Popular Science Monthly/Volume 63/August 1903/Hertzian Wave Wireless Telegraphy III
|HERTZIAN WAVE WIRELESS TELEGRAPHY. III.|
PROFESSOR OF ELECTRICAL ENGINEERING, UNIVERSITY COLLEGE, LONDON.
WE have to consider in connection with this part of the subject the dielectric strength of air under different pressures and for different thicknesses. It was shown by Lord Kelvin, in 1860, that the dielectric strength of very thin layers of air is greater than that of thick layers. The electric force, reckoned in volts per centimeter, required to pierce a thickness of air from two to ten millimeters in thickness, at atmospheric pressure, may be taken at 30,000 volts per centimeter. The same force in electrostatic units is represented by the number 100, since a gradient of 300 volts per centimeter corresponds to a force of one electrostatic unit. It appears also that for air and other gases, there is a certain minimum voltage (approximately 400 volts) below which no discharge takes place, however near the conducting surfaces may be approximated. In this particular practical application, however, we are only concerned with spark lengths which are measured in millimeters or centimeters, lying say between one or two millimeters and five or six centimeters. Over this range of spark length we shall not generally be wrong in reckoning the voltage required to produce a spark between metal balls in air at the ordinary pressure to be given by the rule:
Disruptive voltage spark gap length in millimeters.
If, however, the air pressure is increased above the normal by including the spark balls in a vessel in which air can be compressed, then the spark length corresponding to a given potential difference very rapidly decreases. Mr. F. J. Jervis-Smith found that by increasing the air pressure from one atmosphere to two atmospheres round a pair of spark balls, he reduced the spark length given by a certain voltage from 2.5 em. to 0.75 cm.
Professor R. A. Fessenden has also made some interesting observations on the effect of using compressed air round spark gaps. He found that if a certain voltage between metal surfaces would yield a spark four inches in length, at the ordinary pressure of the air, if the spark balls were enclosed in a cylinder, the air round them compressed at 50 lbs. per square inch, the spark length for the same potential difference of the balls was only one quarter of an inch, or one sixteenth of its former value.
The writer has also made experiments with an apparatus designed to study the effect of compressed air round the spark gap. The experimental arrangements are as follows: A ten-inch induction coil has one of its terminals connected to the internal coating of a battery of Leyden jars. The external coating is connected through the primary coil of an oscillation transformer with the other secondary terminal of the coil, and these secondary terminals are also connected to a spark gap consisting of two brass balls enclosed in a glass vessel into which air can be forced by a pump, the air pressure being measured by a gauge. The balls in the glass vessel are set at a distance of about three millimeters apart. The secondary circuit of the oscillation transformer is connected to another pair of spark balls, the distance of which can be varied.
Suppose we begin with the air in the glass vessel containing the balls connected to the secondary terminals of the induction coil, which may be called the secondary balls, at atmospheric pressure, and create oscillatory discharges in the primary coil of the oscillation transformer, we have a spark between the balls, which may be called the tertiary balls, connected to the secondary terminals of the oscillation transformer. If the secondary balls are placed, say three millimeters apart, the air in the glass vessel enclosing them being at the ordinary atmospheric pressure, then with one particular arrangement of jars used, a spark twenty-five or twenty-six millimeters long between the tertiary balls will take place. Suppose then we increase the pressure of the air round the secondary balls, pumping it up by degrees to 10, 20, 30, 40 and 50 lbs., per square inch, above the atmospheric pressure. We find that the spark between the tertiary balls will gradually leap a greater and greater distance, and when the pressure of the air is 50 lbs. per square inch, we can obtain a fifty-millimeter spark between the tertiary balls, whereas when the air in the glass vessel is at atmospheric pressure, we can only obtain a spark between the tertiary balls of half that length.
This experiment demonstrates that the effect of compressing the air round the secondary terminals of the induction coil is to greatly increase the difference of potential between these balls before the spark passes. In fact, it requires about double the voltage to force a spark of the same length through air compressed at 50 lbs. on the square inch that it does to make a spark of identical length between the same balls in air at normal pressure. This shows that there is a very great advantage in taking the discharge spark in compressed air. A better effect can be produced by substituting dry gaseous hydrochloric acid for air at ordinary pressures.
