How does an ohmmeter measure different resistances?
Generally the meter uses a different reference resistor for each range. 

These could be built with (say) 100 ohm + 900 + 9k + 90k +900k + 9M in series. Starting from the 100 ohm, each tap point gives a total of 100, 1k, 10k etc. The ADC converter in a meter actually measures the ratio of two values -- usually an unknown input voltage and known reference voltage. Either of these can be varied. 

So, if you connect the above R string in series with the unknown resistor and pass any current through it, the ratio of the voltage across the unknown R and the known reference R will equal the resistor ratios. The ADC just displays this (scaled) ratio.

So, most meters apply about 1-3 V across that whole string and display the result.







simulate this circuit &ndash; Schematic created using CircuitLab

Here, depending on which switch is closed, the meter will be on a different range. Some current will flow through the whole string; the voltage across the upper portion will generate the reference for the meter; the voltage on the lower part is measured by the meter w.r.t. the reference and displayed as resistance.