Hertz – clarified and expanded the electromagnetic theory of light

Heinrich Rudolf Hertz

Heinrich Rudolf Hertz, born in Hamburg on February 22, 1857, was the first person to prove electromagnetic waves exist (hence the wavelength doodle) and that electricity can be carried through them. He even lends his name to a unit of measurement, the hertz, which is equal to one cycle per second and is used to gauge frequency.

Hertz was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect radio waves. Heinrich Rudolf Hertz helped establish the photoelectric effect (which was later explained by Albert Einstein) when he noticed that a charged object loses its charge more readily when illuminated by ultraviolet light.

The power of sending messages through space, in any direction, over great distances, is so enormous an addition to the utility of aircraft that a few words must here be said about wireless telegraphy. The discovery was made by the gradual researches of men of science. These researches had their beginning in a famous paper by James Clerk Maxwell, who subsequently became the first professor of experimental physics at Cambridge. His paper, On a Dynamical Theory of the Electro-magnetic Field, read to the Royal Society in 1864, contains a theoretical demonstration that electro-magnetic action travels through space in waves with the velocity of light.

Twenty-three years later, in 1887, Heinrich Rudolf Hertz, of the University of Bonn, published the results of his experiments in producing these waves by means of oscillating currents of electricity.

His investigations confirmed what Clerk Maxwell had proved mathematically. Thereafter progress was rapid, and during the closing years of the nineteenth century the problem of subduing the waves to the service of man was attacked and solved.

In 1889 Professor , was measuring electrical radiation.  At Liverpool  University College he constructed a Hertz radiator to emit the waves, and received them at various points of the building.
Edouard Branly’s invention of the ‘coherer’, an instrument designed to receive Hertzian waves, was communicated to the British Association at Edinburgh in 1893.
During the same year Nikola Tesla published his researches on high frequency currents; on these much of the later work on wireless telegraphy was based.
In 1895-6 William Rutherford set up at the Cavendish Laboratory apparatus by which he received signals in distant parts of Cambridge up to a distance of half a mile from the oscillator.

Many other men of science, among whom was Captain H. B. Jackson, of the Royal Navy, were at work on the problem, when in 1896 Signor Guglielmo Marconi arrived in England with an apparatus of his own construction which ultimately brought wireless telegraphy to the stage of practical and commercial utility. By 1899 signals had been transmitted across the English Channel.
HERTZIAN WAVE.
From the Popular Science Monthly, June-December, 1903

This work involved, not merely the ordinary experience of an electrical engineer, but also the careful consideration of many new problems and the construction of devices not before used. Every step had to be made secure by laboratory experiments before the responsibility could be incurred of advising on the nature of the machinery and appliances to be ordered. Many months in the year 1901 were thus occupied by the author in making small-scale experiments in London and in superintendence of large-scale experiments at the site of the first power station at Poldhu, near Mullion, in Cornwall, before the plant was erected and any attempt was made by Mr. Marconi to commence actual telegraphic experiments. As this work was of a highly confidential nature, it is obviously impossible to enter into the details of the arrangements, either as made by the writer in the first instance, or as they have been subsequently modified by Mr. Marconi. The design of the aerial and of the oscillation transformers and many of the details in the working appliances are entirely due to Mr. Marconi, but as a final result, a power plant was erected for the production of Hertzian waves on a scale never before attempted. The utilisation of 50 H.P. or 100 H.P. for electric wave production has involved dealing with many difficult problems in electrical engineering, not so much in novelty of general arrangement as in details. It will easily be understood that Leyden jars, spark balls and oscillators, which are quite suitable for use with an induction coil, would be destroyed immediately if employed with a large alternating-current plant and immensely powerful transformers.

Wooden Towers supporting the Marconi Aerial at Poldhu Power Station, Cornwall, England.

In the initial experiments with this machinery and in its first working there was very considerable risk, owing to its novel and dangerous nature; but throughout the whole of the work from the very beginning, no accident of any kind has taken place, so great have been the precautions taken. The only thing in the nature of a mishap was the collapse of a ring of tall masts, erected in the first place to sustain the aerial wires, but which now have been replaced by four substantial timber towers, 215 feet in height, placed at the corners of a square, 200 feet in length. These four towers sustain a conical arrangement of insulated wires which can be used in sections and which constitute the transmitting radiator or receiver, as the case may be. Each of these wires is 200 feet in length and formed of bare stranded wire.

