 Since the material of the belt and rollers can be selected, the accumulated charge on the hollow metal sphere can either be made positive (electron deficient) or negative (excess electrons).The friction type of generator described above is easier to build for science fair or homemade projects, since it does not require a high-voltage source.Greater potentials can be obtained with alternative designs (not discussed here) for which high-voltage sources are used at the upper and/or lower positions of the belt to transfer charge more efficiently onto and off the belt.