Sympathetic Vibration and what causes it

Describing Pendulum bob magnetism 

Make sure if your building a stand that it is strong enough to take the weight is one thing,

building a test stand that is rigid enough to prevent it flexing is
another. Here is an explanation you can pass to your customer to
convince him.

Sympathetic vibration happens when two pendulums are connected via their
suspension points, and one is set in motion. Some of that motion will be
transferred to the other pendulum by tiny movements of the mounting board.

If the natural period of the two pendulums is different then there is
little significant effect. However if the two pendulums have the same
natural period (i.e. are the same length) the mutual effect on each
other is magnified greatly, and may eventually stop the clock. (Note
that as far as the physics of the matter is concerned a drive weight
hanging on a line is just another pendulum.)

Therefore to prevent Sympathetic vibration stopping the clock, the seat board and/or the
case must be prevented from moving.

Here is a demonstration of the principle taken to an extreme. Five
metronomes adjusted to the same rate or period are placed on a board
that is free to move on rollers. Note they are started out of phase with
each other, but energy transfer via the seat board gradually synchronizes
them, at which point seat board movement increases

https://www.youtube.com/watch?v=T58lGKREubo

Clifford Coggin
Kent
England