One other incidental advantage is that the noise of the spark is very much reduced. The continual crackle of the discharge spark of the induction coil in connection with wireless telegraphy is very annoying to sensitive ears, but in this manner we can render it perfectly silent. Professor Fessenden also states that when the spark balls are surrounded by compressed air, and if one of the balls is connected with a radiator, the compression of the air, although it shortens the spark gap corresponding to a given voltage, does not in any way increase the radiation. When, however, the air in the spark ball vessel is compressed to 60 lbs. in the square inch, there is a marked increase in the effective radiation, and at 80 lbs. per square inch the energy emitted in the form of waves is nearly three and a half times greater than at 50 lbs., the potential difference between the balls remaining the same.
This effect is no doubt connected with the fact that the production of a wave, whether in ether or in any other material, is not so much dependent upon the absolute force applied as upon the suddenness of its application. To translate it into the language of the electronic theory, we may say that the electron radiates only whilst it is being accelerated, and that its radiating power, therefore, depends not so much upon its motion as upon the rate at which its motion is changing.
The advantage in using compressed air round the spark gap is that we can increase the effective potential difference between the balls without rendering the spark non-oscillatory. In air of the ordinary pressure there is a certain well-defined limit of spark length for each voltage, beyond which the discharge becomes non-oscillatory, but by the employment of spark balls in compressed air, we can increase the potential difference between the balls corresponding to a given distance apart before a discharge takes place, or employ higher potentials with the same length of spark gap. In addition to this, we have, perhaps, the production of a more effective radiation, as asserted by Fessenden, when the air pressure exceeds a certain critical value.
The next element which we have to consider in the transmitting arrangements is a condenser of some kind for storing the energy which is radiated at intervals. Where a condenser other than the aerial is employed for storing the electric energy which is to be radiated by the aerial, some form of it must be constructed which will withstand high potentials. As the dielectric for such a condenser, only two materials seem to be of any practical use, viz., glass and micanite. Glass condensers in the form of Leyden jars have been extensively employed, but they have the disadvantage that they are very bulky in proportion to their electrical capacity. The instrument maker's quart Leyden jar has a capacity of about one five hundredth of a microfarad, but it occupies about 150 cubic inches or more. Professor Braun has employed in his transmitting arrangements condensers consisting of small glass tubes like test tubes, lined on the inside and outside with tinfoil, which are more economical in space. The author has found that condensers for this purpose are best made of sheet glass about one eighth or one tenth of an inch in thickness, coated to within one inch of their edge on both sides with tinfoil, and arranged in a vessel containing resin or linseed oil, like the plates of a storage battery. M, d'Arsonval has employed micanite, but although this material has a considerably higher dielectric strength than glass, it is much more expensive to obtain a given capacity by means of micanite than by glass, although the bulk of the condenser for a given capacity is less.
To store up a certain amount of electric energy in a condenser, we require a certain definite volume of dielectric, no matter how we may arrange it, and the volume required per unit of energy is determined by the dielectric strength of the material. Thus, for instance, ordinary sheet glass can not be safely employed with a greater electric force than is represented by 20,000 volts for one tenth of an inch in thickness, or say a potential gradient of 160,000 volts per centimeter. This is equivalent to an electric force of about 500 electrostatic units. This may be called the safe-working force. The electrostatic capacity of a condenser formed of two metal surfaces a foot square separated by glass three millimeters in thickness is between 1/360 and 1/400 of a microfarad. If this condenser is charged to 30,000 volts, we have stored up in it half a joule of electric energy, and the volume of the dielectric is 270 cubic centimeters. Hence to store up in a glass condenser electric energy represented by one joule at a pressure of 20,000 volts, we require 500 cubic centimeters of glass, and it will be found that if we double the pressure and double the thickness of the glass, we still require the same volume. Hence in the construction of high tension condensers to store up a given amount of energy, the economical problem is how to obtain the greatest energy-storing capacity for the least money. Glass fulfils this condition better than any other material. Although some materials may have very high dielectric strength, such as paper saturated with various oils, or resins, yet they can not be used for the purpose of making condensers to yield oscillatory discharges, because the oscillations are damped out of existence too soon by the dielectric.
