On Sun, Dec 2, 2018 at 11:54 PM Brent Meeker <meeke...@verizon.net> wrote:

>>If you are on the Earth's surface and you raise a clock by one centimeter
>> you've increased its distance from the earth's center by one part in
>> 637,000,000, it is now 1.0000000016 times further away. The intensity of
>> the gravitational field is proportional to the square of the distance so
>> gravity was 1.0000000031 times stronger before you raised raised the clock.
>> Cavendish did not have a scale good enough to measure that, even today the
>> very best (and very expensive) lab weight scale might be able to measure a
>> change of 1.0000001 but the clock can do several hundred times better.
> > He was measuring the change in a much smaller gravitational field.

Cavendish was measuring the displacement of a torsion balance parallel to
the Earth's surface caused by a weak but constant gravitational field,
there was no change whatsoever in the gravitational field parallel to the
Earth's surface at any time during the exparament. If he had 2
*PRECISELY* identical
cannonballs on the ends of a rod, placed a pivot point *PRECISELY* at the
center and place one cannonball one centimeter higher than the other he
would have transformed his torsion balance into a weight balance and
theoretically he could have observed that the balance moved and measured
the small difference in strength in the large field at 2 different places,
but Cavendish couldn't come close to achieving the sort of precision
required to do that 220 years ago, we can't even do that today.

> * > He was measuring the difference between the force on the torsion
> balance with the cannon balls present vs absent.  *

 Cavendish setup the exparament but nothing moved because the torsion
balance was held in place by a thread, he then sealed the room and did
nothing for 2 days to let the air currents settle down. He then carefully
burned through the thread freeing the torsion balance and observed its
movement from far away through a telescope so his own movements wouldn't
disturb anything. At no time did he measure the very small change of
strength of 2 very large gravitational fields because a torsion balance can't
do that, you'd need either a super good weight balance or a super good

 John K Clark

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