# Coil Length

What about using variable length coils? Intuitively it seem sensible that longer coils are needed at higher speed. But practically speaking, it just isn't so.

## Coils of Increasing Length

That is, what about using a sequence of coils with increasing length to help compensate in some form the subsequently smaller time the projectile remains inside each coil.

As an example, look at the photo of a coilgun project posted by Hlyphong. Look closely and you can see the coils at one end are shorter than the coils at the other.

Variable coil length is a technique sometimes used in commercial design of a linear synchronous motor in small transport systems. Done properly, you could simplify the timing control circuits by turning on each coil for the same duration. Suppose the linear motor were installed to move small carts on a track through an assembly area. The coil length would be chosen to let carts move slowly through work areas, and more quickly across the distance between stations.

The use of variable length coils is worthy of consideration, although perhaps inflexible. Once assembled, it is rather difficult to change the length of a coil.

A few things occurred to me when I first considered that approach. The practical matter of winding several coils of various lengths (and then taking measurements of attractive strength profile) was more than I wanted to take on. When I began, I was really struggling with a lot of variables: wire size, coil resistance, tube diameter, choice of projectile, power supply voltage and current, and even the exit velocity goals, to name a few. It was helpful to me to simplify the problem by assuming all coils to be identical. And it was nice to build just one coil-winding jig, and then use it a couple times.

I wonder about power-supply requirements for different sized coils? It would seem to me that coil resistance will double for a coil twice as long. So if you wanted the same current in every coil, then you would need twice the supply voltage. It's certainly not an unsolvable problem, but it would make the design a little more interesting. You may notice in Hlyphong's coilgun that each coil is driven by a separate bench-style power supply.

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