This morning I tried my hand at measuring the inductance of a 10VA toroid to be used as an audio output transformer. Using this method with two multiturn potentiometers in series. The measurements were taken off the secondary with a 4 vpp signal generated from my laptop. The first signal was 70Hz and the measured resistance was 155.2 ohms. Using the formula, this equates to 352.8 mH. Multiplying by the turns ratio squared gives 186H across the primary. This doesn't seem too far fetched from what I have been able to gather from others experience with toroids on this forum. The problem is when I measured at 22Hz, the half voltage was with 136.6 ohms, leading to a calculation of 988.2mH across the secondary and 522H on the primary. This doesn't seem at all right as I was expecting the inductance to go down with the lower frequency. I am trying to understand what I am missing... any help would be appreciated