rickman wrote on 8/5/2017 11:08 AM:
Gareth's Downstairs Computer wrote on 8/5/2017 9:57 AM:
On 05/08/2017 14:34, Chris wrote:

Exactly. The control is single path, master to slave, with no feedback
to the reference, making it an open loop design. The master has no
knowledge of the state of the slave at any time.

Untrue.

The matter starts off when the slave signals to the master and drops
the gravity link in the master, then, when the master pendulum is in
a position to accept the impulse from that dropped gravity link, it
signals back to the slave

But ... I'm still trying to google for the exact mechanisms because
most URLs only hint at what is happening. (I'm also awaiting delivery
of a couple of hope-jones' books about electric clocks)

What you are describing is how the phase measurement of the master is made.
The gravity lever is simply a remontoire providing a consistent push to
overcome the force of friction. It is designed to be invariant of small
changes in timing of its release. You can see that in the animation linked
below. The gravity arm is released at the point when the wheel is directly
under the end of the gravity lever. A small change in timing changes the
force only a tiny amount. This is critical to maintaining the swing of the
free pendulum without affecting its period.

http://www.chronometrophilia.ch/Elec...cks/Shortt.htm

The animation happens in real time so it is hard to see the details of what
is going on. The gravity lever and accompanying control is the magic of the
clock. The rest is pretty straight forward. You need Flash to view this
page. There is a button to see the wires.

One other part of the Shortt clock that requires careful thought is the
relay and spring that perform the phase detection and correction. The slave
pendulum has a leaf spring parallel to the rod and the control relay has a
pick which is activated under control of the master gravity lever. The pick
can intercept the leaf spring or not, depending on the timing. There is an
issue with this which is impossible to eliminate, only minimize and that is
metastability. A decision is being made and it can not be done with
infinite resolution. So the pick and leaf spring must be designed to
minimize the problem, likely done by making the spring thin as possible and
making the edge on the pick as sharp as possible.

We see the same problem in electronics when trying to make decisions on the
state of an input that is changing.

--

Rick C