I'm considering doing part of How to Install an Ammeter in the Black & Decker CMM1200 Cordless Electric Mower but first with just a shunt resistor and voltmeter, versus hooking in a dial ammeter (may hook in a dial later).
But, when examining shunt resistors, I found What Is A Shunt which states: "This shunt is calibrated such that the voltage drop across it is 100mV when the current flowing through it is 100 Amps".
So, in order to acquire such a shunt resistor to directly read current as amps being equivalent to the voltage reading on a voltmeter (in mV), I would need a 100A 100mV current shunt resistor. But, when I search specifically for 100mV 100A current shunt resistors on Ebay all that is returned are 75mV ones (EDIT: A better search link).
What I want to know is why 75mV and not 100mV in those listings? What is significant about them having to be 75mV and not just 100mV?