The
coefficient of proportion is determined when once the units are
chosen, and as soon as we know the numerical values of this force, of
the two masses, and of their distance. But when we wish to make
laboratory experiments serious difficulties appear, owing to the
weakness of the attraction between masses of ordinary dimensions.
Microscopic forces, so to speak, have to be observed, and therefore
all the causes of errors have to be avoided which would be unimportant
in most other physical researches. It is known that Cavendish was the
first who succeeded by means of the torsion balance in effecting
fairly precise measurements. This method has been again taken in hand
by different experimenters, and the most recent results are due to Mr
Vernon Boys. This learned physicist is also the author of a most
useful practical invention, and has succeeded in making quartz threads
as fine as can be desired and extremely uniform. He finds that these
threads possess valuable properties, such as perfect elasticity and
great tenacity. He has been able, with threads not more than 1/500 of
a millimetre in diameter, to measure with precision couples of an
order formerly considered outside the range of experiment, and to
reduce the dimensions of the apparatus of Cavendish in the proportion
of 150 to 1.
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