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The Turn Of The Century
Electrotherapy Museum |
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. PHENOMENA ACCOMPANYING THE TRANSMISSION OF ELECTRICTY THROUGH GASES IT is only static or other very high'-tension electricity which can be transmitted through gases, and the general consideration of the subject may be entered upon at this place. Under ordinary conditions the air or any other gas in contact with a charged body does not become charged and is not a conductor of electricity. If it did so, of course, the body would soon lose its charge by a process of convection; each portion of the air as it became charged being repelled and giving place to another portion, which would in turn take away a certain portion of the charge. Since gases do not ordinarily become charged in this way, it is interesting to note some of the ways in which it can be accomplished and in which they can be rendered conductors of electricity. Gases in contact with the surface of liquids in which splashing or even quiet waves occur become electrified. One of the practicable forms of static machine is dependent upon the charge acquired by a jet of steam. These are some of the ways in which a gas may be ionized, a condition which will be explained later. Other means of ionizing gases and rendering them capable of receiving and transmitting electricity are of greater interest in electrotherapy, among them are especially exposure to the x-ray, the ultraviolet ray, and some of the rays from radio-active substances, also by the passage of a spark from an mduction-coil. This is the way in which the current is started through the mercury vapor in some forms of the Cooper Hewitt and similar lamps. IONIZATION OF GASES By some one of the above processes, or of several others which might be mentioned, some'of the molecules of the gas are dissociated into positively and negatively charged ions. An excess of positive ions in a gas will, of course, cause the gas to have a charge of positive electricity. An example of the way in which this may come about is seen when a gas becomes electrified by contact with an incandescent metal or by the passage of an electric arc through it. In consequence of the high temperature some of the molecules of the gas become dissociated into positive and negative ions. Some 0( these combine with the incandescent metal or with the terminals of the arc. In the resulting compound the metal is the electropositive element and will take negative Ions from the gas and leave the latter with an excess of positive ions. The Atom According to Sir I. I. Thomson.-It is composed of electrons or negative particles grouped in approximately co-planar and concentric circles and in active revolution, the system being within a sphere of positive electricity. The number of electrons in an atom is calculated to be eight times its atomic weight} Ions are atom~ charged by electrons which are supposed to be 1 H. A. Wilson, Phil. Mag., xxi, p. 718, 1911. 620 621 TRANSMISSION OF ELECTRICITY THROUGH GASES ~fi about the size of a hydrogen atom. A positive ion is a group of particles surrounding a positive charge; a negative ion is a group of particles around an electron. In a vacuum tube such as an x-ray tube electrons travel at an average rate of 20,000 miles a second, and under certain other conditions they may travel as fast as 50,000 tniles a second. No matter how complex the chemic formula of a gas may be, each ion is usually a particle of one or other of the single elements which make up the gas. . The ionization of a gas by the ultraviolet ray takes place only when the light is reflected from a fluorescent substance or from the surface of a metal immersed in the gas, and the gas is only able to discharge a charged body in its neighborhood which is not illuminated by ultraviolet rays when the charge on the body is positive. The x-ray, on the other hand, makes the gas through which it passes a conductor of electricity, independently of any reflection of the rays, and the gas thus made tc;> assume a conducting state is able to discharge negatively as well as positively charged bodies when it comes in contact with them. Air ionized by the x-ra'y retains this property if blown through a bellows or if" heated, but it 1oses its condition of ionization if it is made to bubble through a liquid or to pass through a plug of mineral wool, or if a current of electricity is passed through it. A gas ionized by the x-ray rapidly loses that property by contact with either non-conductors (insulators) or conductors. Electropositive metals lose ne~ative charges to the air when exposed to ordinary light and do not require the presence of ultraviolet rays. A gas which has been ionized and rendered a conductor of electricity will transmit electricity at a certain maximum rate which is not exceeded, no matter how much the potential or voltage may be increased. The most satisfactory hypothesis is that each ion of gas can carry only a certain charge of electricity, and with a