Oscillatory Currents and Some of Their Phenomena - IV

The high frequency apparatus, described above, is unquestionably the most powerful type of apparatus for the production of the X-rays. It has advantages over all other types of apparatus, such as induction coils and friction or Holtz machines, in giving a discharge of great quantity, which is practically continuous. Furthermore, the thickness of the discharge admits of being regulated through a wide range, the length of the discharge remaining unaltered.

With high-frequency currents produced, as described above, to excite the X-rays, the limit as to the quantity of rays which can be obtained is determined solely by the construction of the tube. A tube of any particular construction when exhausted for the production of X-rays is capable of dissipating only a certain maximum amount of energy. If more energy in the form of a discharge of greater quantity is sent through the tube the platinum upon which the cathode rays strike, as well as the walls of the tube, becomes dangerously hot. Obviously, if the high-frequency apparatus has sufficient power to furnish the largest and strongest tube upon the market with the maximum amount of energy which it is capable of taking, and this power can be controlled so that it may be diminished to an amount which smaller tubes require, then the apparatus is suited to every range and variety of X-ray work.

The apparatus described above will do all this.

The X-rays are now known to be non-homogeneous; that is, as the exhaustion of the tube proceeds new rays are given off which have greater penetrative power than the rays which first make their appearance when the vacuum is lower. For obtaining the most penetrative rays, with which to view the ribs, the hip joint, etc., the vacuum should be very high. If, however, the exhaustion is carried too far the discharge will prefer the path around the glass outside the tube to the path through the tube. If then the distance between the terminals of the tube is, say, 9 inches, there is no object in having a machine to excite the tube which will give a discharge greater than 9 inches. In point of fact the discharge length need not be more than two-thirds of the distance between the terminals of the tube, because the discharge will run a greater distance over the surface of glass than it will through air. A Thompson double focus tube which has the distance between its terminals about 9 inches will be excited by an apparatus giving a 6-inch discharge of large quantity to its utmost limit.

The 8-inch discharge apparatus, described above, if intended only for X-ray work is, therefore, larger than will be required, although on account of the perfect manner in which the discharge may be regulated it will serve well for this work. If the apparatus is intended solely for X-ray work and it is desirable to have it portable, it may be considerably reduced in size over the one described above. Fig. 17 shows an apparatus made by the author which is intended only for producing X-rays and constructed so as to be portable. The picture shows the apparatus with its various auxiliary parts set up ready for taking an X-ray photograph. The box shown on the left of the picture is the case which is used for protection to the apparatus and for transporting it about. The entire outfit, including the variable inductive resistance and case, weighs 147 pounds. The apparatus is constructed essentially like the one described. The discharge gap of the condenser circuit is enclosed in a small box made of blackened brass on the sides and slate ends. Windows in this box allow the double discharge gap to be seen. The tube which appears on the front of the top of the box is to make the discharge through a series gap, which it is sometimes desirable to have in circuit with the tube, noiseless. The inside dimensions of the box are 12 x 8 x 12 inches deep. The transformer is made like the one described above. The condenser is made of thirty plates of glass, 7½ x 11 inches, and twenty-nine thin sheets of copper. The high-frequency transformer is made 6¾ inches long over all. The inside cylinder, on which the secondary of No. 34 wire is wound, sixty-four turns to the inch, is 5¾ inches in diameter. The outside cylinder, on which the primary of ten turns of copper strip is wound, is 6⅝ inches in diameter. The condenser would have been better if made of fourteen sheets of micanite one-thirty second of an inch thick. On 104 volts when the apparatus is giving its thickest discharge it takes less than five amperes. As the discharge is made thinner and more threadlike by adding inductive resistance in the primary circuit the amount of current required is much less. When the discharge has its maximum thickness it will heat the platinum radiant of a Thompson double focus tube white hot in five seconds.

By putting considerable resistance in the circuit of the primary of the high-potential transformer and increasing the length of the discharge gap enclosed in the small box till the discharge will just be able to pass, the high-frequency discharge may be made to simulate so perfectly the discharge from a friction machine that the eye can detect no difference. Now if the transformer had been made like an induction coil and the condenser had been of very much smaller capacity the discharge across the gap in the condenser circuit would have been of the nature of an arc and the high-frequency discharge could not have been obtained without suddenly interrupting this arc by an air blast or in some other mechanical way. I dwell upon this point, since it seems to be the universal notion that the most perfect form of high-frequency discharge can only be obtained by an artificial interruption of the discharge across the gap in the condenser circuit.

