The Wheatstone bridge and related matters, like what is your greatest achievement in life?


Wheatstone (1802-1875) is a famous UK physicist.  He is best known for the Wheatstone bridge a device to measure resistance.  Using an AC or RF voltage the bridge can also measure impedance. It is here that my story begins.  Impedance is the ratio of voltage to current flowing in a device.  When current is impeded then the device has a high impedance. I have spent most of my life studying physics, especially plasma physics and I got drawn into the area of plasma measurement. The plasma is special in that it has a complex non-linear impedance.  This is due to the conduction of the sheaths. Current can flow from the electrode to the plasma carried by free electrons flowing across the sheath.  However, when the voltage changes sign the current flowing from the plasma to the electrode is limited because electrons are not free to leave the electrode.  In this phase of the RF cycle, the current is carried by positive ions which are less mobile than electrons.  The impedance is very different depending on the direction of the voltage. This produces a non-linear impedance and will create harmonics of the applied RF voltage.  I worked for many years to develop a sensor technology that can measure accurately the plasma impedance at RF frequencies as a way to diagnose and control RF plasma devices. A little bit like developing an advanced version of the Wheatsone bridge.  My company is called impedans.  Pretty obvious name really, if a bit nerdy.

But the story does not end there.  When my granddaughter was born in 2005, I wanted to bring music back into the extended family. Our grand parents were very musical, music and dance were an important part of every day life, but we have lost touch with that way of life.  I used to dance as a child and heard all the great traditional Irish tunes.  But, I never learned to play. I always regretted that,  when I would come across a traditional Irish music session I would be so jealous of the players and their skill.

A physics colleague from the US and his wife visited Ireland about the time of my grand daughters birth. They played guitar and concertina. I loved the sound of the concertina and thought is was such a beautiful little instrument.  I fell in love with it and told my family I would love to learn to play.  That Christmas, to my great surprise my wife and family bought me a concertina as a present. I was shocked but determined to learn. This began a long journey and my love for the instrument was tested by the complexity of mastering Irish traditional music with it speed and ornamentation on the concertina.

Playing concertina at an Irish traditional music session: Dublin, Ireland

Twelve years later, my granddaughter is soon starting secondary school and I have finally graduated to play in sessions.  Above is a picture of me on the concertina playing with some friends, banjo and Uillean pipes.

The original owner of my Wheatstone concertina photographed in the 1920’s

The photo above shows the original owner of my Wheatstone Anglo concertina. The photo is taken in the 1920’s.  I feel this simply photo is a connection back to Wheatstone himself.  Nearly 200 years after the invention of the concertina the design remains the same. In fact the best concertinas come from a period shortly after Wheatstone died, until the 1920’s.  None of the new instruments can match the quality of this period.  I hope my Octiv, impedance and power meter is an improvement on the original Wheatstone bridge but it came with a tremendous effort. A similar effort was needed to get a decent tune out of the Wheatstone concertina. Nothing worth doing comes easy.  One of the most surprising things I have ever been told is that -(despite my international career as a scientist with almost 100 hundred papers and book chapters, building several successful businesses) – my greatest achievement in life was mastering the concertina!

Mike B Hopkins


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