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Measuring Electricity

Real World Current Measurement

In the previous section, we discussed current measurement and connections. Lets go over an example use for it now; using a common light circuit. For the example, lets assume we have a switch, a light bulb, and a battery. We would like to estimate how long we can run our light on a fully charged battery.

The circuit is wired up in a typical fashion with the positive battery lead going to the switch; a connecting wire from the other switch terminal going to the light bulb, and finally a connecting wire from the bulb back to the batteries ground terminal.

Having no specifications on the bulb available, we will start out with the 10A range on our meter to be safe; and we have connected our test leads to the 10A port on our meter. Because we have a simple two terminal switch, we won't need to disconnect anything in the circuit itself.

Current measurement connection diagramFinding the positive battery lead goes to the top terminal of the switch, we know that is the most positive connection so we connect our red test probe to that switch terminal; and then our black probe to the remaining switch terminal.

With the switch off, we find that our meter is reading 0.1A when connected and our light bulb has illuminated. Because the switch is off, we know that all of the current must be passing thru our meter. Such a low reading on a 10A range isn't very accurate, so we decide to use a smaller range for our final reading.

Temporarily, we disconnect our meter from the circuit. We then change the setting dial to a lower range(lets say 200mA) and connect the test probes to the appropriate port(most meters this will be the standard voltage ports in this range). We then reconnect to the circuit, and find that our meter is now showing 125mA. This sounds, and is, much more precise and accurate than we had before so we will use this reading.

Interpreting the results

Our original question was how long can we operate our light on a fully charged battery. For the sake of discussion, we will assume the batteries rated capacity is accurate; and it says the battery is 1000mA/h.

(Please refer to my battery specifications article if needed)

Thus, it is now simple arithmetic to divide the 1000mAh capacity by the 125mA draw ... and we can estimate an 8 hour battery life.

In the real world, our battery may not be exactly 1000mAh but we have a pretty good estimate.

In the next section, we will discuss resistance measurements ...

Next Section (Coming Soon!)