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rubbing of the clothes against the chair provides electrificationand the body of the experimenter and the chair form a sort of capacitor. when he stands up,the capacitance of this capacitor sharply decreases and therefore potential difference sharply increases between the chair[i.e. the earth] and the experimenter`s body.the body should obviously well insulated from the earth[rubber soles]
when the experimenter touches the table,the potential difference between his hand and the earth levels out.an electric current is generated,a negligible part of which is branched off into the galvanometer.to deflect the pointer,the resistance between one end of the galvanometer coil and the earth should be lower than that between the other end and the earth.
the pointer of the galvanometer deflects despite the tremendous resistance of the circuit owing to the great potential difference appearing when the capacitance is reduced.
i am showing the path of the current schematically above.here,`W` is the winding of the galvanometer,`K` is the key and `R` shows a very high but finite resistance between one of the winding ends and earth............................................