Day Six – A brief diversion to answer a question about coils and resistance

I was at work late today, but I feel like I’ve got a good thing going here, and I didn’t want to skip a day entirely. So for today’s post, I’ll pause to answer a question I was asked:

“Does a coil of wire have more resistance than a straight wire?”

The answer is surprising complex, but first, let’s do an experiment to see what we can learn. First, I took a relatively long, thin wire, and attached it to my multimeter:

You can’t see the entire length of the wire, about six feet or so, draped down from the worksurface. You can see that the meter reads 0.6 ohm. Like all electrical components, this wire has resistance, although not a lot. Okay, let’s make a coil:

Here, you can see, I’ve made the wire into a coil; More specifically, what I’ve made here is essentially a form of ferromagnetic core inductor. Those are big words, and the link they form explains them in more detail, but let’s break it down a little. Ferromagnetic means made of a material, like iron, that is capable of forming a permanent magnet, or of being attracted to magnets. In this case, the steel shaft of a screwdriver. Core means that it’s in the middle of the coil, with the wire wrapped around it. An inductor is a type of electrical component, made of a conductor, such as a coil of wire, which resists changes in electric current passing through it (not to be confused with a resistor, which resists the -flow- of current); The flow of current through the coil creates a magnetic field, which acts as a damper against changes in the current. So, with that established, let’s look at the meter. Interesting, 0.9 ohm. It does seem like the resistance is a little higher. What could be happening here? Well, the difference is so small, it could be an error; The clips aren’t gripping the wire ends as tightly, the meter isn’t accurate enough at such small values, etc. However, if we read about ferromagnetic core inductors, we learn that they exhibit some properties that cause what is termed core loss, that is, they lose some of their energy as heat. While the processes that cause this are complex, involving magnetic fields and such, we know that energy loss as heat sounds an awful lot like resistance. So I am comfortable saying that a coil of wire, under most conditions, is likely to have at least slightly more resistance than a straight wire.

I’ll be back tomorrow with a regular circuit post.