Though a discharge through air of 5 inches means a potential difference of about 250,000 volts the physiological effects of the high-frequency discharge are very mild. With an apparatus like the above, having a large condenser, the shock felt by taking hold of both terminals is quite severe, but not at all dangerous to even the life of a cat, as was determined by experiment. If one presents his hand to one terminal, the discharge will go between one-fourth and one-third of the total discharge length between the terminals to reach it, but with the exception of a very slight burn, is not felt. If a piece of metal is held in the hand and presented to the terminal no sensation whatever is noticed. A very severe but probably not fatal shock would be experienced if the current in the primary of the high-frequency transformer were to be passed through the body. It is therefore advisable never to attempt to adjust the discharge gap in this circuit while the machine is in operation.

The limits of this article will not permit of a description of the many striking experiments which can be performed with the high-frequency and high-potential currents. Only a few simple and easily obtained effects may be mentioned, leaving the reader to pursue this fascinating subject by reading the lectures of Tesla and by his own experimenting.

The chief peculiarity of the high-frequency discharge is its ability to pass to an insulated object from one terminal.

Set the coil working and hold a small piece of metal suspended on a thread within an inch or two of one of the terminals. A fine thread-like discharge will pass to the metal, though this be no larger than a small nail. If the piece of metal is increased in size the thickness of the discharge which passes to it is much increased. Decreasing the distance between the terminals of the coil decreases the discharge to the piece of metal and the distance which it will pass. Thus the discharge to the insulated piece of metal depends for its strength upon the size of the metal piece and upon the air resistance which the discharge between the terminals of the coil has to pass through. This action of the high-frequency discharge finds an important application in regulating the vacuum of X-ray tubes. In Fig. 18 t t are the terminals of the high-frequency apparatus, to which the tube T is connected. The tube has a projection v, in the end of which is some caustic potash and a terminal k. F is an ordinary paper fan covered with tinfoil to make its surface conducting. F is attached by a small wire to k. If the vacuum of the tube is very low the potential of the points n and k is nearly that of the earth, and very little current will flow through the wire W.

If, however, the vacuum of the tube becomes very high the potential of the points n and k reaches very high positive and negative values, the changes from positive to negative values occurring with great rapidity. In consequence of this the fan F is charged positive and negative alternately with a like rapidity, and a feeble current flows in the wire W and through the potash in the neck v. This current heats the potash and causes it to give off some of the moisture which is absorbed in it and the vacuum is thereby lowered. The amount of current in the wire W can be controlled by closing up the fan, thereby diminishing its surface. Thus by regulating the surface of the fan the vacuum of the tube may be maintained at a point best suited for the production of X-rays. An arrangement of this character will only operate with a high-frequency apparatus.

Connect a fine wire to each terminal of the high-frequency apparatus. Lead these wires to an insulating support about 10 feet away and adjust the wires so that they are parallel to one another and about 6 inches apart. The current being turned on a beautiful purple sheet of light will fill the space between the wires. In this and other light experiments the room should be darkened to observe the effects at their best. With a powerful apparatus a square metre or more of space may be covered with this purple sheet of electric flame. The effect may be varied by causing a cylinder of light to form between two circular discs of metal connected to the coil with short leads and made to face each other at a distance of a foot or 18 inches apart.

Take a plate of thick glass and paste on opposite sides of it two circular pieces of tinfoil about as large around as a dollar and connect one to one terminal of the machine and the other to the other terminal. The discharge will spread out in streamers some distance over the surface of the glass surrounding the tinfoil discs with a brilliant sheet of light. The high-frequency discharge will not puncture an ordinary window pane at once, but if the two terminals are placed on opposite sides of the pane the discharge spreading out over the surface of the glass will after a short time heat it, when, the glass becoming conducting, the discharge will pass through. The glass shows no hole, but only a spot where it has been melted and again solidified. A 10-inch discharge will in this way eat its way through a cake of paraffin 2 inches thick. If the discharge is passed through a capillary tube the tube becomes brilliantly lighted. If any kind of a vacuum tube is presented to only one terminal of the coil it is quite brightly illuminated. The illumination is much increased by hanging a piece of tinfoil on the free end of the tube. By touching any point of the tube with the finger the illumination of that spot is increased. An incandescent lamp may have its filament brought to incandescence by using only one terminal of the coil in the manner illustrated in Fig. 19. T is the terminal of the coil, to which a plate B is attached. P is a coil of fine wire and around this is wound a coil of a few turns of coarse wire, the ends of which are connected to the lamp L. This experiment may be modified in a variety of ways. But the reader who cares to pursue this side of the subject must be referred to Tesla's lectures.

