Day 5: Digital Multimeters and You

Today was an introductory course to electronics. As it was explained to us we would be getting a short primer on electronic theory before digging deeper into the topic later in the week. Our instructor Mr. Stone, normally teaches the diesel technology classes. He’s a big guy with a handle bar mustache and a lazy eye.

He walked us through voltage, current and resistance. I had a circuit analysis class at New York City College of Technology, so this was largely a refresher for me. I was startled at how quickly he moved through the information, covering nearly half a semester at City Tech in a morning out here. One of the most brilliant things he showed us was something called the thumb chart.

Very quickly, electric circuit theory is fairly easy to understand if you compare it to water in a garden hose. Voltage is like pressure, and your battery provides the voltage, the way a water tower might provide pressure to your garden hose. Current is basically water moving through the hose. And resistance is, as my instructor put it, like kinking the hose.

As they teach in you physics classes or electronic theory, these three elements, voltage, resistance and current are linked by a little thing called Ohm’s law. Ohm’s law states that voltage is equal to resistance times current or V = I x R. With this law and a little bit of algebra you can find the resistance, the voltage and the current as long as you have the other two bits of info. But since my algebra is weak I always moved a little slowly through these algebraic manipulations. But all of that was made incredible easy by the thumb chart.

With this idiotic little graphic, you can find out how to calculate any of the elements in Ohm’s law simply by covering the variable you want. If you want to know the current, cover the letter I and bam, it’s voltage divided by resistance. I quickly realized that you can plug another chart familiar to the electrically inclined which is the power chart. Wattage is an expression of Voltage times Current. So you can make a thumb chart for it. I know what you’re thinking, “Dude, that’s really cool and all but what if you only have the voltage and the resistance, but you still need to calculate power. Well, just turn your attention back to first thumb chart, you can get the current needed for your calculation by dividing the voltage by the resistance.


You either understand what I’ve just written because it is so painfully obvious, or you have no clue what I’m talking about. My point is that these charts provided a eureka moment for me that crystallized a lot of the theory I’d been stumbling over since beginning electrical studies.

Mr. Stone passed out a variety of multimeters. These devices can measure many things, hence the multi in their name. We will likely use them to measure current and voltage. We used them to test a few switches to see if they were working or not. None of them worked. We then watched a video detailing how to use all the other functions. The impact of this video could be measure in decibels, largely coming from a sleeping student.

Later we went to a room with lots of circuit boards set up. These were much bigger and user friendly than the boards they gave us to work with back at City Tech. Here, I learned another important lesson about heat and impedance. We were given some light bulbs and some wires, along with a switch and wires. After wiring these up we used multimeters to make some measurements. My partner and I ran into a problem. We measured the resistance of the light bulb when it was out of a circuit and then we calculated what our current should be. When we wired the circuit up we discovered that our current was much lower than we had anticipated, about 100 times lower. Confused we enlisted the help of our instructor. He tested the equipment, and everything checked out. Then he remembered that the resistance of the bulb in the circuit increases dramatically as current is passed through the filament and the filament heats up. This triggered a memory of my recent physics course where I heard something like the hotter an object gets the less current it can pass, but the name of the phenomenon escapes me.

The week ended early that afternoon, and I went back to Oklahoma City where I would be put to work by my mother trimming trees and bushes around the house. This activity required the use of a ladder, and I would find out the coming week just how many safety violations I committed doing routine things around the house.