The BS407 is fully optimised for accurate measurement of low resistances with a best resolution of 1μΩ. It has applications beyond the testing of components such as measuring the resistance of motor or transformer windings, the properties of materials, the thickness of plates, the security of pipework joints or wiring installations and many others. It uses a Direct Current technique to measure true resistance, rather than the resistive component of impedance which is shown by AC excited RLC bridges. The test current for each range has been carefully chosen to minimise heating of the sample under test while being sufficient to minimise the effects of thermal emf and noise. This gives much greater accuracy at low resistances than can be obtained from the very low test currents used by general purpose high resolution multimeters. The low-noise low-drift bipolar amplifiers employed need much less noise filtering than alternative chopper stabilised technologies resulting in faster settling to the correct reading. The BS407 uses a four terminal measurement system and is supplied with a set of high quality Kelvin clip leads. For speed and convenience front panel switches are provided for current diversion (allowing in-situ zero adjustment) and current reversal (for identifying thermal emf effects). An indicator lamp confirms correct flow of the measuring current thus preventing spurious results from being recorded. The BS407 has eight push-button selected decade measurement ranges from 1999μΩ up to 19.99kΩ. A front panel operated ‘clamp’ switch is available to limit the maximum voltage across the unknown to 20mV. This is a requirement for the measurement of switch contact resistances to international standards. With a lowest range of 1999μΩ the BS407 can resolve 1μΩ. Precision analogue circuitry ensures high measurement accuracy of up to 0.1% of reading ±1 digit. The BS407 is a fully portable instrument which operates from NiMH rechargeable batteries. Stabilised internal operating conditions ensure that readings do not vary with the state of the battery charge. The battery charger is built into the instrument and can be operated while measurements continue