Meters and Stuff
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
3 PREMIERGUITAR OCTOBER 2007
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