What is this large metal bar inside my DMMs? One of them labeled it ST. They appear to be connected to the COM port or fuse. Is this just a big jumper for the 10 A ammeter?
Answer
That is not a simple jumper.
That is the precision resistor used to measure the current. This is also known as a "shunt" - hence the designation ST for shunt.
You measure current by passing it through a known resistance and measuring the voltage across that resistor. Using Ohm's law, you can calculate the current from the voltage and the resistance.
If you look closely, you will see that one of them has been trimmed by making nicks in the wire. That changes the resistance slightly. You measure a known current with a new meter, then whack on the shunt to make your new meter display the known current.
The thick ones like that are usually for the 10A range. The lower current shunts are usually small, precision resistors on the board.
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