# 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? Sep 28, 2014 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. Sep 28, 2014 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. Sep 28, 2014 at 10:41