# GEORGE FITZGERALD (1851-1901) HENDRICK LORENTZ (1853-1928)

1890 – Ireland
1904 – Holland

‘A moving object appears to contract’

The contraction is negligible unless the object’s speed is close to the speed of light.

In 1890 Fitzgerald suggested that an object moving through space would shrink slightly in its direction of travel by an amount dependent on its speed.

In 1904 Lorentz independently studied this problem from an atomic point of view and derived a set of equations to explain it. A year later, Einstein derived Lorentz’s equations independently from his special theory of relativity.

TIMELINE

RELATIVITY

# JAMES CLERK MAXWELL (1831- 79)

1864 Scotland

The Scottish physicist examined Faraday’s ideas concerning the link between electricity and magnetism interpreted in terms of fields of force and saw that they were alternative expressions of the same phenomena. Maxwell took the experimental discoveries of Faraday in the field of electromagnetism and provided his unified mathematical explanation, which outlined the relationship between magnetic and electric fields. He then proved this by producing intersecting magnetic and electric waves from a straightforward oscillating electric current.

‘Four equations that express mathematically the way electric or magnetic fields behave’

In 1831 – following the demonstration by HANS CHRISTIAN OERSTED that passing an electric current through a wire produced a magnetic field around the wire, thereby causing a nearby compass needle to be deflected from north – MICHAEL FARADAY had shown that when a wire moves within the field of a magnet, it causes an electric current to flow along the wire.
This is known as electromagnetic induction.

In 1864 Maxwell published his ‘Dynamical Theory of the Electric Field’, which offered a unifying, mathematical explanation for electromagnetism.

In 1873 he published ‘Treatise on Electricity and Magnetism’.

The equations are complex, but in general terms they describe:

• a general relationship between electric field and electric charge
• a general relationship between magnetic field and magnetic poles
• how a changing magnetic field produces electric current
• how an electric current or a changing electric field produces a magnetic field

The equations predict the existence of electromagnetic waves, which travel at the speed of light and consist of electric and magnetic fields vibrating in harmony in directions at right angles to each other. The equations also show that light is related to electricity and magnetism.

Maxwell worked out that the speed of these waves would be similar to the speed of light and concluded, as Faraday had hinted, that normal visible light was a form of electromagnetic radiation. He argued that infrared and ultraviolet light were also forms of electromagnetic radiation, and predicted the existence of other types of wave – outside the ranges known at that time – which would be similarly explainable.

Verification came with the discovery of radio waves in 1888 by HEINRICH RUDOLPH HERTZ. Further confirmation of Maxwell’s theory followed with the discovery of X-rays in 1895.

JAMES CLERK MAXWELL

Maxwell undertook important work in thermodynamics. Building on the idea proposed by JAMES JOULE, that heat is a consequence of the movement of molecules in a gas, Maxwell suggested that the speed of these particles would vary greatly due to their collisions with other molecules.

In 1855 as an undergraduate at Cambridge, Maxwell had shown that the rings of Saturn could not be either liquid or solid. Their stability meant that they were made up of many small particles interacting with one another.

In 1859 Maxwell applied this statistical reasoning to the general analysis of molecules in a gas. He produced a statistical model based on the probable distribution of molecules at any given moment, now known as the Maxwell-Boltzmann kinetic theory of gases.
He asked what sort of motion you would expect the molecules to have as they moved around inside their container, colliding with one another and the walls. A reasonably sized vessel, under normal pressure and temperature, contains billions and billions of molecules. Maxwell said the speed of any single molecule is always changing because it is colliding all the time with other molecules. Thus the meaningful quantities are molecular average speed and the distribution about the average. Considering a vessel containing several different types of gas, Maxwell realized there is a sharp peak in the plot of the number of molecules versus their speeds. That is, most of the molecules have speeds within a small range of some particular value. The average value of the speed varies from one kind of molecule to another, but the average value of the kinetic energy, one half the molecular mass times the square of the speed, (1/2 mv2), is almost exactly the same for all molecules. Temperature is also the same for all gases in a vessel in thermal equilibrium. Assuming that temperature is a measure of the average kinetic energy of the molecules, then absolute zero is absolute rest for all molecules.

The Joule-Thomson effect, in which a gas under high pressure cools its surroundings by escaping through a nozzle into a lower pressure environment, is caused by the expanding gas doing work and losing energy, thereby lowering its temperature and drawing heat from its immediate neighbourhood. By contrast, during expansion into an adjacent vacuüm, no energy is lost and temperature is unchanged.

The explanation that heat in gas is the movement of molecules dispensed with the idea of the CALORIC fluid theory of heat.

The first law of thermodynamics states that the heat in a container is the sum of all the molecular kinetic energies.
Thermal energy is another way of describing motion energy, a summing of the very small mechanical kinetic energies of a very large number of molecules; energy neither appears nor disappears.
According to BOYLE’s, CHARLES’s and GAY-LUSSAC’s laws, molecules beating against the container walls cause pressure; the higher the temperature, the faster they move and the greater the pressure. This also explains Gay-Lussac’s experiment. Removing the divider separating half a container full of gas from the other, evacuated, half allows the molecules to spread over the whole container, but their average speed does not change. The temperature remains the same because temperature is the average molecular kinetic energy, not the concentration of caloric fluid.

In 1871 Maxwell became the first Professor of Physics at the Cavendish Laboratory. He died at age 48.

TIMELINE

ELECTRICITY

# 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