W. H. Eccles, D. So
Recent commendations of certain new Lodge-Muir-head wireless telegraph stations equipped with insulated oscillators and resonators of the 1897 English patent, make the moment opportune for a brief review of the reasons and facts that have made the use of the earthed radiator almost universal and the use of one unearthed rather exceptional. A plain statement of the present position of the rival cults - the earthed and the unearthed - in the light of widely accepted theory, can be made and is best made without recourse to mathematics.
In the early days of Hertzian space telegraphy, say ten years ago, the solution of the signaling problem was expected in the employment of straight beams of electrical radiation. The electric rays used, produced of course in the Hertzian manner, were to be project ed in any desired direction, and were to be capable of penetrating all obstacles between the sender and receiver. Hence it was that radiation of short wave length generated at the focus of a parabolic mirror was the vogue among the " wireless " experimenters of that day. It was probably in an endeavor to get a bigger output of radiation per spark that Marconi in 1896 connected large insulated conductors - storehouses-to one side of his oscillator. Contrary, perhaps, to what he had expected, signals were better when these insulated conductors were lifted to a distance from the earth (and their capacity thus reduced) and better still when the far side of the spark-gap was earthed. In fact, capacity in itself seems of so little account that the large elevated conductor could be removed and the connecting wire left supported alone in the air without greatly diminishing the strength of signals. Thus Marconi fol lowed Popoff without copying him, to the use of the plain earthed air-wire, originally designed, it seems, by Popoff for other purposes than wireless telegraphy.
The scientific basis, as we understand it, of these rapid successes of Marconi have been given numberless times. We see clearly enough now that it supports Marconi's own opinion that his success was largely due to the earthed air-wire. It is necessary here to write down once again the virtues, as oscillator and radiator, of the long vertical earthed air-wire. Put shortly, its merits are: First, its great length inspires slow vibrations, and therefore long waves; second, it sets up the waves so that the electric force in a wave front is, from the very beginning, in a vertical plane; third, what is partly implied in the last, it attaches to the surface of the earth the free ends of the moving lines of force; fourth, it is a good radiator and absorber.
Long waves are advantageous largely for reasons connected with diffraction; the setting vertically of the electric force naar the earth avoids losses such as would otherwise occur from the generation of profitless currents in the earth's substance; and the attachment of the waves to the earth's surface prevents them straying wholly into space, and gives them such guidance (similar to that given by a wire to the current it carries) as enables ranges of hills to be surmounted without suggestion of penetration. But the air-wire's power as a good radiator, valuable at a time when the object was to "get there," has become in this era of syntonic ambition, its cheafest fault if it is in itself the whole oscillator; for, as has often been explained, rapid radiation shortens enormously the train of effective waves if, as must be in the case of the simple air wire, the energy stored electrostatically, just before each spark, is small. There is no need to do more here than mention that by using a closed (non-radiating) oscillator of great energy capacity to feed the simple air-wire, sustained trains of waves can be omitted and the earthed air-wire thus retained in its old post of honor.
At a very early stage of this story, Lodge seems to have perceived this inadequacy of the simple air-wire for syntonic working; and at least as early as 1897, was using in preference a symmetrical Hertzian wave-maker. The one at Heysham, for example, consists of two horizontal conducting plane areas about 80 feet square, one vertically above the other, and connected to it by a vertical central wire about 80 feet long. The lower square is about three feet from the ground and, like the upper, is insulated. The vertical wire is interrupted at its middle by a spark gap. This structure is from its geometrical configuration, a slower generator than a simple air-wire, and gathers before each spark a much greater charge.
For both reasons it produces trains or waves more sustained than those of the air-wire. Moreover, it possesses most of the merits of the plain air-wire. It makes very long waves, and it sets them up with their moving lines of force in vertical planes. But it fails in this, that it does not attach the waves to the earth's surface. The waves have to do that for themselves, as they may do when free later on. To compensate for this, however, there stands the complete freedom from earth connection. What this means can only be appreciated by recalling one, let us say, of the difficulties that beset the radiator that is earthed, namely the variability in the goodness of the "earth."
This variation of quality of " earth " did not matter so much in untuned working, but in tuned telegraphy it is more important; and when making comparisons with the Lodge Muirhead wave-maker, we must look to tuned systems. Suppose then, that in a syntonic transmitter the radiator is tuned to the closed feeding circuit when there is a good earth, and that by some means the earth goes bad - i. e., acquires an unwonted resistance. Then, inevitably, foreign overtones, of kind and strength depending on the badness of the earth, are introduced into the vibration. These over-tones are produced at the expense of the fundamental by reflection at the resistance of the pulses descending the air-wire.
There is, besides, absorption of energy in the resistance; indeed, it has been shown both theoretically and experimentally, that with a certain critical resistance, the absorption of the vibrations in the resistance may be extremely rapid. If this last condition is ever happened upon in practice, it must be of serious import even when forced vibrations of the correct period, namely, that of the distant receiver, can be maintained upon a transmitter air-wire by a heavy feeding system behind it. All these considerations apply with equal or even greater force to the earthed absorbing wire of the receiving system. Hence it will be seen, to be earth free is something worth striving for. Thus the question is, does the attainment of this desideratum by the Lodge-Muirhead wave-maker, compensate fully for the fact that the waves as at first emitted are free, that is, unattached to the earth's surface?
If one traces in imagination the movements of the lines of electric force near the Lodge-Muirhead radiator when in action, he concludes that the lines which detach themselves from the conductor to form waves appear as rings situated in vertical planes passing through the vertical central wire, moving outward horizontally with the speed of light and expanding as they go. They will resen.ble, in fact, the rings familiar to us in diagrams of the ordinary Hertzian oscillator, but will probably be more distorted than those in the pictures and will include a smaller proportion of the whole number of lines of force initially present in the fully charged oscillator. As each ring moves forward and expands, its lower bend strikes the earth and is reflected or absorbed there. If the earth's surface were of very good conducting material, the lower half of each ring would ultimately be folded up towards coincidence with the upper half.
Now, in this process of reflection, the direction of the horizontal component of electrical force is reversed. Thus, high reflection at the earth's surface would involve considerable strengthening of the waves near the earth - just where the strength is wanted. As a matter of working experience, this strengthening by reflection seems not to be experienced at the earth's surface in any degree so completely as has just been indicated; but it is well admitted that the earth's surface cannot be supposed a perfect conductor. The folding up of the lower half of each looped line of force is probably very incomplete. Allowing for this, it seems possible that perhaps one-half of the whole wave energy - the half carried in the lower portion of each ring of force - may be wasted ohmically in the earth comparatively near the radiator.
On the other hand, the earthed radiator, placed normally over a conducting sheet (the earth to which it is connected) detaches, in the popular theory, half rings, which move outwards in vertical radial planes, with their cut ends sliding perpendicularly over the conducting plane. This arrangement seems ideally perfect. But here again the earth's surface is, in reality, not a good conductor. Therefore, in the very act of electrical vibration - which maybe interpreted, as is well known, in terms of positive and negative reflections of electricity at the top and bottom of the earthed air-wire - great wastage may occur before the detachment of the waves.
In both cases lines of force travelling over a bad conductor must acquire a forward slope which will involve the propagation of energy into the earth or, to put it in another way, will involve the creation of currents in the earth's substance with consequent ohmic losses. Taking all this into account, however, as well as may be done, by these descriptive methods of examining the question, the advantage, on the whole, seems to lie with the earthed radiator. The conclusions we are driven to amount, in fact, to these: That in the case of the Lodge-Muirhead radiator working over the badly-conducting earth a 50 per cent, ohmic Ioss of the whole output of radiation is possible; whereas with the earthed radiator there are losses in vibrational energy amounting probably to a smaller figure.