Finding a better value for G

The gravitational proportionality constant G, although one of the first constants measured and perhaps the most fundamental and universal constant in Nature, is now the least accurately known. G, which relates the force F between two particles to their masses $m_1$ and $m_2$ and the distance d between them $${\mathrm{F\hspace{0.17em}=\hspace{0.17em}-Gm}}_1{m}_2{\mathrm{/d}}^2$$ is now known only with an accuracy of about one half of one percent. Because many contemporary techniques, such as in geophysics, require a good knowledge of G, as well as for purely theoretical interests, a group of us at the University of Virginia, Charlottesville, have been developing the apparatus shown in figure 1 to measure the absolute value of G as well as its possible variation with time or other factors.