Induction

A hundred years or so separates the invention of the electric guitar and the discovery of the principle behind the electric guitar. This seems like a rather long time but as with most new ideas in science the gap between a new discovery or concept and its technological and commercial application can last several years, if not decades. We are now getting closer to having a working fusion reactor (please see my previous blog ITER for more on this). But the concept of producing energy from controlled nuclear fusion originated in the late 1910s, early 1920s – again, nearly a hundred years ago.

The electric guitar is useless without the application of electromagnetic induction. What does that mean precisely? Well, back in 1831, Michael Faraday discovered that electricity could be produced using a magnet. What was thought to be two distinct phenomena – i.e. electricity and magnetism – turned out to be related. More specifically, it turned out to be the same phenomenon but simply expressed differently. For decades electricity and magnetism were studied as separate subjects until Faraday came along to demonstrate that they are in fact connected to one another. And it wasn’t until about 30 years after Faraday’s discovery that James Clerk Maxwell provided a rigorous mathematical foundation to the relationship between electricity and magnetism. Maxwell showed that electricity and magnetism were essentially the two faces of the same coin, namely electromagnetism. His description of the electromagnetic field in terms of his now famous equations also provided us with another interesting fact: that the speed of light, denoted by the letter c, is a universal constant whose exact value can be determined mathematically. (Please see my blog Relativity for more on this.) Light is but one slice of the electromagnetic spectrum as described in Spectrum.

Maxwell Equations showing the relationship between electric field, E, and magnetic field, B.

Maxwell Equations showing the relationship between electric field, E, and magnetic field, B.

Back to Faraday. What he demonstrated was that a moving magnet can induce an electric current in an adjacent circuit. Not only that but, if an electric current was switched on and off, this change in the current from on to off or off to on, would cause another current to flow in an adjacent circuit. He devised several ways to show this phenomenon but it all depended on the rather curious relationship between changing electric currents and moving magnets. Now, whether the magnet is moving relative to the electric circuit or whether it is the magnet that’s stationary and the circuit in motion, it doesn’t quite matter. What’s important is the relative motion of one system to the other. (A principle which would prove key in Einstein’s development of his Theory of Relativity and the subsequent derivation of the world’s most famous equation: E = … well, you know the rest.)

An electric current in a circuit produces both an electric and a magnetic field. But varying the current causes the magnetic field to vary. And, as Faraday found out, a varying magnetic field can induce a current in another circuit. This is the principle of electromagnetic induction. It’s very simple but very powerful – like most fundamental principles in science. It’s even more relevant because it has several applications. The electric guitar is one of them. And here’s how it works: the guitar steel strings are magnetised by a series of permanent magnets located in the pickup. The pickup also comprises of a number of coiled wires which make up the electric circuit. As the magnetised guitar strings are strummed, they vibrate over the electric coils. And according to the principle of electromagnetic induction, these moving magnets or strings induce a current in the coils. The current thus induced can be amplified and sent to a loudspeaker to give a roaring or whining sound of the electric guitar.

Science expressed as an art

Science expressed as an art

Other applications of electromagnetic induction include electrical generators which convert mechanical energy (movement) into electrical energy (electricity). For instance, a waterfall can be used to turn a turbine which turns a magnet over a coil (or coil over a magnet) to produce electricity. Graphics tablets are also based on electromagnetic induction: the pen act as a magnet moving over a grid of wires on the tablet which detect the changing magnetic field and thus able to induce a current to carry the signal or information about the location and movement of the pen. Some devices can now be charged wirelessly using electromagnetic induction. The charging base station induces an electric current in the device and thus charges the battery. Even the kitchen can benefit when this principle is applied to cooking. Induction cooking transfer energy form the base to the pots directly without the use of heated stoves or gas. The downside is that the pots have to be magnetic in order to respond to the energy transfer by induction but the upside is that this is significantly more efficient than traditional cooking methods using gas and, more importantly, is safer as the stove itself remains cool while only heating the pot. Some travel cards are also based on this principle. They contain a coil which when waved over a detector induces a current which is then interpreted as a signal to allow the user of the card to pass through the barrier.

As you can imagine, there are other practical ways to apply the principle of electromagnetic induction. What is interesting is how such a basic and simple phenomenon has transformed the world. So much of what we do relies on this principle. A world without electricity and magnetism and their relationship is truly inconceivable. We can only hope that the discoveries and inventions being made currently will have far-reaching applications decades from now and will completely revolutionise the way we live. This is the power of science.

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