Question about Cavendish experiment I got a question. In Cavendish experiment was used lead in both of spheres. Can we repeat that experiment with balls made from stone  or anything else. Because lead could be a byproduct of radioactive decay and by that emit ionizing radiation. And by ionizing lead it can create positive charge and to be attracted. 
 A: Naturally found lead is not radioactive. Its longest lived isotope has a half-life of approx. 15 million years, which means that over the course of geological time this isotope would have disappeared. Unless it gets contaminated by radioactive isotopes of other elements or its produced by decay, as you mentioned, it does not radiate.
Having said that, you are correct that one has to be very careful about these kinds of error sources in precision experiments, especially when the error, as in this case, would have the exact same functional dependence as the effect that we want to measure! In case of the Cavendish experiment this is easy: you make the entire apparatus conductive so that all electrical charges are shorted out and the electric field it produces is zero. 
The experiment, more exactly, it's modern equivalents, have been repeated with many types of materials to test the equivalence principle. That is one of the most important experiments in all of physics because general relativity and with that the entirety of modern cosmology depends on the equivalence principle being valid. 
A: You can, of course, do the experiment with any kind of weights.  However, heavier weights give a stronger force and thus a better signal-to-noise ratio.  Charge non-neutrality due to radioactive losses are totally negligible, since macroscopic matter tends to neutralize very swiftly.  I have actually done the Cavendish experiment with much more radioactive depleted Uranium and the improvement in sensitivity over less dense lead was substantial.
