“Warning: Science Content”

First, let me acknowledge that I am a geek, why else would I write about flashlights?

I was asked about failing flashlights recently and thought that an explanation might be interesting reading for other people who have wondered about this, so here goes.

When you touch the two pieces of metal in a switch together and they do not make electrical contact, the usual fault is because of dry contact resistance.

Even with minute currents, there is a molecular wetting effect on working contacts where the molecules allow what we think of as current flow at the point of contact closure to take place.

Surface oxidation, which occurs naturally as a result of free contact with the atmosphere, increases resistance, thus reducing or stopping current flow.

It’s the reason that unused flashlights don’t work. It’s also the reason we kick TV sets, bounce radios and punch starter buttons.

Metal oxides make really good insulators, and if the two pieces of metal have some surface oxide, it actually can be difficult to make an electrical contact, even under pressure.

That is why a good switch is designed to apply both pressure and a sliding motion as the contacts are closed. Also, the contact surface will be made from a metal that resists developing oxidation such as tungsten, unlike your average flashlight.

Flashlights fail because virtually every point of electrical contact is subject to dry contact resistance. Typically, the bulb leans against a copper spring, which has a wire that goes to a slide switch that merely presses two pieces of metal together. The other side of the switch connects to a spring where the battery sits; batteries are stacked with simple touching contacts between them with the end pressed on another spring, which connects to the other side of the bulb … sometimes.

Slide the switch and nothing happens. Slap it in the palm of your hand, and it probably will light because you’ve now added some sliding motion to the contact through the surface oxides.

If it doesn’t, you slap it again, and again, until you’ve broken the bulb’s filament.

The new light emitting diode technology tends to partially fix the problem, where the contact resistance may allow the LED to turn on, but at reduced brightness, which, I will admit, drives me nuts.

At this point, you probably toss the flashlight into the trash and buy another, only to start the process yet again.

The cure? Maintenance!

It’s much better to periodically open the flashlight and clean all of the contact points with a pencil eraser. Don’t use emery cloth or sandpaper, though, as this only aggravates the problem by exposing more of the switch contact to the atmosphere, resulting in still more oxidation and resistance.

So, instead of yelling at the flashlight, yell at dry point contact resistance. You now know what the problem is, though your family will be convinced you’ve finally been pushed over the edge into true geekdom.

Howie Soule of Fairfield is an electrical engineer.

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