Day 4 Meters Voltage and Current

On this day we meet electric circuits and make measurements on them using multi-meters. Meters that can measure voltage and current.

We use two kinds of meters, both costing under $20.

The more modern digital multimeters.

And, the more traditional multimeter with a needle and a dial.

Red and Black Leads

On both meters start by attaching a black lead to the COM input, which stands for common, and by attaching a red lead to the volt/ohm/amp input. This is standard practice with the black lead representing the reference point and the red lead representing the measurement point.

What Positive and Negative readings mean

If the digital or the dial scale read a positive number then you are measuring current flowing into the red lead or voltages which are higher at the red lead than the black lead.

If the digital meter reads negative number, the dial scale may not read at all, then you are measuring a current flowing out of the red lead, or a voltage at the red lead that is lower than the reference point.

Electricity is mostly invisible and abstract so we make models of electricity to explain it. Here are a few words on models.

On Models of Electricity

With the theory behind us, let's make some measurements.

We'll measure voltages and currents on a circuit constructed on a board made by Don Rathjen. This is a board with 8 alligator clips screwed onto it. The alligator clips can be attached to batteries, lamps, wires and a variety of other circuit components.

Start by measuring voltage

Meters, Voltage

The "What's Going On" section of this activity discusses what voltage is. But it is important to measure voltage first before encountering its definition.

Meters Current

Next we use both the current and the voltage measurement capacity of the meters to measure both the open circuit voltage and the short circuit current made by a battery.

Meters Battery

And, finally,we use the meters to measure the resistance of a circuit element and the power dissipated by a circuit element.

Meters Resistance

On this day we measured and measured and measured.

Don Rathjen points out that his high school students start out lost in a maze of connections, and dial settings, and scale readings when they first start to make readings on meters. But that after making 3 sets of measurements they become comfortable with and capable of making measurements using a multimeter.

Scientific Explorations with Paul Doherty

© 2000

2 August 2000