Cavendish Experiment

The Cavendish Experiment, performed in 1797–1798 by British scientist Henry Cavendish, was alleged to be the first experiment to measure the force of gravity between masses in the laboratory. The Cavendish Experiment is often held up as evidence for the universal attraction of mass, and as a proof for gravity. The experiment involves two spherical lead balls attached to a torsion balance, which is alleged to detect the faint gravitational attraction between the masses.

When institutions have reproduced this experiment with modern methods involving lasers and instruments of the highest precision, however, the detection of gravity has been fraught with difficulty, giving erratic results.

Gravity Not a Constant
Scientific American provides an assessment of a large number of Cavendish Experiments conducted by prestigious laboratories and institutions and explains that, unlike other fundamental forces in physics, gravity cannot be accurately measured.

Puzzling Measurement of "Big G" Gravitational Constant Ignites Debate (Archive)

Measuring the Very Faint
Physicist Jens Gundlach explains that gravity is very hard to measure and would require measuring the force equivalent of the weight of a few human cells on two one-kilogram masses that are one meter apart:

Gundlach explains that there are many effects that could overwhelm the gravitational effects. Static attraction, air viscosity, air particles, static drag, other forces, &c, can easily overcome such gravitational attraction.

Wildly Erratic
The article explains that the results are wildly erratic.

The values of these sophisticated laboratory experiments differ from one another by as much as 450 ppm of the gravitational constant. The weight of a few cells as compared to the masses involved in the experiments, what they should be measuring, for context, is smaller than 450 ppm. The uncertainty for measuring the gravity of the opposite mass with the equipment should be only about 40 ppm, yet the values observed are far more erratic.

450ppm is not accurate. The effect from gravity is a small portion of that. The results need to be consistent, and they need to match gravity. As stated, there are plenty of forces and effects stronger than gravity that it might be detecting.

If it can't detect something that matches gravity, then it's not gravity. One cannot merely assume that the experiment is detecting a multitude of effects to cause the inconsistent results, but that gravity is in there somewhere.

Whatever effects one can imagine is modifying the results could also be creating them as well. One quickly sees that the experiments need to be accurate and consistent for a valid test of gravity.

Cannot Be Measured
The end sentence is plain in its understanding, and tactfully admits that they cannot measure gravity.

Forbes Article
From a Forbes piece titled Scientists Admit, Embarrassingly, We Don't Know How Strong The Force Of Gravity Is (Archive) we read the following about the issue:

Gravity 'Oscillates'
Due to the mysterious readings and problems, some are now calling gravity part of "Dark Energy."

https://www.newscientist.com/article/dn24180-strength-of-gravity-shifts-and-this-time-its-serious/ (Archive)

History
According to physicist George T. Gillies the difficulties in measuring G have been a recurring theme in science's study of gravity.

The Newtonian gravitational constant: recent measurements and related studies (1996) (Archive) George T. Gillies

Abstract:

Concluding Remarks - p212

The Newtonian Gravitational Constant: An Index of Measurements (1983) (Archive) George T. Gillies