Meters and Stuff
September 10, 2007
Last month I alluded to the idea of using a multimeter to test pickups and configurations. If you aren’t aware of these handy little gadgets, here’s a brief introduction.
Multimeters are so named because they perform multiple functions. They can typically test AC voltage, DC voltage, amperage, and resistance. For our purposes we will use the resistance function. Resistance is measured in ohms, so multimeters are sometimes referred to as ohmmeters.
There’s no need to have a thorough grounding in electronics theory to use a multimeter, so feel free to go buy one today if you don’t already have one; you’ll be using it in no time. The basic premise is this: insert the meter’s two leads into a circuit and you can read the current, amperage, or the resistance of whatever lies between them.
Electrons, or electricity, can flow through various materials, such as wire. When electrons are flowing, we call this an electric current. Every material presents some degree of resistance to current flow. Some, like gold wire, have a low resistance, so current will just zip right through. Others, like a Snickers bar, have a high resistance. You’re not going to get a lot of current flowing through a Snickers bar. The material’s size is also a factor; for instance, a fat gold wire has less resistance than a skinny gold wire. It’s like a garden hose; more water will flow through a fat hose than a skinny one since the fat hose provides less resistance.
Also, a circuit is essentially an unbroken electrical path. If we put a single coil pickup on the table, making sure the leads aren’t contacting each other, that is an incomplete circuit. The two leads are just opposite ends of a single piece of wire, and electrons won’t flow through this piece of wire – they don’t like dead ends. But when we touch the two leads together, we now have a complete circuit, or an unbroken path. Electrons can now flow through this circuit.
Pull the two leads apart again, stopping any current flow. If we then insert our meter into the circuit, touching one of its leads to one of the pickup’s wires, and the other meter lead to the pickup’s other wire, we complete the circuit through the meter, which reads the resistance of the coil. In this case, the meter would actually generate a current through the circuit, which would allow it to measure the circuit’s resistance. Since the only component in this particular circuit is the pickup, essentially one long piece of wire, the only thing being measured is the resistance of that long piece of wire.
We just checked a coil with the meter to see whether it was intact. Sure, we measured the resistance of the coil, but that doesn’t necessarily tell us anything of much value – something we’ll explore in-depth in a future column. What is of value is that our measurement was not miniscule – or nearly zero, and also not infinity.
This indicates that the coil is intact, and the pickup should work. If you stumble across an old pickup, a multimeter will very quickly allow you to determine the condition of the coil. What the meter is telling you here is, “Man, I’m shoving a lot of electrons through this wire, and every electron I push through one end comes out of the other, giving 100% return. I didn’t lose any electrons, but it was really hard work, meaning there’s a lot of resistance.” Well, it should be hard work, that’s a lot of wire, and did we mention how skinny that wire is?
But what if the meter reads essentially no resistance? What if, instead of something like 6.8k, or 6800 ohms, you read something like .3 ohms? Even though this is fairly unusual with a pickup, it would indicate a short in the winding, meaning the electrons have found a shortcut or a path that will allow them to bypass most of that long piece of wire, and they’re taking it! Hey, why go the long way if there’s something easier? For example, if the two solder joints on the bottom of the pickup somehow got jumpered by something like a blob of solder, the electrons would head up one lead toward the coil, but just before they got there, they would see the path to the other lead, and they’d come right back down, bypassing the coil altogether. So the .3 ohms would be telling you that all you were reading was the resistance of the two leads but not the coil, which would show minimal resistance.
It’s more likely that instead of reading nearly zero ohms, you would read infinite ohms. We’ll discuss what that would mean next month. Until then!
Founder, Acme Guitar Works