Searches of fractionally charged particles in matter with the magnetic levitation technique

A survey is given of the experiments performed with the magnetic levitation electrometer to search for stable fractional charges in matter. After a general introduction, illustrating the principle of the method, an analysis is presented of the electric forces acting on the levitating object (whose knowledge is essential to determine the residual charge). This is followed by a short account of the historical evolution of the subject, after which we proceed to a detailed description of the ferromagnetic levitation electrometer used in our laboratory and of the results obtained. The main differences with the Stanford diamagnetic (superconducting) instrument are also illustrated. The final section contains a concise summary of the other experiments of search of quarks in matter, in particular of the Millikan automated ones. (orig.)