Apparatus for measuring cleanliness of a molten metal, the apparatus comprising: one or more rechargeable ultra-capacitors (11a, 11b) operable at a discharge voltage of 2.7 volts or less; at least two electrodes (12a, 12b); a metal cleanliness probe having an interior, a wall made of electrically resistive material and a passage in the wall interconnecting the interior with an exterior of the probe to allow molten metal to pass therethrough, wherein one of the at least two electrodes (12a, 12b) is positioned in the interior of the probe as an interior electrode and another of the at least two electrodes (12a, 12b) is positioned outside the probe as an exterior electrode; a device registering voltage across the interior and exterior electrodes and generating a voltage signal; characterized by the apparatus comprising: for the or for each of the one or more ultra-capacitors (11 a, 11b), an associated resistor ladder network circuit (Na, Nb) interconnecting its associated ultra-capacitor (11a, 11b) with one of the electrodes (12a, 12b), the or each resistor ladder network circuit (Na, Nb) comprising two or more resistors (23a, 33a, 43a, 53a, 63a, 73a, 23b, 33b, 43b, 53b, 63b, 73b) connected in parallel to each other, each resistor (23a, 33a, 43a, 53a, 63a, 73a, 23b, 33b, 43b, 53b, 63b, 73b) being in a circuit leg (22a, 32a, 42a, 52a, 62a, 72a, 22b, 32b, 42b, 52b, 62b, 72b) including one or more field effect transistors (21a, 31a, 41a, 51a, 61a, 71a, 21b, 31b, 41b, 51b, 61b, 71b) capable of being switched directly between a non-conductive OFF condition and a fully conductive ON condition, and the resistor ladder network circuit or circuits (Na, Nb) having resistance values effective to maintain a measurement current of no more than 100 amps through molten metal present in the passage when the circuit or circuits (Na, Nb) are exposed to the discharge voltage from the one or more ultra-capacitors (11a, 11b); and a controller adapted for individually switching the field effect transistors (21a, 31a, 41a, 51a, 61a, 71a, 21b, 31b, 41b, 51b, 61b, 71b) of the circuit legs (22a, 32a, 42a, 52a, 62a, 72a, 22b, 32b, 42b, 52b, 62b, 72b) of the or each resistor ladder network circuit (Na, Nb) between the non-conductive OFF condition and the fully conductive ON condition according to a sequence effective for maintaining the measurement current within a pre-determined current range at least for a time required for measurement of cleanliness of the molten metal. for the or for each of the one or more ultra-capacitors (11 a, 11b), an associated resistor ladder network circuit (Na, Nb) interconnecting its associated ultra-capacitor (11a, 11b) with one of the electrodes (12a, 12b), the or each resistor ladder network circuit (Na, Nb) comprising two or more resistors (23a, 33a, 43a, 53a, 63a, 73a, 23b, 33b, 43b, 53b, 63b, 73b) connected in parallel to each other, each resistor (23a, 33a, 43a, 53a, 63a, 73a, 23b, 33b, 43b, 53b, 63b, 73b) being in a circuit leg (22a, 32a, 42a, 52a, 62a, 72a, 22b, 32b, 42b, 52b, 62b, 72b) including one or more field effect transistors (21a, 31a, 41a, 51a, 61a, 71a, 21b, 31b, 41b, 51b, 61b, 71b) capable of being switched directly between a non-conductive OFF condition and a fully conductive ON condition, and the resistor ladder network circuit or circuits (Na, Nb) having resistance values effective to maintain a measurement current of no more than 100 amps through molten metal present in the passage when the circuit or circuits (Na, Nb) are exposed to the discharge voltage from the one or more ultra-capacitors (11a, 11b); and a controller adapted for individually switching the field effect transistors (21a, 31a, 41a, 51a, 61a, 71a, 21b, 31b, 41b, 51b, 61b, 71b) of the circuit legs (22a, 32a, 42a, 52a, 62a, 72a, 22b, 32b, 42b, 52b, 62b, 72b) of the or each resistor ladder network circuit (Na, Nb) between the non-conductive OFF condition and the fully conductive ON condition according to a sequence effective for maintaining the measurement current within a pre-determined current range at least for a time required for measurement of cleanliness of the molten metal.