Re: [Phys-L] Allan deviation

[Bernard Cleyet <bernard@cleyet.org>]

On 2015, Jul 12, , at 15:45, Bennett <bennett@oakland.edu> wrote:

> I am visiting my Niece in New Jersey.
>
> I am observing her Grandfather clock.The pendulum hangs by a  very thin
> brass strip,  about 3/4" wide in the top 15 inches, and an array of
> parallel rods below, and a large disk.

Is the strip the suspension, i.e. its top is attached to the support.   Or is it part of the rod?  Some rods are as you described.

The array of rods is called a grid iron —an early temperature compensation.**  If they’re not of different metals they are fake.. I suspect about 99.9% are fake.  Incidentally, the big Ben clock uses a modified “grid iron”.  [iron rod 14’5’; zinc tube 10’5” and an outer iron tube of the same length, which carries the 230kg bob; {p48 Rawlings 3rd ed.}  The Westminster web site, IIRC has much more, including the fortuitous failure; fortuitous, as allowed some horologists to make measurements.

More:  The zinc over the decades gradually compressed requiring added pennies to keep the clock “in beat”.  Finally, in ’38 the existing pennies were replace by a single cast weight.  

> When I look from the side. the disk tends to twist, at a faster rate and
> seemingly erratically.

Yes this drives repairers crazy.  The fault may be in a bent, or bowed suspension spring.  Since you wrote brass, it’s a thin rod.  Did you obtain a pic. of the suspension?   How about the escapement?  Does the crutch engage the thin brass?  (unusual)



>   Very Interesting!
>
> I have taken pictures and will try to enlarge them when I get home.


** A successful invention by John Harrison, of longitude fame.  

bc wonders why Bennett didn’t bring it with him home.