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Jim Drake’s Entertainment Blog; Multi-meters: A lesson in Resistance and electrical theory



An extremely famous equation that describes electrical properties is called Ohm’s Law. Scientists figured out — a long time ago, in a galaxy far, far away — that the quantity of electrical current, or “amperage” is equal to the ratio of “voltage” divided by something called resistance (unfortunately not the Star Wars Resistance). The resistance is in a unit called “Ohms.”

This inconveniently and confusingly labeled equation I=E/R, or “Ohm’s Law,” was the first topic covered in last week’s “Learn how to use a Multi-meter seminar” at the library, which I, and my trusty multi-meter, attended.

There were 10 people in the class, and I must say it was fun to see everyone bring their own multi-meter. Come to think of it, that was the most multi-meters I’ve ever seen in one place, except for that time I was in Section 5, Aisle 27 of Home Electronic World Depot, trying to decide which one to buy.

I’ve been using my multi-meter mainly for measuring batteries, audio cables and instrument cables. One time, I successfully verified that an electronic component in a video game, a voltage regulator, had failed. I bought a new one and soldered it in, and like magic, the game came back to life. Perhaps this free community course could give me even more insight about what the heck I was actually measuring.


Shannon Vance, our instructor, moved to the Gorge in 2002, and spent a career as a telecom test equipment engineer in Santa Cruz. Now, he works on internet programming and software with his company, Greenlight Development. He is also a member of the Gorge Technology Alliance (which is not part of the Star Wars Alliance, although he did keep referring to his multi-meter as R2D2 every time it made a beeping sound).

Vance brought several working visual displays to class. The first one was a plywood structure containing an electrical outlet wired to a switch and a light bulb (a green light bulb, for his company’s namesake) that could be tested by our multi-meters. The second one was a bicycle pump and a small innertube. Why would someone bring that to a multi-meter class?

Well, the air pump and tube were brought in to demonstrate the different aspects of Ohm’s Law. He pumped up the inner tube, and said that electrical current can be thought of as the volume of air in the inner tube. The air pressure that exists in the inner tube can be thought of as the voltage. The higher the pressure, the more voltage. And the air valve in the inner tube can represent the resistance. If the valve is closed, that’s high resistance. Open it a little, and the resistance goes down. Open it all the way, there’s no more resistance. So now, replace air with electrons, and you begin to have an idea on the various quantities of Ohm’s Law that can be measured.

In addition to learning basic household electrical tests, our group learned how to interpret all of the various electrical symbols on the meter. This is important because you need to know if your meter can even do a certain test in the first place (not all multi-meters can measure AC current, for example). We also learned how to plug the test leads in correctly, some basic safety tips, and we learned that when measuring Ohms of resistance, the multi-meter is putting a small voltage across the system using a 9V battery that’s inside. And that’s what’s been telling me about my guitar cables, all this time.


Unable to resist waiting for the latest DVD to be released, Jim rang in the New Year by catching up with all nine episodes of the Star Wars saga.



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