In the not-too-distant past, voltages and currents were measured using "moving needle" analog meters (ie, moving-iron and moving-coil meters). The current shunt was developed to allow these meters to measure currents that were higher than their basic sensitivity. For example, if a meter needed 1mA to give a full-scale reading (ie, 1mA FSD), it could be used to measure currents up to say 1A by connecting a low resistance 'shunt' across its terminals. The resistance was chosen so that it would carry 99.9% of the current, leaving just 0.1% to flow through the meter itself. This effectively converted the 0-1 mA meter into a 0-1A meter.
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