Doing the experiment with various materials is a way of testing the assumption that the gravitational field produced by an object is exactly proportional to its

Doing the experiment with various materials is a way of testing the assumption that the gravitational field produced by an object is exactly proportional to its mass. It would be a huge discovery if there were deviations from the law. It is quite hard to test how much, say, electrons contribute to gravitational fields because you can't get enough of them together (without a corresponding number of protons) to create a measurable gravitational effect that isn't overwhelmed by the large electric fields in such an experiment.