoine Nicolas Caritat de Condorcet. This was one of the three final options considered by the French Academy of Sciences committee. However, on March 19, 1791 the committee instead chose to base the metre on the length of the meridian through Paris. A pendulum definition was rejected because of its variability at different locations, and because it defined length by a unit of time. (However, since 1983 the metre has been officially defined in terms of the length of the second and the speed of light.) A possible additional reason is that the radical French Academy didn't want to base their new system on the second, a traditional and nondecimal unit from the ancien regime.
Although not defined by the pendulum, the final length chosen for the metre, 10⁻⁷ of the pole-to-equator meridian arc, was very close to the length of the seconds pendulum (0.9937 m), within 0.63%. Although no reason for this particular choice was given at the time, it was probably to facilitate the use of the seconds pendulum as a secondary standard, as was proposed in the official document. So the modern world's standard unit of length is certainly closely linked historically with the seconds pendulum.

Copied from Wikipedia. http://en.wikipedia.org/wiki/Pendulum

I think its there!

Sorry its been a while since the last installment, but its not like I have stopped development. Quite the contrary! I wasn't so happy with the last prototype of the RB2 from the point of view that it still had a degree of non-user-friendliness.
The biggest problem I think was the multitude of pulleys. The pulleys are there to allow the clock run for a number of days. For instance, you can have a 1 kg weight driving the clock where it may take the weight 1 day to reach the ground, or you can have a 5 kg weight going through 4 pulleys (which gives a reduction of 5:1) which will allow the clock to run for 5 days. The problem is that the pulleys make rewinding, setup and construction difficult, they also add more and more friction every time another is added. So I worked out that I could use a extra set of gears on the drive train to do the gear reduction for me instead of using pulleys. Heaps easier to make 2 gears, and way less complicated to setup.

I have also developed a clutch type system so that when setting the time on the minute hand, the hour hand moves with it the appropriate amount!

The third big change with this version of the RB2 is the redesign of the Grasshopper. This design changes the pendulum swing from 6 degrees to 3 degrees, which reduces the amount of energy the clock needs to run and also is more accurate. I think it looks nicer too.

The 4th design change is the use of different tooth profiles on the gears and pinions. This version doesn't have the full compliment of new teeth, but other design changes made to the RB3 have shown the new designed teeth to introduce less friction.

The 5th design change was also part of the re-gearing of the drive and getting rid of most of the pulleys. The grasshopper needs a constant drive pressure for accuracy, since it is a recoiling escapement. The problem with the old drive system was that it used a large amount of string around a 30mm barrel. So fully wound the barrel's diameter would build out to around 40mm. So unwound the torque exerted by the barrel was created by a 15mm radius, and fully wound the torque was created with a 20mm radius. That is around a 33% difference in torque! That difference in torque didn't make the clock's accuracy pathetic, but it could be made better. The new geared drive uses 2/5ths of the amount of string and it is wound on a 60mm barrel, so the torque change from wound to unwound is now around 5%. I am very happy with that.

The 6th design change to the clock is yet to be prototyped, but does exist in CAD form. It is an inside out Gravity Ratchet. The original design has been explained in an earlier blog entry. The reason for changing the design is simply for aesthetic reasons. The new barrel which holds the string has been made to have a 60mm diameter instead of a 30mm diameter as per the earlier explanation of the design change #5. Because the barrel is now so large, it blocks the view of being able to see the original enclosed ratchet working, so our with the original and in with the inside out version me thinks!

This version has been a real success. It sprung to life the first time I swung the pendulum and just kept going! It has been running constantly now for at least a month with 1 little hickup in the middle, so I am very pleased with it.
All in all, the clock may look pretty much the same, but it is very different.
Hope you enjoyed the read.

cheers
rosco