Perhaps of all the striking phenomena of oscillatory currents the most remarkable are the phenomena of impedance which these currents exhibit. To show these arrange matters as shown in Fig. 20. Here T is a high-potential transformer, like the one described above. C is a condenser, preferably of small capacity. s is a double discharge gap, w is a heavy copper wire, l l are incandescent lamps of high resistance and I₁ l₁, are lamps of lower resistance. When the transformer is working discharges will take place across the air gap s, and the high-resistance lamps l l will burn more brightly than the lamps of lower resistance, l₁ l₁. These lamps burn in spite of the fact that they are short circuited by the heavy wire w. If the lamps are dispensed with and the points p₁ p₁ of the circuit are approached to within a small fraction of an inch of each other while the rest of the circuit to the right of these points forms a loop of some size a discharge will pass between the approximated points of the circuit, which indicates a potential difference between these points of several hundred volts. The wire w, which would offer scarcely any resistance to the passage of a direct current, impedes these oscillatory currents as if it were of dry wood.

These impedance phenomena increase with the frequency of the oscillations, which must be made very great by making the condenser C of small capacity and the circuit of wire short. A Leyden jar will answer very well for the condenser and an induction coil may be used for the transformer. In this case, however, a continuous arc is apt to occur at the discharge gap, and as the resistance of an arc is small the condenser will not be highly charged and the oscillations will be poor. This difficulty may be overcome either by blowing out the arc with a moderate blast of air or sometimes by inserting another condenser, C₁(shown in dotted line), in the circuit. This condenser should have a little greater capacity than the condenser C. In this way C can become charged to its full capacity while C₁ holds back the current from the transformer so that a flaming arc cannot occur across the discharge gap. In this and other experiments on the oscillatory discharge it is desirable to arrange matters so that the condenser in the circuit carrying the oscillatory discharge is supplied with electricity from the transformer more slowly than it empties itself of its charge when the air of the discharge gap breaks down. In this case the low-resistance arc at the gap is automatically broken. It is hard, however, to say just what conditions are required to produce the best oscillations in any particular case without blowing out the arc. This would be a very useful matter to experiment upon till the principles are determined clearly which govern the automatic interruption of the arc. It may be that adding a suitable ohmic or inductive resistance at the point P, Fig. 20, would serve the same purpose as the condenser C₁.

No mention has been made of the use of iron in the coils to increase the inductive action between a primary and a secondary circuit. A simple experiment will show why iron cannot with advantage be used. Take a glass tube and wind on it a few turns of wire and send through this wire, which will correspond to the primary of the high-frequency transformer, an oscillatory current. If now a fine iron wire is inserted in the tube it will in a few seconds become too hot to handle. A large number of wires put in the tube will also become very hot, but not so quickly as one. This heating of the wires is probably due not only to hysteresis or molecular friction, which resists the rapid reversals of the magnetization, but also to currents which are induced in the metal. Heating of the iron represents a great loss of energy and as the inductive effects can be obtained without it nothing is to be gained by its use.

If the circuits are arranged like that shown in Fig. 1, the condenser being made of about fifteen glass plates a foot square, and the wire w is not over 6 inches long, when the oscillatory discharge is passing the short wire w, even if it is as large as No. 14, will become sufficiently hot to burn its insulation, while the No. 32 wire of the transformer supplying the energy to the transformer remains perfectly cool. This indicates that a very large current flows in the condenser circuit when oscillations are taking place. The discharge through the gap is entirely different in character from what it is when no condenser shunts the gap. In the latter case it is a species of flame, having a yellow hue and nearly noiseless. In the former case the discharge becomes very noisy, extremely brilliant and very blue. By waving a stick in the light of the discharge the light is seen to be discontinuous, as the moving stick gives rise to a large number of multiple images. As the balls of the gap are more and more separated the intervals between the separate discharges become greater. This light undoubtedly has powerful actinic properties, and the idea suggests itself that kinetoscopic views might be taken with its aid, especially of moving microscopic objects. An arrangement something like that shown in Fig. 21 might be adopted.

Here S is the discharge gap, giving rise to the brilliant rapidly interrupted light, l is a condensing lens, S ₁ is the slide of the microscope containing the moving microscopic object, M the microscope; N₁ N_₂ are two rolls, on which a sensitive film runs, B a box to exclude light. The microscope can be easily arranged to throw the magnified image on the film at I. If the film be now rolled from one roll to the other while the rapidly intermittent discharge is passing at S, kinetoscope views ought to be obtained on the film without great difficulty. At any rate the brilliant and intermittent character of the light ought to find useful application in some way similar to the above.