A Calculated Risk: Going Where No Foam Has Gone Before

Mark Valentine, Electrical Engineer

A few months ago, Forrest M. Mims III, the editor of The Citizen Scientist, received an inquiry from an upper-level physics student that led to a brief (or perhaps not so brief) account of how I discovered the “Steam Battery.” Essentially, this is an arrangement that produces a voltage across a sample of black ESD (Electro Static Discharge) foam when human breath is applied to one side of it.

After I gave my account, I began thinking about well-documented effects that behave similarly to the steam battery. The most prominent is the “hot-point-probe” effect I suggested previously in the “Backscatter” column, in which a p-type or n-type silicon wafer generates a current between two probes when one is hot, and the other is cold. It's similar to the Seebeck effect, but a bit more complicated, since the current can flow in either direction depending on the type of wafer.

Figure 1. A digital calculator can be powered by a steam battery.

When the wafer is p-type, the hot probe is negative, and the cold probe is positive, because the free charge carriers are positive and flee from heat, leaving the stationary lattice under the hot probe negatively charged. To see if this might help explain the steam battery, I did a bit of internet research and found that graphite is weakly p-type.

This supported the theory that the hot-point probe effect was at work in the steam battery, because ESD foam seems to be composed of carbon (at least in part), and the side that acquired the negative polarity was the same side that was directly exposed to humid air (and most likely condensation and heat transfer taking place is more intense on this side than the opposite side).

However, after writing my previous update on the steam battery, I was still puzzling over one thing: the voltage between the electrodes persists long after humid airflow is applied (on the order of several minutes). I began mulling over this, thinking perhaps that humid air remains trapped within the foam after airflow stops, and water vapor in this trapped air continues to condense. However, the decay of the voltage across the foam seems to last far too long for this to be the case.

Instinctively, perhaps based on the perception that the ESD foam samples were always a bit damp after experiments, I placed a drop of tap water on top of one of the paper clips of the original steam battery apparatus. Immediately, the voltage on the meter jumped to about 0.5 V and remained there. This hinted at some kind of electrochemistry.

Next, I placed a mechanical pencil lead under the sample and used this as a third “center-tap” electrode. (Recall from basic electrolysis experiments that pencil lead is used for its low chemical reactivity). The potential between the pencil lead and the dry paper clip was 0.3 V, but it was nearly 0.8 V between the pencil lead and the wet paper clip on which the water drop sat. This explained the 0.5 V between the two paper clips, since the two electrochemical cells in the steam battery apparatus formed by the foam/paper clip junctions are in reverse series.

Incidentally, 0.8 V was the voltage measured when I first observed the steam battery effect over two years ago. So, the steam battery seemed to be an electrochemical device after all!

I dismantled the original steam battery, and constructed a crude apparatus using new paper clips, two pieces of 0.5-mm mechanical pencil lead, and two pieces of ESD foam cut from a single larger piece. For some reason the new cells only gave about 0.6 V each, but the three in series gave 1.8 V. The short-circuit current of this stack was too small to measure on my digital multimeter, but I wondered if it might be enough to power the technological miracle of our age, the $1.00 solar calculator.

I purchased one of these units, which are dual powered from an internal button-cell battery or a low-current, but high-voltage solar cell. I cut the leads to both power sources inside the calculator, and then ran the red and black leads that were once connected to the battery to the outside of the case. I then stripped the ends of these wires. Next, I rigged up a new ESD foam battery stack with three cells using the new paper clips and the same mechanical pencil lead and connected it to my multimeter with probe clips; the stack was still reading 1.8 V.

I pulled the plugs out of the meter's test-lead jacks, and fumbled around with them until I was able to pin down the ends of the wires running out of the calculator so that matching colors were in good electrical contact. The calculator swiftly displayed a dark “0,” and never before had I been so happy to see this most mysterious number, which is both something and nothing. At that instant, it meant everything! I did a quick check to make sure that 2 + 2 = 4, and the calculator confirmed my elementary-school education.

I then reconstructed this entire setup into a single, robust demonstration platform (including precision spring-clip jumpers, the most expensive part of the new device), which is shown in Figure 1 after having computed the square root of two (1.4142135).

Figure 2 shows a close up of each cell in the battery stack of the demo platform. The paper clips are negative (so the leftmost paper clip is connected to the black probe clip), and the pencil leads in contact with the foam samples are positive (so the rightmost pencil lead is connected to the red probe clip via the rightmost paper clip). As can be seen in the figure, a drop of water is placed on top of each paper clip, and bulges over on either side to touch the upper surface of the foam.

Figure 2. A close-up of the ESD foam battery stack that powers a calculator in the demonstration platform.

While many questions remain about the chemistry that might be going on in this new arrangement (Is it air breathing? Is it rechargeable? Why do some foam samples work when others don't? What causes each water drop to develop an opaque film on its surface?), the mystery of the steam battery would seem to be solved.

Apparently, humid air seeps through the ESD foam sample, and reaches the junctions between the foam and the paper clips. When humid air enters closer to one paper clip than the other (as it does in the steam battery), more condensation must be forming at this nearby junction, creating a greater amount of liquid water there than at the opposite junction. This would create a stronger cell voltage in that nearby junction that could overcome any potential of the reverse-series cell formed by the opposite junction, allowing a substantial voltage to develop between the two paper clips.

This phenomenon eluded my explanation because my brain refused to accept the possibility of a battery with two identical metals for electrodes (as opposed to the classic battery created when two dissimilar metals, such as copper and zinc, are stuck in a lemon). Speaking of elusive phenomena, none of this begins to explain how Dr. Christopher M. Sorensen's carbon aerosol gel sandwiched between two paper clips produces no voltage, yet drops in resistance when exposed to air, only to return to its nominal value (after drying out?). Looks like another job for mechanical pencil lead.

Alas, whatever comes of these further investigations, the steam battery would seem a mythical beast. Yet, it seems to have given birth to something equally bizarre, and I can prove the existence of this new creature mathematically . . . sort of.