A Tale of Two Meters

Not sure where this thought come from. Perhaps to add more clarity on how these digital multi-meters work. In order to help illustrate how mult-meters work, I enlisted the aid of two meters, connected together. One set for voltage, and the other set to resistance. Let's see what they tell us.

The meter on the left , set to measure voltage, shows us the meter on the right is forcing a voltage, in the resistance mode, or ohms mode. In this case, the reading on the volt meters reads 2.8 volts dc. More than enough voltage to measure value of a resistor, or other passive impedances. Side note, this being a calibrated meter, thus the output voltage is carefully adjusted to this voltage.

The meter on the right, set to measure resistance, shows the input impedance of the meter on the left, at 11 Meg ohm, with the scale set to Auto-Ranging. Case in point, newer digital Multimeters advertise their input impedances at 10 Meg ohm or better ; as not to load down the circuit as compared to the 50 micro ampere jeweled movement of an analog meter, rated at 20 k ohms per volt. That is to say, the coil on a 50 micro ampere jeweled movement requires 50 micro amps of forward current for full scale deflection. This ultra high input impedance, also the reason these digital meters detect false measurements, sometimes referred to as "stray voltages". One method I use, in some cases, I will strap a resistor across the test leads, 20 k ohm, 50 k ohm, to intendedly put a some load on what I am measuring, if I am suspecting a stray voltage or not. Further, when I am measuring , say a common 9 volt battery, if will insert a 100 ohm resistor across the battery, and then measure battery voltage under a load. Gives you a better idea the SoC, or the "State Of Charge" of the cell.