In arranging condensers to attain a given capacity, regard has to be taken of the fact that for a given potential difference there must be a certain total thickness of dielectric, and that if condensers of equal size are being arranged in parallel, it adds to their capacity, whilst joining them in series divides their capacity. If N equal condensers or Ley den jars have each a capacity represented by C and if they are joined n in series and m in parallel, the joint capacity of the whole number is mC/n, where the product mn N.
Passing on next to the consideration of oscillation transformers of various kinds—these are appliances of the nature of induction coils for transforming the current or electromotive force of electrical oscillations in a required ratio. These coils are however destitute of any iron core, and they generally consist of coils of wire wound on a fiber, wooden or ebonite frame, and must be immersed in a vat of oil to preserve the necessary insulation. No dry insulation of the nature of indiarubber or guttapercha will withstand the high pressures that are brought to bear upon the circuits of an oscillation transformer. In constructing these transformers, we have to set aside all previous notions gathered from the design of low frequency iron core transformers. The chief difficulty we have to contend against in the construction of an effective oscillation transformer is the inductance of the primary circuit and the magnetic leakage that takes place. In other words, the failure of the whole of the flux generated by the primary circuit to pass through or be linked with the secondary circuit. Mr. Marconi has employed an excellent form of oscillation transformer, in the design of which he was guided by a large amount of experience. In this transformer the two circuits are wound round a square wooden frame. The primary circuit consists of a number of strands of thick insulated cable laid on in parallel, so that it consists of only one turn of a stranded conductor. The secondary circuit consists of a number of turns, say ten to twenty, of thinner insulated wire laid over the primary circuit and close to it, so that the transformer has the transformation ratio of one to ten or one to twenty. In the arrangements devised and patented by Mr. Marconi, these two circuits, with their respective capacities in series with them, are tuned to one another, so that the time-period of each circuit is exactly the same, and without this tuning the device becomes ineffective as a transformer. There is no advantage in putting a number of turns on the primary circuit, because such multiplication simply increases the inductance, and, therefore, diminishes the primary current in the same ratio which it multiplies the turns, and hence the magnetic field due to the primary circuit remains the same. Where it is desired to put a number of turns upon a coil, and yet at the same time keep the inductance down, the writer has adopted the device of winding a silk or hemp rope well paraffined between the turns of the circuit, so as to keep them further apart from one another, and as the inductance depends on the turns per centimeter, this has the effect of reducing the inductance.
The next and most important element in any transmitting station is the aerial or radiator, and it was the introduction of this element by Mr. Marconi which laid the foundation for Hertzian wave telegraphy as opposed to mere experiments with the Hertzian waves. We may consider the different varieties of aerial which have been evolved from the fundamental idea. The simple single Marconi aerial consists of a bare or insulated wire, generally about 100 or 150 feet in length, suspended from a sprit attached to a tall mast. As these masts have generally to be erected in exposed positions, considerable care has to be taken in erecting them with a large margin of strength. To the end of a sprit is attached an insulator of some kind which may be a simple ebonite rod, or sometimes a more elaborate arrangement of oil insulators, and to the lower end of this insulator is attached the aerial wire. As at the top of the aerial we have to deal with potentials capable sometimes of giving sparks several feet in length, the insulation of the upper end of the aerial is an important matter.
