| An Introduction To Electrical Measurements |
Measuring Current
Current is measured
with an ammeter
While voltage is a more common measurement, it is often necessary to measure
current. When measuring current, it is important to remember that
current is a flow variable.Current flows through electrical elements, and if you want to measure current
you have to get it to flow through the ammeter. Here's the same circuit we used
in the example above. Consider what we would have to do to measure the current
flowing through element #4.
If we want to measure the current through element
#4, we have to get that current to flow through the ammeter. Here's a way to
insert an ammeter into the circuit to measure that current.
However, this doesn't give the whole picture.
Remember that polarity is important. In the circuit the polarity for the
voltage across element #4 is defined, but the current polarity is not defined.
In the diagram below, we have defined the direction of that current, and given
it an algebraic name, Im.
As with the voltmeter, you need to pay attention
to the polarity, and you also want to remember this.
- When you measure current, the
ammeter should not disturb the circuit where you are attempting to measure
the current. In the circuit above, that disturbance is the voltage across
the ammeter. You want that voltage to be as close to zero as it can
possibly be. That means that you need to have the resistance of the
voltmeter as small as possible. Ideally, the resistance of an ammeter would
be zero.
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