I conduct an experiment to find the emf of the copper-zinc system . I connect the copper to the positive terminal of the batery and a potentiometer and the zinc plate to the galvanometer.The galvanometer is connected to a jockey . The battery I use is 3V .I touch the jockey to the potentiometer so that the galvanometer show no current but suprisingly , no matter where I move the jockey , the galvanometer show only a litte change(smaller than the smallest scale of the galvanometer) . Is it true ? I also try to connect a voltmeter across the copper-zinc system to check the emf of the system . The voltmeter show only about 0.5V but from the calculation it should be around 1.1 V . What wrong with my experiment . The solution I use is 2.0 mol/dm3 copper(II) sulphate and 1.0 mol/dm3 zinc(II) sulphate . Because of unsuccessful conducting the experiment , I tried to dip the zinc plate to the copper(II)sulphate with the copper plate still inside the copper(II) sulphate . Something black solid precipitates at the zinc plate . What is this solid .