the nature of a metal sample , such as the purity of precious metals and metal alloys , such as gold , is determined using a handheld probe having an electrode embedded in an electrolyte in a reservoir of the probe and arranged to conduct electrically with the sample through the electrolyte and a fibrous tip . the tip can resemble the point of a marker or writing instrument . the tip is in fluid communication with the reservoir containing the electrolyte , and preferably has the electrode embedded therein . an electrical connection with the electrode is made at the rear of the probe . for testing gold purity , the electrode is preferably a thin platinum wire extending through the reservoir and into the tip to just behind the tip surface . a battery is coupled to a calibration potentiometer whereby the voltage on a millivoltmeter is set to full scale when the probe is open circuited . when coupled to the sample in a circuit , the galvanic cell formed by the same , the electrode and the electrolyte reduces the voltage sensed at the meter , as a function of the extent of galvanic action . preferably the electrolyte is a weak acid including nitric acid and ammonium chloride salt in aqueous solution .