PARTS AND MATERIALS
6-volt incandescent lamp
Basic circuit construction components such
as breadboard, terminal strip, and jumper wires are also
assumed to be available from now on, leaving only components
and materials unique to the project listed under "Parts and
Lessons In Electric Circuits, Volume
1, chapter 1: "Basic Concepts of Electricity"
Lessons In Electric Circuits, Volume
1, chapter 8: "DC Metering Circuits"
How to measure current with a multimeter
How to check a multimeter's internal fuse
Selection of proper meter range
Current is the measure of the rate of
electron "flow" in a circuit. It is measured in the unit of
the Ampere, simply called "Amp," (A).
The most common way to measure current in a
circuit is to break the circuit open and insert an "ammeter"
in series (in-line) with the circuit so that all
electrons flowing through the circuit also have to go
through the meter. Because measuring current in this manner
requires the meter be made part of the circuit, it is a more
difficult type of measurement to make than either voltage or
Some digital meters, like the unit shown in
the illustration, have a separate jack to insert the red
test lead plug when measuring current. Other meters, like
most inexpensive analog meters, use the same jacks for
measuring voltage, resistance, and current. Consult your
owner's manual on the particular model of meter you own for
details on measuring current.
When an ammeter is placed in series with a
circuit, it ideally drops no voltage as current goes through
it. In other words, it acts very much like a piece of wire,
with very little resistance from one test probe to the
other. Consequently, an ammeter will act as a short circuit
if placed in parallel (across the terminals of) a
substantial source of voltage. If this is done, a surge in
current will result, potentially damaging the meter:
Ammeters are generally protected from
excessive current by means of a small fuse located
inside the meter housing. If the ammeter is accidently
connected across a substantial voltage source, the resultant
surge in current will "blow" the fuse and render the meter
incapable of measuring current until the fuse is replaced.
Be very careful to avoid this scenario!
You may test the condition of a multimeter's
fuse by switching it to the resistance mode and measuring
continuity through the test leads (and through the fuse). On
a meter where the same test lead jacks are used for both
resistance and current measurement, simply leave the test
lead plugs where they are and touch the two probes together.
On a meter where different jacks are used, this is how you
insert the test lead plugs to check the fuse:
Build the one-battery, one-lamp circuit
using jumper wires to connect the battery to the lamp, and
verify that the lamp lights up before connecting the meter
in series with it. Then, break the circuit open at any point
and connect the meter's test probes to the two points of the
break to measure current. As usual, if your meter is
manually-ranged, begin by selecting the highest range for
current, then move the selector switch to lower range
positions until the strongest indication is obtained on the
meter display without over-ranging it. If the meter
indication is "backwards," (left motion on analog needle, or
negative reading on a digital display), then reverse the
test probe connections and try again. When the ammeter
indicates a normal reading (not "backwards"), electrons are
entering the black test lead and exiting the red. This is
how you determine direction of current using a meter.
For a 6-volt battery and a small lamp, the
circuit current will be in the range of thousandths
of an amp, or milliamps. Digital meters often show a
small letter "m" in the right-hand side of the display to
indicate this metric prefix.
Try breaking the circuit at some other point
and inserting the meter there instead. What do you notice
about the amount of current measured? Why do you think this
Re-construct the circuit on a breadboard
Students often get confused when connecting
an ammeter to a breadboard circuit. How can the meter be
connected so as to intercept all the circuit's current and
not create a short circuit? One easy method that guarantees
success is this:
Identify what wire or component terminal
you wish to measure current through.
Pull that wire or terminal out of the
breadboard hole. Leave it hanging in mid-air.
Insert a spare piece of wire into the hole
you just pulled the other wire or terminal out of. Leave
the other end of this wire hanging in mid-air.
Connect the ammeter between the two
unconnected wire ends (the two that were hanging in
mid-air). You are now assured of measuring current
through the wire or terminal initially identified.
Again, measure current through different
wires in this circuit, following the same connection
procedure outlined above. What do you notice about these
current measurements? The results in the breadboard circuit
should be the same as the results in the free-form (no
Building the same circuit on a terminal
strip should also yield similar results:
The current figure of 24.70 milliamps (24.70
mA) shown in the illustrations is an arbitrary quantity,
reasonable for a small incandescent lamp. If the current for
your circuit is a different value, that is okay, so long as
the lamp is functioning when the meter is connected. If the
lamp refuses to light when the meter is connected to the
circuit, and the meter registers a much greater reading, you
probably have a short-circuit condition through the meter.
If your lamp refuses to light when the meter is connected in
the circuit, and the meter registers zero current, you've
probably blown the fuse inside the meter. Check the
condition of your meter's fuse as described previously in
this section and replace the fuse if necessary.