This was explained to me once but I don't remember the explanation. Maybe you can help!

I built a controller for a heater - a purely resistive load. The supplier didn't have an AC ammeter in stock so I used a DC ammeter with a bridge rectifier. Only the Ammeter is DC. The load is still fed with AC. So, the rectifier and ammeter are in series with the load.

An AC ammeter reads 4.5 Amps. The DC ammeter reads 4 Amps, which is correct. 4.5 Amps AC has a peak-to-peak sine wave of 6.36 Amps (4.5 / .707) The DC equivalent is 4 amps (6.36 x .636).

Just to keep this on track, I don't care that the DC ammeter reading is low. I'm not trying to fix that. I'm just trying to remember why the DC current is lower than the AC current. I think the explanation I was given was to do with harmonics but it was so long ago that I'm not sure.