In the original Marconi system, the lower end of the aerial was simply attached to one spark ball connected to one terminal of the induction coil, and the other terminal and spark ball were connected to the earth. In this arrangement, the aerial acted not only as radiator, but as energy-storing capacity, and as already explained, its radiating power was on that account limited. The earth connection is an important matter. For long distance work, a good earth is essential. This earth must be made by embedding a metal plate in the soil, and many persons are under the impression that the efficiency of the earth plate depends upon its area, but this is not the fact. It depends much more upon its shape, and principally upon the amount of its 'edge.' It has been shown by Professor A. Tanakadate, of Japan, that if a metal plate of negligible resistance is embedded in an infinite medium having a resistivity r, the electrical conductance of this plate is equal to 4π/r times the electrostatic capacity of the same plate placed in a dielectric of infinite extent. Hence in designing an earth plate, we have to consider not how to give it the utmost amount of surface, but how to give it the greatest electrostatic capacity, and for this purpose it is far better to divide a given amount of metal into long strips radiating out in different directions, rather than to employ it in the form of one big square or circular plate. The importance of the 'good earth' will have been seen from our discussion on the mode of formation of electric waves. There must be a perfectly free access for the electrons to pass into and out of the aerial. Hence if the soil is dry, or badly conductive in the neighborhood, we have to go down to a level at which we get a good moist earth. In fact, the precautions which have to be taken in making a good earth for Hertzian wave telegraphy are exactly those which should be taken in making a good earth for a lightning conductor.
Whilst on the subject of aerials, a word may be said on the localization of wireless telegraph stations on the Marconi system. For reasons which were explained previously, the transmission of signals is effected more easily over water than over dry land, and it is hindered if the soil in the neighborhood of the sending station is a poor conductor. Hence all active Hertzian wave telegraph stations, like all active volcanoes, are generally found near the sea. In those cases in which a multiple aerial has to be put up consisting of many wires, one mast may be insufficient to support the structure, and several masts arranged in the form of a square or a circle have to be employed. The illustrated papers have reproduced numerous pictures of the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova Scotia, and Cape Cod in the United States. In these stations, after preliminary failures to obtain the necessary structural strength with ordinary masts, tall lattice girder wooden towers have been built, about 215 feet in height, well stayed against wind pressure, and which so far have proved themselves capable of withstanding any storm of wind which has come against them.
An important question in connection with the sending power of an aerial is that of the relation of its height to the distance covered. Some time ago Mr. Marconi enunciated a law as the result of his experiments, connecting these two quantities, which may be called Marconi's Law. He stated that the height of the aerial to cover a given distance, other things remaining the same, varies as the square root of the distance. Let D be the distance and let L be the length of the aerial, then if both the transmitting and receiving aerial are the same height, we may say that D varies as L2. This relation may be theoretically deduced as follows: Any given receiving apparatus for Hertzian wave telegraphy requires a certain minimum energy to be imparted to it to make it yield a signal. If the resistance and the capacity of the receiver is taken as constant, this minimum working energy is proportional to the square of the electromotive force set up in the receiving aerial by the impact on it of the electric waves. This electromotive force varies as the length of the receiving aerial, and as the magnetic force due to the wave cutting across it, and the magnetic force varies as the current in the transmitting aerial, and therefore for any given voltage varies as the capacity, and therefore as the length of the transmitting aerial. If, therefore, the transmitting and receiving aerial have the same length, the minimum energy varies as the square of the electromotive force in the receiving aerial, and therefore as the fourth power of the length of either aerial, since the electromotive force varies as the product of the lengths of the aerials. Hence when the distance between the aerials is constant, the minimum working energy varies as the fourth power of the height of either aerial, but when the lengths of the aerials are constant, the energy caught up by the receiving aerial must vary inversely as the square of the distance D between the aerials. Hence if we call e this minimum working energy, e must vary as 1/D2 when L is constant, or as L4 when D is constant, and since e is a constant quantity for any given arrangements of receiver and transmitter, it follows that when the height of aerial and distance vary, the ratio L4/D2 is constant, or, in other words, D2 varies as L4 or D varies as L2, i. e., distance varies as the square of the height of the aerial, which is Marconi 's Law. The curve therefore connecting height of aerial with sending distance for given arrangements is a portion of a parabola.
Otherwise, the law may be stated in the form where a is a numerical coefficient. If L and D are both measured in meters, then for recent Marconi apparatus as used on ships , roughly. (See a report on experiments made for the Italian navy 1900-1901, by Captain Quintino Bonomo—'Telegrafia senza fili,' Home, 1902.)
