Introduction

When you measure something you change it.

Here is a simple way to estimate the size of the change you make when you use a meter to make a measurement.

The left meter measures the short circuit current through a solar cell.

Material

Solar cell any size will do.

2 identical meters. older analog meters like the ones shown work best.

Clip leads as needed

To Do and Notice

Measure the short circuit current produced by the solar cell, to do this.

Place the solar cell under light that doesn't change.

Set the meter to measure DC electric current.

Connect the two leads from the meter to the two leads from the solar cell.

Measure the current.

However, adding the meter into the circuit actually changes the value of the short circit current. Here is a way to estimate the amount of the change.

Take the second meter, set it to the same DC current scale as the first meter then connect it in series with the first meter.

Connect two current meters in series with a solar cell.

Measure the current on the first meter again.

Notice that the current value measured by the meter may have changed when you added the second meter.

What's Going On ?

When you measure something you change the thing you are measuring.

If the change is small you can estimate the change by introducing a second measurement.

If placing one meter into a circuit reduces the current by d percent then placing a second meter in series will reduce it by 2d percent.

Thus the change in the measurement of the current when you add the second meter is an estimate for the amount the first meter changed the system.

For Example in room light our solar cell produced 30 microamps as measured by the first meter. Adding the second meter reduced the measured current to 22 microamps.

Thus we can estimate that the current before our first meter was added to the circuit to make the measurement was 38 microamps.

Going Further

You can also measure the open circuit voltage made by the solar cell.

To estimate the error introduced by the meter add a second meter in parallel with the first one.

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Scientific Explorations with Paul Doherty

© 2001

15 Jan 2001