How Ammeters and Shunts Work

Measuring flow (amperage)
In a water system, rate of flow is measured by installing a paddle wheel in the pipe and seeing how fast it spins. Amperage is measured in the same way by installing an ammeter in the electrical circuit.

With larger volumes of water it is impractical to measure the whole flow rate - the paddle wheel would have to be huge. A small restriction is made in the pipe to cause a slight pressure drop. The slight pressure drop in the main pipe causes water to flow through the bypass pipe, and a paddle wheel in this pipe measures the rate of flow. This is multiplied by a suitable factor to determine the rate of flow in the main pipe.

High amperage is measured in a similar fashion. A specially calibrated. low-resistance shunt is installed in the main wire (cable). A small circuit containing a voltmeter is connected across the terminals of the shunt The voltmeter measures the voltage drop across the shunt. which is then multiplied by a suitable factor and reconfigured to give the overall rate of flow (amperage).

     Illustration by "Blue Sea Systems"
Text from Boat owner's Mechanical and Electrical Manual/ Nigel Calder -2nd ed
(Nigel really gets around, don't he)

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