Hi , I've been wondering about this one thing. Well a transformer works on indcution , so a increase/decrease in voltage amplitude and/or periodical polarity reversal is needed to induce a current and voltage in the secondary. Now if we leave the current reversal aside for a bit and look at the increase then highest point and then decrease of a waveform that " feeds" the transformer primary , I wonder what's the lowest frequency a transformer can still be able to work ? The bigger the wavelength the longer it takes for the voltage to reach and then drop drom a certain level in one half period, but since we can also say that AC is just DC if looked at from a " point in time" perspective then since the time it takes for a very low frequency half wave to change is pretty long does the transformer secondary can still sense that? Or maybe I should ask , what is the lowest rate of change in voltage/current that a transformer can still work with ?