The Steam Battery: A Low-Cost Science Experiment Performed with Ordinary Materials

Mark Valentine, Electrical Engineer

Introduction

Most semiconductor components are vulnerable to static electricity, also referred to as electrostatic discharge (ESD). These components are usually packaged with a piece of black, electrically conductive foam (referred to hereafter as “ESD foam”) placed over their electrical leads as a form of protection from ESD. Because ESD foam has a large internal surface area within a relatively small volume, it is plausible to assume that certain changes in the environment surrounding a sample of ESD foam might cause observable changes in the electrical resistance of the sample.

Initial investigations revealed that a sudden change in humidity from my breath did indeed result in a change in resistance for several ESD foam samples. However, the resistance readings appeared erratic and occasionally read as negative on the digital multimeter (DMM) used for the measurements. I soon found that the ESD foam samples were producing a DC voltage, which led to the formulation of a new hypothesis and experiment to address the means by which humidity could be generating the observed DC voltage in an apparently uniform ESD foam sample. Specifically, the experiment was designed to determine whether the DC voltage in the ESD foam settles into a steady state with the continuous localized application of humid air. It was also designed to determine the relationship between the polarity of the DC voltage across the sample and the relative position of the region on the sample directly exposed to the high-humidity airflow. If the experiment were to be framed within the guidelines of the scientific method, it might be expressed as follows:

Problem : Explain how an apparently uniform sample of ESD foam can produce a DC voltage in response to sudden exposure to high-humidity air.

Hypothesis: The polarity and persistence of the observed DC voltage between two electrodes in contact with a sample of ESD foam depends on the humidity gradient created and maintained within the sample with respect to the position of the electrodes.

Experiment: First, verify that the DC voltage between two electrodes in contact with the ESD foam sample can be sustained by constant and localized application of humid air (breath) on the sample (which should create a humidity gradient within the sample with respect to the two electrodes). Second, apply localized humid air to the opposite end of the sample (which should reverse the previous humidity gradient with respect to the two electrodes) and verify that the generated DC voltage is sustained with reversed polarity.

Conclusions: The application of humid air, and thus the formation of a humidity gradient, was not carefully controlled in the previous resistance experiments in which DC voltage generation was first observed in ESD foam samples. If the expected results of the present experiment do not occur, the hypothesis is incorrect, because the uncertainty in the humidity gradient that plagued the previous experiment has been eliminated. If the expected results do occur, then they lend qualitative support to the hypothesis but reveal further development is needed to explain the detailed relationship between the high-humidity air stream and the polarity and magnitude of the generated DC voltage.

Building the Apparatus

Figure 1 shows the apparatus I used to conduct the experiment previously described. There are many other possible designs. The only real requirements are that the ESD foam sample under test be secured firmly to a non-conducting surface by two electrodes that use the same metal (such as aluminum foil, which can be seen in the photograph) to make electrical contact with the sample, and that humid air can be directed to opposite ends of the sample.

Figure 1. Sample pieces and apparatus (shown with one sample piece installed) used to perform the ESD foam experiment.

The following supplies are required for constructing the apparatus shown in Fig. 1:

•  Perfboard—available from Radio Shack.

•  Two large paperclips.

•  Two pieces of aluminum foil, each cut to 5 cm x 2.5 cm (2” x 1”).

•  Ten cm (2 ft) of fishing line.

•  One new soda straw (please see important WARNING below under “Conducting the Experiment”).

•  One paper towel.

•  Five black ESD foam samples cut to approximately 2.5 cm x 1.25 cm x 0.6 cm (1.0” x 0.5” x 0.25”).

The following tools will be required to build the apparatus and conduct the experiment:

•  Digital Multimeter (manual ranging DMM preferred in order to prevent erratic data readings that can occur with auto ranging; analog meters will not work due to their relatively low input impedance).

•  Stopwatch (or clock with second hand).

•  Spring-loaded probe clips.

•  Needle-nose pliers.

•  Scissors.

The first step in building the apparatus is to fashion two supporting electrodes from the two paper clips. Use the needle-nose pliers to bend the paperclips into the shape illustrated in Figure 2, noting for reference that the electrodes should be about 2 cm (0.75 in) wide.

Figure 2. Illustration of paper clip electrode.

Insert the free ends of the electrodes through holes in the perfboard so that the two electrodes will be situated in a symmetrical fashion with the innermost segments of the electrodes being slightly less than 2.5 cm (1 in) apart, as shown in Fig. 1. Bend the inserted ends of the electrodes (resembled by the dashed lines in Fig. 2) flat against the bottom side of the perfboard so that they point toward the outermost perfboard edges. Then, using the needle-nose pliers, tightly compress the resulting elbow-shaped bends that pass through the holes so the electrodes will be held tightly in place.

Wrap one piece of aluminum around the innermost segment of each electrode. It should be wrapped tightly, but it is acceptable for the aluminum to roll on the electrode. Install a piece of ESD foam with opposite ends secured lightly under these aluminum coverings. Adjust the shape of the electrodes so the contact force is firm enough to secure the foam against the perfboard, but light enough to allow the foam to be removed and replaced easily.