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Transversal electromagnetic waves, according to Heinrich Hertz's 1887 experiments

Nothing is more remarkable, however, than the small amount of energy which, if properly utilised in electric wave making, will suffice to influence a sensitive receiver at a distance of even one or two hundred miles. Suppose, for instance, that we charge a condenser consisting of a battery of Leyden jars, having a capacity of one seventy-fifth of a microfarad, to a potential of 15,000 volts; the energy stored up in this condenser is then equal to 1·5 joules, or a little more than one foot-pound. If this energy is discharged in the form of a spark five millimetres in length through the primary coil of an oscillation transformer, associated with an aerial 150 feet in height, the circuits being properly tuned by Mr. Marconi’s method, then such an aerial will affect, as he has shown, one of Mr. Marconi’s receivers, including a nickel silver filings coherer tube, at a distance of over two hundred miles over sea. Consider what this means. The energy stored up in the Leyden jars cannot all be radiated as wave energy by the aerial, probably only half of it is thus radiated. Hence the impartation to the ether at any one locality of about half a foot-pound of energy in the form of a long Hertzian wave is sufficient to affect sensitive receivers situated at any point on the circumference of a circle of 200 miles radius described on the open sea.

Hertzian wave telegraphy is sometimes described as being extravagant in power, but, as a matter of fact, the most remarkable thing about it is the small amount of power really involved in conducting it. On the other hand, Hertzian wave manufacture is not altogether a matter of power. It is much more dependent upon the manner in which the ether is struck. Just as half an ounce of dynamite in exploding may make more noise than a ton of gunpowder, because it hits the air more suddenly, so the formation of an effective wave in the ether is better achieved by the right application of a small energy than by the wrong mode of application of a much larger amount. If we translate this fact into the language of electronic theory, it amounts simply to this. It is the electron alone which has a grip of the ether. To create an ether wave, we have to start or stop crowds of electrons very suddenly. If in motion, their motion implies energy, but it is not only their energy which is concerned in the wave making, but the acceleration, positive or negative—i.e., the quickness with which they are started or stopped. It is possible we may discover in time a way of manufacturing long ether waves without the use of an electric spark, but at present we know only one way of doing this—viz., by the discharge of a condenser, and in the discharge of large condensers of very high potentials it is difficult to secure that extreme suddenness of starting the discharge which we can do in the case of smaller capacities and voltages.

How strange it is that the discharge of a Leyden jar studied so profoundly by Franklin, Henry, Faraday, Maxwell, Kelvin and Lodge should have become an electrical engineering appliance of great importance!

Whilst there are many matters connected with the commercial aspect of Hertzian wave telegraphy with which we are not here concerned, there is one on which a word may properly be said. The ability to communicate over long distances by Hertzian waves is now demonstrated beyond question, and even if all difficulties are not overcome at once, it has a field of very practical utility, and may even become of national importance. Under these circumstances, we may consider whether it is absolutely necessary to place the signalling stations so near the coast. The greater facility of transmission over sea has already been discussed and explained, but in time of war, the masts and towers which are essential at present in connection with transmitting stations could be wrecked by shot or shell from an enemy’s battleship at a distance of five or six miles out at sea, and would certainly be done within territorial waters. Should not this question receive attention in choosing the location of important signalling stations? For if they can, without prejudice to their use, be placed inland by a distance sufficient to conceal them from sight, their value as a national asset in time of war might be greatly increased.

It has been often contended that whilst cables could be cut in time of war no one can cut the ether; but wireless telegraph stations in exposed situations on high promontories, where they are visible for ten to fifteen miles out at sea and undefended by any forts, could easily be destroyed. The great towers which are essential to carry large aerials are a conspicuous object for ten miles out at sea; and a single well-placed shell from a six-inch gun would wreck the place and put the station completely out of use for many months. Hence if oceanic telegraphy is ever to be conducted in a manner in which the communication will be inviolable or, at any rate, not be capable of interruption by acts of war, the careful selection of the sites for stations is a matter of importance. A small station consisting of a single 150-foot mast and a wooden hut can easily be removed or replaced, but an expensive power station, the mere aerial of which may cost several thousand pounds, is not to be put up in a short time

 

 

Reference:
“THE WAR IN THE AIR” By WALTER RALEIGH – OXFORD THE CLARENDON PRESS (1922)

“HERTZIAN WAVE WIRELESS TELEGRAPHY” – By Dr. J. A. FLEMING, F.R.S.
[From the Popular Science Monthly, June-December, 1903.]
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http://en.wikipedia.org/wiki/Heinrich_Hertz