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Re: The Cavendish experiment



There was an intellectual reason for wanting to know the density at the
time, but I don't recall what it was. Can anyone suggest why the
interest in the density?

On Thu, 24 Feb 2000, Laurent Hodges wrote:

...
G isn't that easy to measure in the lab. It was first achieved by
Cavendish in (I think) 1792. He published under the flamboyant
title "Weighing the Earth"....
Leigh

It's amusing to catch a rare slip in the eponymous article describing
this very diffident, retiring person (14th ed. Enc. Brit. 1957 printing):

"Cavendish's last great achievement was his series of experiments
to determine the destiny of the Earth (Phil Trans 1798)..."

"...The figure he gives for the specific gravity
of the Earth is 5.48."

(A modern estimate gives 5.497)


The Encyclopedia is right! (The currently accepted density is actually a
little over 5.5.) No where in Cavendish's paper does he speak of G or
anything of the sort. The only number he gives is the density of the earth
(relative to water). That's what he was interested in. Why? The
experiment wasn't Cavendish's. It was John Michell, who wanted to know the
density, who devised the experiment and built the apparatus for it
(including inventing the torsional pendulum, which Coulomb also
independently invented). Michell died before completing the experiment,
the apparatus eventually was given to Cavendish, who had to rebuild it
because the great wood beam had warped. All this is described by Cavendish
in his paper.

By the way, although Cavendish does give the density as 5.48, his numbers
don't average to that, so he made an error. Reaveraging I get 5.448, so I
assumed he accidentally lost a "4" digit. He was still remarkably close,
as you will appreciate if you've ever tried to reproduce this experiment to
find G.

Laurent Hodges, Professor of Physics
12 Physics Hall, Iowa State University, Ames, IA 50011-3160
lhodges@iastate.edu http://www.public.iastate.edu/~lhodges