Measuring Current

Ammeter

An ammeter is an instrument which is used to measure the current which flows through a wire. Always remember the following:

• An ammeter is always placed in series with the resistor(s) when measuring the current.
• An ammeter always has a low impedance (resistance) so that there is a negligible voltage across it, and so does not disturb the circuit.

Instructions:

You are given three resistors which are connected to the positive terminal of a battery which maintans an overall voltage V. You are also given an ammeter to measure the current through each of the resistors. One terminal of the ammeter is connected to ground (the green dot) and to the negative terminal of the battery. The other probe is floating, and you can connect it to the end (shown as red dots) of any resistor in the circuit to measure the current throught that resistor. (Hold down the left mouse button to drag the probe. As you make a measurement the ammeter display turns red.)

Measure and note all of the currents, then fill in the answers to the questions below. When you have finished, you can check your answers, with the responses appearing in the boxes to the right.

Try to answer the following questions: What is the current through the left resistor? mA What is the current through the middle resistor? mA What is the current through the right resistor? mA If you connected wires from each of the resistors (red dots) to the ground (green) dot, what would be the total current taken from th battery? mA and then Check the answers.	This page requires a Java compatible browser.

Resistors in Parallel

You also know that for resistors in parallel, the voltage across each resistor is the same, and equal to the voltage of the battery. Now that you know all the currents, and all of the resistances, use Ohm's Law to calculate the voltages separately, to verify that they are indeed all the same.