Next, cut the fishing line into four equal pieces, and cut the soda straw in half. Place a round stopper made from a tightly rolled piece of paper towel snugly into one end of each straw half (the stopper should restrict foam fibers from traveling back through the straw; it also restricts the speed of the airflow through the straw). Position each piece of straw so that the end with the stopper is pressed against one end of the foam sample, and the other end projects out beyond the edge of the perfboard. Secure both pieces of straw into these positions with fishing line wrapped over the ends of the straws and threaded through the underlying holes in the perfboard. Tie each piece of fishing line in a double knot so the straws are held firmly enough to the perfboard to prevent slipping but not so tightly that the straws cannot be replaced. Trim all the free ends of string. This completes the assembly of the apparatus.

Conducting the Experiment

WARNING: Only one person should use the apparatus. If more than one person uses the same straws, there is a risk of contracting bacterial meningitis. Young people, including college students, should not share drinking straws, cups, or eating utensils for the same reason.

The above warning is issued because it is necessary to blow through the straws to conduct the experiment. Breath has high relative humidity, which is the reason condensation forms on a mirror brought in close proximity to exhaled air. Breath therefore provides a convenient and safe source of humid air if the above warning is heeded.

The experiment is conducted according to the steps that follow. Following this procedure, there is a section devoted to data collection that shows my data for the experiment in Table 1. This might prove useful, because a formatted table for data collection should be prepared before performing the experiment. Also, it may be helpful to refer to Fig. 3, a photograph of the complete apparatus used for the experiment.

Figure 3. Apparatus connected to DMM.

Perform the following experimental procedure on each of the five ESD foam samples:

•  Secure an ESD foam sample to the apparatus.

•  Attach the probes of the DMM—using the spring-loaded clips—to the electrodes of the apparatus (the portion of the electrodes that are perpendicular to the face of the perfboard work well for this).

•  Measure and record the resistance of the sample and record.

•  Reverse the probe connections at the DMM jacks; repeat step (3).

•  Change the mode setting and the probe jack connections of the DMM to measure DC voltage (black to “-“; red to “+”).

•  Using a pressure similar to that for blowing up a balloon, apply slow deep breaths to the straw closest to the electrode connected to the black probe of the DMM for five minutes, recording the voltage observed initially and then every 30 seconds. Bear in mind that some samples will not produce a large voltage, so excessive pressure is not necessary. Also, if this activity results in light-headedness, please slow down!

•  After five minutes, stop applying breaths. Record the elapsed time between the time of the last applied breath and the time required for the magnitude of the voltage to decay to the magnitude of the initial voltage, or to the magnitude of 25 mV, whichever comes first.

•  Apply slow deep breaths to the other straw for 5 minutes, recording the voltage observed initially, and then every 30 seconds.

•  After 5 minutes, stop applying breaths. Record the elapsed time between the time of the last applied breath and the time required for the magnitude of the voltage to decay to the magnitude of the initial voltage or to the magnitude of 25 mV, whichever comes first.

Data Collection

Table 1 shows the data I collected while performing the previously described experiment. When performing this experiment, note that readings on the DMM will fluctuate, and that the voltage data shown here are “snapshots” of the values displayed on the DMM at the designated time intervals.

* Note:For sample #5, the Decay Time was measured as the time for the sample to reach a magnitude of 6 mV, since the voltage magnitudes for this sample were mostly below 25 mV except for a few outliers.

Table 1: Data collected from five samples during the experiment.

Analysis and Conclusions

For all samples, the voltage was observed to fluctuate but not decay during the portion of the experiment where the humid air was steadily applied. For certain samples the resistance was sufficiently high to allow for the possibility that the meter was registering electrical noise. However, for other samples the resistance was low enough to eliminate that effect as the one responsible for voltage buildup. Furthermore, the polarity of the voltage did reverse for each sample when the flow of high-humidity air was reversed, which is critical to the accuracy of the hypothesis. Therefore, the trends in the data do not challenge the hypothesis.

There are, of course, other general observations that lend support to the hypothesis by eliminating other mechanisms that might be producing the observed DC voltage. For example, the condensed water from breath should be relatively distilled, preventing it from acting as an electrolyte for an electrochemical reaction. Furthermore, high humidity would tend to suppress the buildup of static electricity, and because the airflow through the straws is restricted, static electricity probably would not develop even if the ambient air were sufficiently dry. These facts tend to refute the possibility that the observed DC voltage is a form of triboelectricity. However, while these observations are encouraging, it is obvious that much more detailed experimentation is needed before the hypothesis can be confirmed.

Going Further

There are many possibilities for more sophisticated experiments. For one thing, it would be interesting to include the detailed chemical composition for each sample of ESD foam as part of the experimental data. Another interesting experiment would be to monitor the buildup and decay of the DC voltage on an oscilloscope to get a clearer picture of the voltage fluctuations on small time scales. Yet another experiment might simultaneously monitor and record the voltages at several points on the surface of the ESD foam sample. Mechanical variations on the experiment are also possible. For instance, steam from heated water could be drawn through the ESD foam sample using a weak vacuum pump, which might produce a more reliable humidity gradient within the sample. However, while there are many possibilities for new experiments, they would all share the goal of testing any hypothesis that explains the generation of the observed DC voltage in the ESD foam.

I hope that you will find this experiment enjoyable and inspiring. However, the real purpose of this experiment is to evoke appreciation for the scientific method, which is not a mere formality for confirming an expected result, but rather a process that equips the experimenter to make new discoveries.