An electret carries a permanent electric field just as a permanent magnet
carries a permanent magnetic field. One way to think of an electret is as
a capacitor that always carries a charge and that you never have to recharge
(except when you make it initially or something bad happens to it
such as it is melted.)
Needing some electrets to test one of my theories for how the
testatika might work I tried
making some electrets.
Making Electrets
Working with high voltage is dangerous so take necessary
precaustions. Further, wear suitable goggles and gloves when making electrets.
The basic steps to making an electret at home are:
- heat the electret material to the melting point and keep it hot for a while,
- turn off the heat, heat up the top electrode (I use a heat gun), move it to above
the electret
material and turn on the high voltage power supply thereby placing
an electric field across the electret material,
- once the electret has cooled, turn off the power supply, and
- quickly wrap the electret material completely in aluminium foil to short
out both sides,
- wait a few weeks before unwrapping and testing.
By the time the few weeks are up, many of the dipolar molecules (molecules with
two poles: positive and negative) within the electret will have lined up in such
a way that the resulting electric field has the opposite polarity of the electric
field that was used in step 2.
Looking closely at the above steps you see that you need a heat source,
high voltage power supply, ... Here's a setup I've used in the past.
In the above picture are the following:
1. 30kV DC power supply,
2. variac voltage control for power supply,
3. high voltage probe,
4. oscilloscope for reading voltage using above probe,
5. high voltage wire (positive) feeding top electrode,
6. ground connection (negative) feeding bottom electrode,
7. electric heater,
8. ventilation hose.
Make sure you have good ventilation! When the electret material is
heated up it will likely give off toxic fumes or at least ones you don't want
to breath.
Closeup of the electret on the heater. Note the ventilation
hose on the right. The electret is an acrylic disc (Lucite, transparent) in this case.
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Closeup of the necessary electrodes and the electret,
a wax mix in this case. Notice that because the high voltage was on while this
picture was taken, the material under the top electrode has risen up to touch it.
This is due to dielectrophoresis.
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Just how you make the top electrode is up to you. It will have to be something
that you can put in place after the electret material has been heated. Also,
it will have to be detachable from the wires that feed it in case you find you
can't easily separate the electret material from the electrode once it is ready
to wrap up. This has happened to me on occasion so I ended up making a new
electrode for the next electret while the old one sat wrapped up with the electret
for a month.
The electrode should have well rounded edges to prevent
ionization and arcing. I've drilled four small holes angled inward that don't
go all the way through. On the right you can see the 14AWG wire ends that will slide
into the four holes. That's all that'll be holding on to the electrode so that
the wires can be easily detached.
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The wires are in the holes and some tape has been wrapped
around to make sure the wires press firmly into the holes. Since the wire ends
are in the holes, ionization from them is minimized, if not eliminated. Look
again at the pictures above to see the wire/electrode in place.
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Step 2 is to apply a high voltage across the electret. This can be dangerous
if an electrical arc will cause an explosion from whatever material you are
using for your electret. If that's possible then it's best to pick
a different material. With wax, an arc can cause the wax to splatter, causing
a burn if it hits you. For advice on handling this, see the
Scientific American article "How to make an electret" by C.L.Stong, November 1960,
on Richard Haider's website, specifically starting in column one on the
first page, then the second page, endng on the third.
In making some electrets, I turned off the heat and turned on the voltage
to 6kV. At this point the electret was still liquid. When wax is heated, its
internal resistance goes down. As wax cools its internal resistance goes up.
So the voltage would slowly creap up and at around 6.5kV I'd turn it back down.
I had to do this repeatedly until the wax cooled sufficiently for its internal
resistance to stabalize.
In the case of wax, when you turn off the heat and turn on the high voltage
you want the wax to take a long time to cool. This is why
C.L.Stong in his Scientific American article uses an oil bath surrounding
the electret mold. However, I had trouble making such a setup and went without it.
Mototaro Eguchi, in his article,
doesn't use an oil bath and had good results so I felt okay doing without it,
even though my results may not be as good.
Steps 4 and 5 were to wrap up the electrets in aluminium foil, shorting out the
two sides. Wierd, but that's what you have to do if you want a long lasting electret.
Two wrapped electrets sitting on the shelf. The
cylinders are just weights to keep the wrapping on well.
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