Member postings for John Haine

I gather that the RAS Regulator reproduction under test at Upton Hall (one of several built), based on the same design principles as Clock B but much more representative of Harrison's methods and materials, is performing very well though I don't think details have been published yet.

Dave, I've put a pot on the frequency control input and just left it half way up for the moment. It would be nice to set it more precisely but it's likely to be near enough for all practical purposes. Now having a picPET on the board, if I had a GPS 1pps signal I could just feed that in and adjust the pot to get precisely a 1 second period as measured at the data output...

I'm using a BME280 also, I had some problems with the recommended Python library but found you actually need to use an old version to make it work!

Very promising Dave! I've reached the point of re-writing my Python logging code for the new clock, having lashed up a pP07 and one of those OCXOs on a bit of veroboard. What environmental sensor are you using? With luck I can start logging tomorrow....

Dave, I sent you a PM.

The key aspect of Clock B is that the pendulum automatically compensates for barometric pressure variation by balancing the variation in rate caused by buoyancy against the opposite change in rate caused by amplitude change resulting from varying air drag. It is of course temperature compensated by using an invar pendulum rod and by other means. It's a complex system that is very hard to "design" as not all aspects of it can be modelled but have to be set up on test.

Any pendulum that is allowed to vary its amplitude according to air drag has a nominal amplitude where it has zero first-order sensitivity to air density variations. So it would be possible to apply the same method as Clock B does, though with much less complication.

An alternative could be to post your complete article to a file sharing site such as Dropbox or Google Drive, and then post a link to it from here (possibly with a couple of tantalising photos). Of course the site owners would rather you submitted an article for publication which might be why the site doesn't allow uploading of any files other than jpgs!

Just a thought, but today's super-magnets might be a good alternative. As well as being more powerful than traditional types, they retain magnetism extremely well. Might be good enough to make a low-friction pendulum bearing. Anyone tried?

Alas, it is impossible to make a stable fully constrained bearing using static magnetic fields because of something called Earnshaw's Theorem. You can suspend diamagnetic materials, such as a frog, but they are weakly magnetic and unable to take a large load. It is possible to make a bearing which has one mechanical constraint, for example a knife edge suspended from a strong magnet pole, which works just like a knife edge but can have much reduced load on the knife itself. I recall seeing a balancing device that had a ground steel shaft with a concentric point at each end, which was "suspended" horizontally between the poles of a strong magnet so that one point was sitting on a pole face and the other held in mid-air by the field.

But anyway, I think it is true that the energy dissipated in the suspension is orders of magnitude less than air resistance for normal arcs. I have done some run-down measurements that suggested that pendulum Q increases slowly as the arc decreases, then drops quite quickly for very low amplitude, which might be because the spring loss is becoming dominant.

and look where that got 'em! All degrading rapidly now...  I used to work for Post Office Telecommunications in the early 70s when they were building Milton Keynes.  A great opportunity for the PO to showcase technology so we developed and installed an advanced CATV system, alongside the brand-new aluminium phone cables, so the new residents would have a good phone service and what was then the state of the art in "broadband".  FF 50 years and the aluminium cables are all degraded and I understand that the broadband service in MK is awful.

Edited By John Haine on 03/02/2023 10:50:37

I think you need the same thread handedness at each end? You rotate the screw, it moves into one part and out of the other but different amounts.

To be clear, I meant a screw which has different threads at each end, one screws into one part while the other screws out of the other. So if the pitches were 1mm at one end and 0.9mm at the other, 1 turn would move the parts relatively by 0.1mm. I've seen these use a lot on (electron) microscopes. The trick could be to find two standard pitches that are very close, quite possibly one metric and the other imperial so their pitch difference is very small.

Differential screws?

Lots of ideas for solutions but far too little information on symptoms!! Please post your video on YouTube, I don't think anyone is going to want to ask you to send it then view it then puzzle out what might be wrong.

There was a recent Which? review of AA/AAA batteries which is worth looking up. Overall Aldi were much the best value for money!

There's a certain amount of post-hoc rationalisation here. The shape of the "impulse slope" of the Synchronome pallet is essentially a circular arc or 0.75" radius. Apparently that's what they were made with but when F H-J was presenting a paper at the IEE, William Shortt who was in the audience worked out what the shape should be to apply a "raised cosine" impulse that started gradually, built up to a maximum, and then rolled off again to zero, but that shape isn't the circular arc. With the circular arc I think the idea is that the roller drops on to the pallet just before the start of the slope but that depends on the clock being very precisely set up. There's a time-lapse video on-line (or used to be, the page seems to have been removed from Brian Mumford's site) that shows this is an idealisation. Actually, except for considerations of mechanical "ringing", the shape of the impulse is virtually irrelevant for any reasonably high-Q pendulum, all that matters is the position of its phase centre. Nasty spiky forces just amount to higher order harmonics that the pendulum filters out.

Alas, Fedchenko's clock that impulses one per cycle at the base of the bob, rather shows that F H-J was wide of the mark in his conclusion.

Just a data point, my small clock has a 25cm pendulum with a tiny neo magnet on the end swinging above a small air cored coil which has a resistance of about 200 ohms. It's impulsed for about 15ms every other swing or so from an Arduino output through another 220 ohm resistor. You really don't need much energy or force to impulse a reasonably high Q pendulum.

Impulse timing matters if the strength or phase of the impulse changes. If the phase centre of the impulse is at the centre of the swing it doesn't change the pendulum phase.

Sweet! You can buy 1/8 dia shank carbide engraving cutters in 10s for not much on ebay.

There was one looking a bit like this on the contactor box on my VMB mill. Does it have a flexible silicone cap? I think it's actually an air-pressure operated emergency switch, which can be linked by a tube to some sort of bellows to link up a remote e-stop. It was one of the several bits of the contactor that gave up the ghost!

Hi Vasantha, that sounds very interesting, thanks for sharing this. Google fails to find the software from the description, please could you post a direct link?

In my view one should avoid any magnetic material in the fixed part of the coil system. There clearly has to be some somewhere to be attracted by the coil, usually on the pendulum. But you want to avoid any spurious forces from remanent magnetism. With any reasonable Q the force needed for impulse is so tiny that a core should not be needed.