# Mutual inductance of two loops

I got to measure inductance of circuit with two loops connected like this:

simulate this circuit – Schematic created using CircuitLab

simulate this circuit

The box measures $L$. I have this formula $$M=\frac{L_1-L_2}{4}$$ and when I compute $M$, it's negative. Is that ok? What does it mean? Should I make the result absolute?

In one measurement, I got $L_1=-1394~ \mu H$ and $L_2=-1392~ \mu H$. That seems wierd. I measured like 10m long cables. Isn't that suspicious? What could make the result wrong like this?

I was measuring with frequency $200 kHz$ and cables are flat. PNLY and Twist. One loop was made by connecting two nearest wires in the cable.

• Where is the formula from? And yes, it looks suspicuous. At which frequency did you measure it? Do the inductors have cores? – venny Sep 28 '14 at 2:19
• frequency 200 kHz. I was measuring inductance of cables. Like PNLY and Twist flat cable. Inductors in the schema are there as symbols for inductance of loops made by wires od the cable. – user50222 Sep 28 '14 at 10:39
• The formula is taken from relations of inductance of two coils in serie and "antiserie" as in the schemas. $L = L_1 + L_2 - 2M$ and so. – user50222 Sep 28 '14 at 10:41