Science Buddies: "Ask an Expert"

The written instructions provided online for this project (https://www.sciencebuddies.org/science- ... p053.shtml) appear to be conflicting with one of the figures included with it.

Step 6 and Step 7 of the "Making a Simple Conductance Sensor" section state the following:

6. Attach one of the free copper wires on the conductance sensor (Figure 1) to the positive terminal of the 9-V battery, using the wire with one of the alligator clips.

7. Attach the other copper wire from the sensor to the black terminal of the multimeter, using the other alligator clip.

Figure 2, which appears below Step 8, depicts the right conductance sensor wire connected to (presumably) the black terminal of the multimeter, per Step 7. But it depicts the other (left) conductance sensor wire connected to the negative terminal on the 9V battery, and the positive terminal of the 9V battery connected directly to what I assume is the red terminal of the multimeter. There is no explicit reference to Figure 2 within the written instructions, but what it depicts seems to conflict with the written instructions.

We have been unable to obtain any reading other than zero on the multimeter. What is the correct configuration?

Hello and welcome to the forums!

The figure appears to be correct. The list does not include some of the connections. If you follow the picture (figure 2), you should get some data. Is your solution conductive? Remember that pure water does not conduct. If you have a solution with plenty of ions in it, and it doesn't seem to conduct, try the following. First, is your copper wire on the sensor insulated? It may have some insulating material painted on it. If so, you will need to scrape and or burn it off (be careful if you burn it off - it gets hot!). If you are sure that the wire is conducting (try shorting the ends together - do you see movement on the multimeter?), then move the 2 coils on your sensor closer together and try again.

You could test your medium by just sticking the probes from the multimeter in the solution. Set the multimeter on a very high ohms scale. Clean off the probes! They will corrode. Let us know if any of this helps.

Keith

There IS a discrepency between the incomplete schematic in Figure 2 and the picture. The picture shows the red (+) battery terminal attaching to the top end of the conductance probe while the schematic shows the + battery terminal attached to the milliamp meter.

Because there is no inherent "+" or "-" lead on the conductance probe, the only requirement to get positive current readings from the milliamp meter is that you hook opposite polarities of one battery lead and a milliamp meter lead together. The picture shows the black negative (-) battery lead connecting to the red positive (+) milliamp meter lead and the other leads connecting to the conductivity probe. Hooking the red positive (+) battery lead to the black negative (-) milliamp meter lead would also provide positive current readings with the other leads on the conductivity probe.

If you hook meter + to battery + (or meter - to battery -) with the other leads connected to the conductivity probe, you will get negative current readings.

Thank you both for your replies. Turns out we did not have the multimeter leads plugged into the proper set of jacks on the multimeter itself. We managed to track down a neighbor, who is an electrician, and he got us straightened out. Thanks again.