4-terminal shunt resistor
Take a look at this. It shows two terminals as the main current flow (thick tracks) and uses 2 more terminals for measurement of the volt drop generated by that current: -
The "better" way is on the left because it takes measurement connections from a defined place and at no point on those measurement connections is there load current flowing.
The picture on the right shows the measurement connections at some small distance from the shunt resistor terminals and therefore there is a small volt drop that forms an error - in effect you can't rely on the stated value of the shunt resistor in order to convert the measured voltage to an assumed current flow.
I will just go out on a limb and say that it's probably because of this:
- 2 terminals for the current to flow through.
- 2 terminals for measuring.
By having 4 connections, the supplier of the shunt resistor can make sure that what you are measuring across will be the shunt resistor where the current will flow which you are trying to measure.
If there was only two terminals, then you would have to decide for yourself where you want to branch off for measuring. There are many viable positions to measure across, but which one will most correctly measure the voltage across the shunt resistor? When you are making high quality stuff, this is no simple question.
Shunt resistors are used where there are high currents. As such the point at which you tap of the voltage to be measured is important. Moving it by even 1mm may make a big difference especially if there are large currents.
The four tap shunt is built to have the voltage points there where they represent the correct conversion from amps to voltage. See it is part of the calibration.
On your question of putting them in parallel.
That would be possible if the two resistors where really equal in value. In that case you can measure one of them and multiply the result by two.
But we all know components are never exactly accurate. Thus a small difference in resistance will make the current split unequal. I would say you have to measure both voltages and add them up.