Hello friends ! I have many different E , EI , ER, R cores from different manufacturers I bought sometime. I need to determine mu permeability by using a simple method, using a sine wave generator, and oscilloscope. Earlier today I took one E 30/15/7 no gap , and calculated an inductor. Proposed L = 703 uH Frequency f = 60KHz N = 21 turns I connected a 0.01 uF known value ceramic capacitor in parallel with the coil, applied a 60 KHz low level sine wave to this circuit and monitored on my scope. With these values of L and C , the circuit showed perfect resonance at 60 KHz with maximum amplitude of the sine wave. Then used the formula: u = L / Lo ... where L is the value of the inductance using the core , Lo is the value of the inductance without the core (air core) The value of the air core resonance occurred at 954.55 KHz that means L = 2.78 uH , so u = 703 uH / 2.78 uH = 252.8 I find that this must be wrong. When I bought these cores the material was Epcos N27 and u should be arround 1500. So I am doing something wrong here, or the formula or method is not correct. Could you please enlighten me ! Thank you.