Would analog multimeters measure correct dimmer's output?
Figure 1. Waveform from SCR phase-angle controlled voltage.

Moving coil and moving iron analog multimeters have their scales calibrated to give the true RMS reading on full-wave sinusoidal waveforms. They will not read correctly on non-sinusoidal waveforms such as those shown in Figure 1.

Higher quality digital meters will have a True RMS converter on the AC modes. This will measure and display the correct reading.



Power in a circuit is given by \$ \frac {V^2}{R} \$. The significant aspect of this is that the parts of the waveform where the voltage are high have much greater effect due to the square term. True RMS readings tell you the equivalent DC voltage that would give you the same power.

There are many tutorials on the web that will give you a primer on true RMS and why we use it.