Experiment of the Month

Introduction to the Oscilloscope as a Voltmeter

The MU Physics Department does not claim that we invented these labs. Actually, the origin of these labs are currently unknown to us. Our labs do not have written instructions. In keeping with this spirit, the description given here will be brief and general. The intent is that each performance of the lab will be unique; in each nature will reveal a slightly different face to the observer.

In this exercise we measure the electric potential of "9 volt" batteries with oscilloscopes. We set the 'scope to dc, with the time base at about .1msec/division. We have students draw a detailed picture (in their lab notebook) of the 'scope front panel, with all the settings identified. They then measure the potential of a single battery, noting the effect on the display of reversing the polarity of the connection. They measure using the 5V/div scale, the 2V/div scale, and the 1V/div scale. On our 'scopes, at 1V/div the line goes off scale; an important experience for the students.

Two batteries are then connected in series as sketched in the figure.

figure 5

Students have already measured the voltage of each battery, and estimated the uncertainty of those results. Now they calculate the sum of the two voltages, and the uncertainty in the sum. This is compared to the measured value of the two batteries in series, to test the proposed rule that the voltage of batteries in series is the sum of the voltages of the individual batteries.

Students continue adding batteries in series and measuring the resulting batteries, working in larger and larger groups. (Each student has one battery.) The resulting data are listed on the blackboard and the blackboard data are used to make a graph of voltage versus number of batteries. The slope is the voltage of the average single battery.

This lab acts as an introduction to the oscilloscope and as a review of uncertainties and graphing techniques from the previous semester.