[@NickDay2]

@NickDay2 asked the Naked Scientists:
   
If the period of a pendulum is dictated by its length, not the weight of the bob, why do they put pennies on Big Ben's?

What do you think?

[MikeS]

In all honesty I don't know but I suspect it could be something to do with the the inefficiently of the escapement.

[Geezer]

Adding pennies to the bob effectively shortens the length of the pendulum. The center of mass of the bob with the pennies is slightly closer to the fulcrum, so the period will be reduced, very slightly.

[yor_on]

That one is very clever.

"The pendulum is installed within an enclosed windproof box sunk beneath the clockroom. It is 3.9m long, weighs 300 kg and beats every 2 seconds. The clockwork mechanism in a room below weighs 5 tons. On top of the pendulum is a small stack of old penny coins; these are to adjust the time of the clock. Adding a coin has the effect of minutely lifting the position of the pendulum's centre of mass, reducing the effective length of the pendulum rod and hence increasing the rate at which the pendulum swings. Adding or removing a penny will change the clock's speed by 0.4 seconds per day."

From Big Ben.

[MikeS]

Obvious really,I should have thought of that.

[Geezer]

It wasn't that obvious to me. I had to think about it for about five minutes to figure it out  []

[imatfaal]

I'll admit that even after I had read Geezer's answer and Yor-on's confirmation I still didn't believe it could make enough difference on a 300 kg pendulum; but I did the maths.  On an ideal pendulum of the size in Yoron's post a 10gramme coin about quarter/half way up would make a change of the right magnitude.