1831 – England

‘A changing magnetic field around a conductor produces an electric current in the conductor. The size of the voltage is proportional to the rate of change of the magnetic field’

This phenomenon is called ‘electromagnetic induction’ and the current produced ‘induced current’. Induction is the basis of the electric generator and motor.

Faraday developed HANS CHRISTIAN OERSTED’s 1820 discovery that electric current could deflect a compass needle. In his experiment Faraday wrapped two coils of insulated wire around opposite sides of an iron ring. One coil was connected to a battery, the other to a wire under which lay a magnetic compass needle. He anticipated that if he passed a current through the first wire it would establish a field in the ring that would induce a current in the second wire. He observed no effect when the current was steady but when he turned the current on and off he noticed the needle moving. He surmised that whenever the current in the first coil changed, current was induced in the second. To test this concept he slipped a magnet in and out of a coil of wire. While the magnet was moving the compass needle registered a current, as he pushed it in it moved one way, as he pulled it out the needle moved in the opposite direction. This was the first production of electricity by non-chemical means.

In 1831, by rotating a copper disc between the poles of a magnet, Faraday was able to produce a steady electric current. This was the world’s first dynamo.

NEWTON, with his concept of gravity, had introduced the idea of an invisible force that exerted its effect through empty space, but the idea of ‘action-at-a-distance’ was rejected by an increasing number of scientists in the early nineteenth century. By 1830, THOMAS YOUNG and AUGUSTIN FRESNEL had shown that light did not travel as particles, as Newton had said, but as waves or vibrations. But if this was so, what was vibrating? To answer this, scientists came up with the idea of a weightless matter, or ‘aether’.

Faraday had rejected the concept of electricity as a ‘fluid’ and instead visualised its ‘fields’ with lines of force at their edges – the lines of force demonstrated by the pattern of iron fillings around a magnet. This meant that action at a distance simply did not happen, but things moved only when they encountered these lines of force. He believed that magnetism was also induced by fields of force and that it could interrelate with electricity because the respective fields cut across each other. Proving this to be true by producing an electric current via magnetism, Faraday had demonstrated electromagnetic induction.

When Faraday was discovering electromagnetic induction he did so in the guise of a natural philosopher. Physics, as a branch of science, was yet to be given a name.

The Russian physicist HEINRICH LENZ (1804- 65) extended Faraday’s work when in 1833 he suggested that ‘the changing magnetic field surrounding a conductor gives rise to an electric current whose own magnetic field tends to oppose it.’ This is now known as Lenz’s law. This law is in fact LE CHATELIER‘s principle when applied to the interactions of currents and magnetic fields.

Fluctuating Electromagnetic Fields and EM Waves

It took a Scottish mathematician by the name of JAMES CLERK MAXWELL to provide a mathematical interpretation of Faraday’s work on electromagnetism.

Describing the complex interplay of electric and magnetic fields, he was able to conclude mathematically that electromagnetic waves move at the speed of light and that light is just one form of electromagnetic wave.
This led to the understanding of light and radiant heat as moving variations in electromagnetic fields. These moving fields have become known collectively as radiation.

Faraday continued to investigate the idea that the natural forces of electricity, magnetism, light and even gravity are somehow ‘united’, and to develop the idea of fields of force. He focused on how light and gravity relate to electromagnetism.
After experimenting with many transparent substances, he tried a piece of heavy lead glass which led to the discovery of the ‘Faraday Effect’ in 1845 and proved that polarised light may be affected by a magnet. This opened the way for enquiries into the complete spectrum of electromagnetic radiation.

In 1888 the German physicist HEINRICH HERTZ confirmed the existence of electromagnetic waves – in this case radio waves – traveling at the speed of light.

The unit of capacitance, farad (F) is named in honour of Faraday.

TIMELINE

ELECTRICITY

# CHRISTIAN JOHANN DOPPLER (1803- 53)

1842 – Austria

‘Any source of sound or light moving away from an observer changes in frequency with reference to the observer’

DOPPLER

The pitch of the whistle of a train is higher when the train is approaching an observer standing on a platform and lower when it is moving away from the observer.

Doppler explained the effect by pointing out that when the source of sound is moving toward the observer, sound waves reach the ear at shorter intervals, hence the higher pitch. When the source is moving away the waves reach the ear at longer intervals, hence the lower pitch. The Doppler effect also occurs when the source of sound is stationary and the observer is moving.

Doppler predicted that a similar effect would apply to light waves.

Different colours are the optical equivalent of notes of different pitch; blue light vibrates at roughly twice the pitch of red light.

In 1929 EDWIN HUBBLE suggested that the Doppler effect applied to light coming from distant stars gives a measure of the distance and speed of distant galaxies.

TIMELINE

# HEINRICH RUDOLPH HERTZ (1857- 94)

1888 – Germany

‘Radio waves can be produced by electric sparks. They have the same speed as light and behave as light’

In 1864 MAXWELL‘s equations predicted the existence of electromagnetic waves.
His thinking had shown that electromagnetic waves could be refracted, reflected and polarized in the same way as light. Hertz was able to measure the speed of these waves and to show that the speed is the same as that of light.

Hertz hypothesised that he could experimentally examine the waves by creating apparatus to detect electromagnetic radiation. He devised an electric circuit with a gap that would cause a spark to leap across when the circuit was closed. If Maxwell’s theory was correct and electromagnetic waves were spreading from these oscillator sparks, appropriately sensitive equipment should pick up the waves generated by the spark.
Hence he constructed the equivalent of an antenna.
His simple receiver consisted of two small balls at the ends of a loop of wire, separated by a small gap. This receiver was placed several yards from the oscillator and the electromagnetic waves would induce a current in the loop that would send sparks across the small gap. This was the first transmission and reception of electromagnetic waves. He called the waves detected by the antenna ‘Hertzian waves’.

We are now familiar with all the types of electromagnetic waves that make up the complete electromagnetic spectrum. They all travel with the speed of light and differ from each other in their frequency. We measure this frequency in hertz.

It was left to the Italian electrical engineer GUGLIELMO MARCONI to refine this equipment into a device that had the potential of transmitting a message and to develop technology for the practical use of Hertzian  waves – when they became commonly known as radio waves.

Further experimentation showed that these waves had the properties that Maxwell had predicted.
As well as being important as a newly discovered phenomenon, Hertz’s discovery helped to prove that Maxwell had been correct when he suggested that light and heat were forms of electromagnetic radiation.

Radio waves are electromagnetic waves. Other main kinds of electromagnetic waves are: gamma rays; X-rays; ultra-violet radiation; visible light; infrared radiation and microwaves.

This radiation was behaving in all the ways that would be expected for waves, the nature of the vibration and the susceptibility to reflection and refraction were the same as those of light and heat waves. Hertz found that they could be focused by concave reflectors.

Experimenting further, Hertz spotted that electrical conductors reflect this electromagnetic radiation and that non-conductors allow most of the waves to pass through.

In honour of Hertz’s achievements, the SI unit of frequency, the hertz (Hz), was named after him.

Hertz’s discoveries came at an early age. The German physicist died at the age of thirty-six.

TIMELINE

ELECTRICITY