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| Vertical section of Cavendish balance |
In 1798, the English
physicist and chemist Henry Cavendish was the first to measure Newton's universal
gravitational constant (G) using a spectacularly ingenious
method, which has been scarcely improved later. The method was devised by John
Michell, who died without being able to carry it out, so Cavendish performed the
experiment. In fact, his objective was not to measure the constant, but the
mass of the Earth, but the value of the constant could be inferred from the
result.
Cavendish’s
instrument was a torsion balance
from which two identical balls of lead hung. Next to these balls, one on one
side and one on the other, hung two much larger lead spheres, 175 kg each,
which attracted the first two, producing a slight twist of the balance, which
Cavendish could observe by means of a small telescope located outside the
enclosure, to avoid observer interference. He thus detected a displacement of about
4 mm, which he measured with a precision of ¼ mm. This allowed him to calculate
that the density of the Earth is 5.448 times greater than that of water, from
which it is possible to deduce the mass of the Earth and the value of G:
G=6.674×10-11N.m2/kg2
This is
the official value, which is known with quite a low accuracy (1 in 10,000), compared
with other universal constants.