This law, however, must not be used without discretion. After Mr. Marconi had transmitted signals across the British Channel, some people, forgetting that a little knowledge is a dangerous thing, predicted that aerials a thousand feet in height would be required to signal across the Atlantic, but Mr. Marconi has made such improvements of late years in the receiving arrangements that he has been able to receive signals over three thousand miles in 1903, with aerials only thirty-three per cent, longer than those which, in 1899, he employed to cover twenty miles across the British Channel.
We turn, in the next place, to the consideration of those devices for putting more power into the aerial than can be achieved when the aerial itself is simply employed as the reservoir of energy. Professor Braun of Strasburg, in 1899, described a method for doing this by inducing oscillations in the aerial by means of an oscillation transformer, these oscillations being set up by the discharges from a Leyden jar or battery of Leyden jars, which formed the reservoir of energy. The induction coil is employed to produce a rapidly intermittent series of electrical oscillations in the primary coil of an oscillation transformer by the discharge through it of a Leyden jar. Mr. Marconi immensely improved this arrangement, as described by him in a lecture given before the Society of Arts, on May 17, 1901, by syntonizing the two circuits and making the circuit, consisting of the capacity of the aerial and the inductance of the secondary circuit of the oscillation transformer, have the same time-period as the circuit consisting of the Leyden jars, or energy-storing condenser, and the primary circuit of the oscillation transformer, and by so doing immensely added to the power and range of the apparatus.
Starting from these inventions of Braun and Marconi, the author devised a double transmission system in which the oscillations are twice transformed before being generated in the aerial, each time with a multiplication of electromotive force, and a multiplication-of the number of groups of oscillations per second. This arrangement can best be understood from the diagram (see Fig. 15).
Fig. 15. Alternating Current Double Transformation Power Plant for Generating Electric Waves (Fleming), a, alternator; H1H2, choking coil; K, signaling key; T, step-up transformer; S1S2, spark gap; C1C2, condensers; T1T2, oscillation transformers; A, aerial; E, earth plate.
In this case, a transformer T or transformers, receive alternating low frequency current from an alternator a, being regulated by passing through two variable choking coils, H1 and H2, so as to control it. This alternating current is transformed up from a potential of two thousand to twenty, forty or a hundred thousand, and is employed to charge a large condenser C1, which discharges across a primary spark gap S1 through the primary coil of an oscillation transformer T1. The secondary circuit of the oscillation transformer is connected to a second pair of spark balls S2, which in turn are connected by a secondary condenser C2, and the primary circuit of a third transformer T2, and the secondary circuit of this last transformer are inserted between a Marconi aerial A and the earth E. When all these circuits are tuned to resonance by Mr. Marconi's methods, we have an enormously powerful arrangement for creating electric waves, or rather trains of electric waves, sent out from the aerial, and the oscillations are controlled and the signals made byone of the choking coils.
Another transmitting arrangement, which involves a slightly different principle, and employs no oscillation transformer, is one duo also to Professor Braim. In this case, a condenser and inductance are connected in series to the spark balls of an induction coil, and oscillations are set up in this circuit. Accordingly, there are rapid fluctuations of potential at one terminal of the condenser. If to this we connect a long aerial, the length of which has been adjusted to be one quarter of the length of wave corresponding to the frequency, in other words, to make it a quarter wave resonator, then powerful oscillations will be accumulated in this rod. The relation between the height (H) of the aerial and the frequency, is given by the equation , where n is the frequency of the oscillations and H the height of the aerial in centimeters. The frequency of the oscillations is determined by the capacity (C) and inductance (L) of the condenser circuit, and can be calculated from the formula
That is, the frequency is obtained by dividing into the number 5,000,000, the square root of the product of the capacity in microfarads, and inductance in centimeters, of the condenser circuit. It will be found, on applying these rules, that it is impossible to unite together any aerial of a length obtainable in practise with a condenser circuit of more than a very moderate capacity. It has-been shown that for an aerial two hundred feet in height the corresponding resonating frequency is about one and a quarter million. As we are limited in the amount to which we can reduce the inductance of a discharge circuit, probably to something like a thousand centimeters, a simple calculation shows that the largest capacity we can employ is about a sixtieth of a microfarad. This capacity, even if charged at 60,000 volts, would only contain thirty joules of energy, or about 22.5 foot-pounds, which is a small storage compared to that which can be achieved when we are employing the above described methods, which involve the use of an oscillation transformer. In such a case, however, it is an advantage to employ a spark gap in compressed air, because we can then raise the voltage to a much higher value than in air of ordinary pressure without lengthening the spark so much as to render it non-oscillatory.
