The Galvanometer
One of the earliest and most significant transducers used for instrumentation is the galvanometer. Practically, it is one of the most important inventions ever produced by humankind, but it had rather humble beginnings.
Its first, and rather gruesome form, was the frog leg galvanoscope. Many of us have heard that one of the earliest descriptions of electricity was via frog legs that twitched when exposed to a coincidental saline battery. What is less well known is that someone looked at this, and thought that this was a spectacular idea for a sensor. Granted the best alternative at the time for detecting electric charge was poking an object and seeing if you got a shock.
The frog galvanoscope was a device consisting of an amputated frog’s leg, with a nerve left exposed, placed in a glass tube. Electrical contact is made with the exposed nerve. The frog leg will then twitch when a current is passed on the exposed nerve. I guess there are far worse things in the history of most professions right? This instrument was highly sensitive, but did not indicate how strong the ‘galvanic’ force was, hence ‘scope’ rather than ‘ometer’. The first move away from this device came when André-Marie Ampère described the mathematics underpinning the creation of a magnetic field about a wire with a flowing current, building on the work of Hans Christian Ørsted. Ampere very humbly named the instrument after Galvani, who had discovered the frog-leg effect, rather than himself. Later on, international agreement named the unit of current, the ampere or amp, after him.
The magnetic field of the wire is used to deflect a compass needle, with the earth’s magnetic field providing the force that returns the needle to ‘zero’, a so-called ‘tangential’ device. Naturally, having to align the equipment with the earth’s magnetic field was a significant limitation.
Many tried to improve this design, with several significant leaps forward, but the peak form was the D’Arsonoval/Weston type. Not only did this provide its own restoring force, but it also deflected linearly with extreme precision.
This works by suspending a coil on spiral springs, these are metallic and provide the electrical connection to the coil which is energised by the current to be measured. This coil is contained within a circular channel through a magnet. This means that as the coil rotates in response to the current, it doesn’t move away or toward the centre of the magnetic field keeping the relationship constant. A needle or mirror can be attached to the coil to measure the deflection. When calibrated, such that a deflection matches a known current, this galvanometer can be termed an ‘ammeter’ as it indicates the amount of amperes flowing through a conductor.
But why is this so significant? Why is the galvanometer fundamental to the field of instrumentation? It’s because so many other things could suddenly be measured by it. If the resistance of your galvanometer is known, for a given current, the voltage is also known, and a reliable voltmeter is born. If you attach a pen to the end of the needle and move paper past the needle at a constant rate, you now have a way of recording material over time. Polygraphs and electro-cardio-grams (heart traces) depend on this so too do dynamometers, built to measure the performance of trains and cars. This allowed speed and dynamics to be recorded with far greater accuracy than ever before possible. Previous attempts had depended on a human with a stopwatch counting the time taken between set distances, now it was possible to measure the eddy current produced by a rotating magnet attached to the wheel in minute detail and obtain speed from this. For anything that needed resistance changes to be measured, a galvanometer in the heart of a full bridge would be used, this unlocked our ability to accurately measure strain, with our old friend the strain gauge whilst also allowing heat to be measured electrically rather than relying on the expansion of mercury or alcohol. Whenever you see a gauge on an old piece of equipment, if it’s driven electrically, a galvanometer will be at its core.
In a very real sense, the galvanometer gave birth to the modern world, so much of our understanding, so many of our human achievements, have been unlocked by them and what they allowed us to record and measure. Now, in the computer age, with extremely accurate analogue-to-digital converters, the digital voltmeter has taken the galvanometer’s place as the fundamental transducer for sensing. Just as a calibrated galvanometer could measure voltage, a calibrated voltmeter can measure current and record it digitally to boot.
Yet even now, galvanometers are still used at the forefront of science. By attaching a mirror to a galvanometer mechanism, the mirror can be pointed with extreme precision by varying the current flowing through the galvanometer. This allows scanning lasers to be driven with stunning accuracy using a fundamental mechanism first developed hundreds of years ago in the first days of the study of electricity.
Here at Flintmore, we work extensively with the galvanometer’s successor, the analogue to digital voltmeters which are at the core of our instrumentation equipment. Yet we still appreciate the galvanometer as fundamental to the history of our profession, and the history of the world.
Further Reading
This article by the education organisation Byju’s dives into the underlying mathematics that describe the effects critical to the galvanometers function: https://byjus.com/jee/galvanometer/
Ferrovial gives a great summary article: https://www.ferrovial.com/en/stem/galvanometer/#:~:text=What%20is%20a%20galvanometer%3F,it%20experiences%20an%20electrical%20overload.
And, as with many transducers and non-contentious scientific phenomenon, the galvanometer article on wikipedia is of high quality and very detailed:
