Part 5

The important point in connection with the working of a coil used for charging a condenser is not the length of spark which the coil can give alone, but the length of spark which can be obtained between small balls attached to the secondary terminals, when these terminals are also connected to the two surfaces of the condenser. Thus, a coil may give a ten-inch spark if worked alone, but on a capacity of one-thirtieth of a microfarad it may not be able to give more than a five-millimetre spark. Hence, in describing the value of a coil for wireless telegraph purposes, it is not the least use to state the length of spark which the coil will give between the pointed conductors in air, but we must know the spark length which it will give between brass balls, say, 1 centimetre in diameter, connected to the secondary terminals, when these terminals are also short-circuited by a stated capacity, the spark not exceeding that length at which it becomes non-oscillatory.

A good way of describing the value of an induction coil for wireless telegraph purposes is to state the length of oscillatory spark which can be produced between balls one centimetre in diameter connected to the secondary terminals, when these balls are short-circuited by a condenser having a capacity, say, of one-hundredth of a microfarad, and also one-tenth of a microfarad.

If a hammer or motor interrupter is employed with the coil, then a primary condenser must be connected across the points between which the primary circuit is broken. This condenser generally consists of sheets of tinfoil alternated with sheets of paraffin paper, and for a ten-inch coil may have a capacity of about 0·4 or 0·5 of a microfarad.[15]

Lord Rayleigh discovered that if the interruption of the primary circuit is sufficiently sudden and complete, as when the primary circuit is severed by a bullet from a gun, the primary condenser can be removed and yet the sparks obtained from the secondary circuit are actually longer than those obtained with the condenser and the ordinary break.[16]

In the use, however, of the coil for Hertzian wave telegraphy, with all interrupters except the Wehnelt break a condenser of suitable capacity must be joined across the break points.

Turning in the next place to the primary key, or signalling interrupter, it is necessary to be able to control the torrent of sparks between the secondary terminals of the coil, and to cut them up into long and short periods in accordance with the letters of the Morse alphabet. This is done by means of the primary key. The primary key generally consists of an ordinary massive single contact key with heavy platinum contacts. As the current to be interrupted amounts to about ten amperes and is flowing in a highly inductive circuit, the spark at break is considerable. If the attempt is made to extinguish this spark by making the contacts move rapidly away from one another through a long distance, in other words, by using a key with a wide movement, then the speed at which the signals can be set is greatly diminished. The speed of sending greatly depends upon the time taken to move the key up and down between sending two dots, and hence a short range key sends quicker than a long range key. If it is desired to use a short range key, then some method must be employed to extinguish the spark at the contacts. This is done in one of three ways: Either by using a high resistance coil to short-circuit these contacts, or by a condenser, or by a magnetic blow-out, as in the case of an electric tramcar circuit controller. Of these, the magnetic blow-out is probably the best.

Mr. Marconi has designed a signalling key which performs the function not only of interrupting the primary circuit, but at the same time breaks connection between the receiving appliance and the aerial.

The author has designed for signalling purposes a multiple contact key which interrupts the circuit simultaneously in ten or twelve different places. The particular point about this break is the means which are taken to make the twelve interruptions absolutely simultaneous. If these interruptions are not simultaneous, the spark always takes place at the contact which is broken first, but if the circuit is interrupted in a dozen places quite simultaneously, then the spark is cut up into a dozen different portions, and the spark at each contact is very much diminished. By this break, voltages up to two thousand volts may be quite easily dealt with.

Various forms of break have been devised in which the circuit is broken under oil or insulating fluids, but, generally speaking, these devices are not very portable, and a dry contact between platinum surfaces with appropriate means for cutting up the spark and blowing it out so that the mechanical movement of the switch may be small is the best thing to use.

The signalling key is really a very important part of the transmitting arrangement, because whatever may be the improvements in receiving instruments, it is not possible to receive faster than we can send. A great many statements have appeared in the daily papers as to the possibility of receiving hundreds of words a minute by Hertzian wave telegraphy, but the fact remains that whatever may be the sensibility of the receiving appliance, the rate at which telegraphy of any kind can be conducted is essentially dependent upon the rate at which the signals can be sent, and this in turn is largely dependent upon the mechanical movement which the key has to make to interrupt the primary circuit, and so interrupt the secondary discharge.

In order to make the separation of the contact points of the switch as small as possible, and yet prevent an arc being established, various blow-out devices have been employed. The simplest arrangement for this purpose is a powerful permanent magnet so placed that its inter-polar field embraces the contact points and is at right angles to them.

As already explained, the applicability of the induction coil in wireless telegraphy is limited by the fact of the high resistance of the secondary circuit and the small current that can be supplied from it. Data are yet wanting to show what is the precise efficiency of the induction coil, as used in Hertzian wave telegraphy, but there are reasons for believing that it does not exceed 50 or 60 per cent.

Where large condensers have to be charged--in other words, where we have to deal with larger powers--we are obliged to discard the induction coil and to employ the alternating-current transformer. But this introduces us to a new class of difficulties. If an alternating-current transformer wound for a secondary voltage, say, of 20,000 or 30,000 volts, has its primary circuit connected to an alternator, then if the secondary terminals, to which are connected two spark balls, are gradually brought within striking distance of one another, the moment we do this an alternating-current arc starts between these balls. If the transformer is a small one, there is no difficulty in extinguishing this arc by withdrawing the secondary terminals, but if the transformer is a large one, say, of ten or twenty kilowatts, dangerous effects are apt to ensue when such an experiment is tried. The short circuiting of the secondary circuit almost entirely annuls the inductance of the primary circuit. There is, therefore, a rush of current into the transformer, and if it is connected to an alternator of low armature resistance the fuses are generally blown and other damage done.

Let us supppse, then, that the secondary terminals of the transformer are also connected to a condenser. On bringing together the spark balls connected with the secondary terminals we may have one or more oscillatory discharges, but the process will not be continuous, because the moment that the alternating-current arc starts between the spark balls it reduces their difference of potential to a comparatively low value, and hence the charge taken by the condenser is very small, and, moreover, the circuit is not interrupted periodically so as to re-start a train of oscillations.

When, therefore, we desire to employ an alternating-current transformer as a source of electromotive force, although it may have the advantage that the resistance of the secondary circuit of the transformer is generally small compared with that of the secondary circuit of an induction coil, yet, nevertheless, we are confronted with two practical difficulties: (1) How to control the primary current flowing into the transformer, and (2) how to destroy the alternating-current arc between the spark balls and reduce the discharge entirely to the disruptive or oscillatory discharge of the condenser.

The control over the current can be obtained, in accordance with a plan suggested by the author, by inserting in the primary circuit of the transformer two variable choking coils. The form in which it is preferred to construct these is that of a cylindrical bobbin standing upon a laminated cross-piece of iron. These bobbins can have let down into them an =E=-shaped piece of laminated iron, so as to complete the magnetic circuit, and thus raise the inductance of the bobbin. By placing two of these variable choking coils in series with the primary circuit, the current is under perfect control. We can fix a minimum value below which the current shall not fall, by adjusting the position of the cores of these two choking coils, and we can then cause that current to be increased up to a certain limit which it cannot exceed, by short-circuiting one of these choking coils by an appropriate switch. Several ways have been suggested for extinguishing the alternating current arc which forms between the spark balls connected to the secondary terminals when these are brought within a certain distance of one another. One of these is due to Mr. Tesla. He places a strong electromagnet so that its lines of magnetic flux pass transversely between the spark balls. When the discharge takes place the electric arc is blown out, but if the balls are short-circuited by a condenser the oscillatory discharge of the condenser still takes place across the spark gap. Professor Elihu Thomson achieves the same result by employing a blast of air thrown on the spark gap. This has the effect of destroying the alternating-current arc, but still leaves the oscillating discharge of the condenser. The action is somewhat tedious to explain in words, but it can easily be understood that the blast of air, by continually breaking down the alternating-current arc which tends to form, allows the condenser connected to the spark balls to become charged with the potential of the secondary circuit of the transformer, and that this condenser then discharges across the spark gap, producing an oscillatory discharge in the usual manner. The author has found that, without the use of any air blast or electromagnet, simple adjustment of the double choking coil in the primary circuit of the transformer, as above described, is sufficient to bring about the desired result, when the capacity of the condenser is adjusted to be in resonance.

Another method, which has been adopted by M. d'Arsonval, is to cause the spark to pass between two balls placed at the extremities of metal rods, which are in rapid rotation like the spokes of a wheel. In this case, the draught of air produced by the passage of the spark balls blows out the arc and performs the same function as the blast of air in Professor Elihu Thomson's method. When these adjustments are properly made, it is possible, by means of a condenser and an alternating-current transformer supplied with current from an alternator, to create a rapidly intermittent oscillatory discharge, the sparks of which succeed one another so quickly that it appears almost continuous. When using a large transformer and condenser, the noise and brilliancy of these sparks are almost unbearable, and the eyes may be injured by looking at this spark for more than a moment. In the construction of transformers intended to be used in this manner, very special precautions have to be taken in the insulation of the primary and secondary circuits, and the insulation of these from the core.

It may be remarked in passing that experimenting with large high-tension transformers coupled to condensers of large capacity is exceedingly dangerous work, and the greatest precautions are necessary to avoid accident. In the light, however, of sufficient experience there is no difficulty in employing high-tension transformers in the above-described manner, and in obtaining electromotive forces of upwards of a hundred thousand volts supplied through transformers capable of yielding any required amount of current.

On occasions where continuous current alone is available, a motor generator has to be employed converting the continuous current into an alternating current. This is best achieved by the employment of a small alternator directly coupled to a continuous-current motor; or by providing the shaft of a continuous-current motor with two rings connected to two opposite portions of its armature, so that when continuous current is supplied to the brushes pressing against the commutator, an alternating current can be drawn off from two other brushes touching the above-mentioned insulated rings.

The next element of importance in the transmitting arrangement is the spark gap. In the case of those transmitters employing an ordinary induction coil, the secondary spark, or the discharge of any condenser connected to the secondary terminals can be taken between the brass balls about half an inch or one inch in diameter, with which the terminals of the secondary coil are usually furnished; and it is generally the custom to allow this spark discharge to take place in air at ordinary pressure. In the very early days of his work Mr. Marconi adopted the discharger devised by Professor Rhigi, in which the spark takes place between two brass balls placed in vaseline or other highly insulating oil.[17] But whatever advantage may accrue from using oil as the dielectric in which the spark discharge takes place, when carrying out simple laboratory experiments on Hertzian waves, there is no advantage in the case of wireless telegraphy. The Rhigi discharger was, therefore, soon discarded. If discharges having large quantity are passed through oil, it is rapidly decomposed or charred, and ceases to retain the special insulating and self-restoring character which is necessary in the medium in which an oscillating spark is formed. The conditions when the discharges of large condensers are passed between spark balls are entirely different from those when the quantity of the spark, or to put it in more exact language, the current passing, is very small. In the case of Hertzian experiments it is necessary, as shown by Hertz, to maintain a high state of polish on the spark balls when they are employed for the production of short waves of small energy, but when we are dealing with large quantities of energy at each discharge, those methods which succeed for laboratory experiments are perfectly impracticable. The conditions necessary to be fulfilled by a discharger for use in Hertzian wave telegraphy are that the surfaces shall maintain a constant condition and not be fused or eaten away by the spark, and, next, that the medium in which the discharge takes place shall not be decomposed by the passage of the spark, but shall maintain the property of giving way suddenly when a certain critical pressure is reached, and passing instantly from a condition in which it is a very perfect insulator to one in which it is a very good conductor; and, thirdly, that on the cessation of the discharge, the medium shall immediately restore itself to its original condition.

When using the ordinary ten-inch induction coil, and when the capacity charged by it does not exceed a small fraction of a microfarad, it is quite sufficient to employ brass or steel balls separated by a certain distance in air, at the ordinary pressure, as the arrangement of the discharger. When, however, we come to deal with the discharges of very large condensers, at high electromotive forces, then it is necessary to have special arrangements to prevent the destruction of the surfaces between which the spark passes, or their continual alteration, and many devices have been invented for this purpose. The author has devised an arrangement which fulfils the above conditions very perfectly for use in large power stations, but the details of this cannot be made public at the present time.

* * * * *

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.[18] The electric force, reckoned in volts per centimetre, required to pierce a thickness of air from two to ten millimetres in thickness, at atmospheric pressure, may be taken at 30,000 volts per centimetre. The same force in electrostatic units is represented by the number 100, since a gradient of 300 volts per centimetre 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 millimetres or centimetres, lying, say, between one or two millimetres and five or six centimetres. 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_ = 3,000 × _spark-gap length in millimetres_.

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[19] 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 to 0·75 centimetre.

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 50lb. 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 millimetres 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 millimetres 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 millimetres long between the tertiary balls will take place. Suppose, then, we increase the pressure of the air round the secondary balls, pumping it by degrees to 10, 20, 30, 40 and 50lb. 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 50lb. per square inch, we can obtain a fifty-millimetre 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 50lb. 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 60lb. in the square inch, there is a marked increase in the effective radiation, and at 80lb. per square inch the energy emitted in the form of waves is nearly three and a-half times greater than at 50lb., 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.