When employing methods involving the use of an oscillation transformer, it is possible to use multiple aerials having large capacity, and hence to store up a very large amount of energy in the aerial, which is liberated at each discharge. The most effective arrangement is one in which the radiator draws off gradually a large supply of energy from a non-radiating circuit, and so sends out a true train of waves, and not mere impulses, into the ether, and as we shall see later on, it is only when the radiation takes place in the form of true wave trains that anything like syntony can he obtained.
There are a number of variants of the above methods of arranging the radiator and associated energy-storing in circuit. Descriptions of these arrangements will be found in patents by Mr. Marconi, Professor Slaby and Count von Arco, Sir Oliver Lodge, Dr. Muirhead, Professor Popoff, Professor Fessenden and others. In all cases, however, they are variations of the three simple forms of radiator already described.
Returning to the analogy with the air or steam siren suggested at the commencement of this article, the reader will see, in the light of the explanations already given, that all parts of the air wave producing apparatus have their analogues in the electrical radiator as used in Hertzian wave telegraphy. The object in the one case is to produce rapid oscillations of air particles in a tube, which result in the production of an air wave in external space; in the other case, the arrangement serves to produce oscillations of electrons or electrical particles in a wire, the movements of which create a disturbance in the ether called an electrical wave. Comparing together, item by item, it will be seen, therefore, that the induction coil or transformer used in connection with electric wave apparatus is analogous to the air pump in the siren plant. In the electrical apparatus, this electron pump is employed to put an electrical charge into a condenser; in the air wave apparatus, the air pump is employed to charge an air vessel with high pressure air. From the electrical condenser the charge is released in the form of a series of electrical oscillations, and in the air wave producing appliance, the compressed air is released in the form of a series of intermittent puffs or blasts. In the electrical wave producing apparatus, these electrical oscillations in the condenser circuit are finally made to produce other oscillations in an air wire or open circuit, just as the puffs of air finally expend themselves in producing aerial oscillations in the siren tube. Finally, in the one case we have a series of air waves and in the other case, a series of electrical waves. These trains of electric waves or air waves, as the case may be, are intermitted into long and short groups, in accordance with the signals of the Morse alphabet, and therefore the Hertzian wave transmitter, in whatever form it may be employed, when operated by means of a Marconi aerial, is in fact an electrical siren apparatus, the function of which is to create periodic disturbances in the universal ether of the same character as those which the siren produces in atmospheric air.
(To be continued.)
- See Proc. Roy. 80c., London, February 23 and April 12, 1860; or reprint of papers on electrostatics and magnetism, p. 247.
- See Phil. Mag., August, 1902, Vol. IV., p. 224, 6th Series. Mr. Jervis-Smith has also described an experiment to show how much the use of compressed air round a spark gap is of advantage in working an ordinary Tesla coil. In his British specification No. 12,039 of 1896, Mr. Marconi had long previously mentioned the use of compressed air round the spark gap.
- This energy storage is at the rate of 44 foot-pounds per cubic foot of glass. This figure shows what a relatively small amount of energy is capable of being stored up in the form of electric strain in glass. In the case of an air condenser, it is only stored at the rate of one foot-pound per cubic foot.
- See British specification No. 7,777 of 1900—G. Marconi, 'Improvements in Apparatus for Wireless Telegraphy.'
- That this number really does represent the order of this oscillation frequency in an aerial has been shown by C. Tissot, Comptes Rendus, 132, p. 763, March 25, 1901, by photographs taken of the oscillatory spark of a Hertzian wave telegraphic transmitter. (See Science Abstracts, Vol. IV., Abs. 1518.) He found frequencies from 0.5 million to 1.6 million.