definite number of ions liberated in the gas only a certain rate of transmission of the current is possible. An ion which has performed its function of carrying an electric charge apparently becomes neutralized or bound again and is no longer capable of carrying electricity. Hence, a layer of ionized gas Qeases after a time to transmit the current, and a thin layer ceases sooner than a thick layer. The maximum rate at. which a gas will transmit electricity is different in various gases and is called their saturation current. That of mercury vapor is about twenty times the saturation current of air. It is interesting to note that the absorption of the x-ray by different gases is in proportion to their saturation currents. There are two different ways in which the extent to which air has been ionized is used in practical therapeutic measurements. One method is by observing the time which an electroscope requires to become discharged after having received a standard charge and being exposed to ionized air. This method has been used in the measurement of the amount of x-ray applied in therapeutics, the electroscope being placed at a certain distance from the x-ray tube and exposed to the direct rays from it at the same time that the patient is -being treated. The rapidity with which the electroscope becomes discharged certainly does indicate the degree of ionization of the surrounding air, but whether this is due exclusively to the influence of the x-ray or even bears such a practical relation to it as to form a reliable means of x-ray dosage is a serious question. Another method of measuring the electric 622 MEDICAL .ELECTRICITY AND RONTGEN RAYS conductivity of ionized air is by having a thin layer of air between metal plates which are kept at a constant difference of potential by a galvanic battery , and ionizing the air by exposure to radium or other rays. The ionization of the air allows a current to pass across the air space and complete the circuit. The strength of this current as shown by a galvanometer indicates the degree of ionization of the air. This method is in constant use for measuring the radio-activity of uranium, polonium, thorium, and radium. The conductimty of ionized air is influenced by pressure, but varies either as the pressure or as the square root of the pressure. Hertz discovered in 1887 that when ultraviolet light falls upon a spark gap the discharge is facilitated. This was the basis of photoelectric signalling. The artificial light richest in the ultraviolet ray was found to be an arc light of which one pole was zinc or cadmium. Cathode, Lenard, and x-rays all render any gas through which they pass a conductor of electricity. An ionized gas is an electrolyte, i. e., a substance through which electricity may pass and in which the current is formed by the motion of positively charged ions in one direction, and negatively charged ions in the other direction. In the case of a liquid, which is really the most characteristic electrolyte, the accumulation of electropositive ions at one pole and of electronegative ions at the other pole is so great that there is a demonstrable change in molecular composition. The liberation of hydrogen gas at one pole and of oxygen gas at the other when electricity is paSsed through water is an example of this; the water being an electrolyte, and the chemic change being called electrolysis. The motion of the ions toward one pole or the other may be called phoresis. Cataphoresis, or the motion of electropositive ions through an electrolyte toward the negative pole, has important uses in electrotherapeutics. This same process of electrolysis takes place in solids and gases, though the molecular change or the change demonstrable by chemic analysis is of far less importance than is the transmission of electricity and its secondary effects, radiant and otherwise, produced by the transmission of the current. If a platinum wire is heated red hot in hydrogen gas, the platinum becomes positively, and the hydrogen negatively, charged. The same is true of iron or palladium wires. Ait and all other gases differ from hydrogen in being positively charged, except mercury vapor, which is not charged at all. If an electric arc is passed through oxygen gas the oxygen becomes positively charged and will discharge a negatively charged body, or will give a positive charge to an uncharged body. The reverse effect is produced when an electric arc is passed through hydrogen. Positive and Negative Ions at the Same Discharging Point.-J. Zelengl finds that ions of both signs can be detected near a point from which a static current of unvarying polarity passes through the air to a plate. With a discharge of 7 micro-amperes and the point positive, the number of positive ions is 250 times more than the negative ions Gases which arise from flames are electrified and are conductors of electricity. Both positive and negative ions are to be found in a flatl}e ; these make aflame an excellent conductor of electricity. 1 Phys. Rev., xxxiii, 1911, 70. 623 TRANSMISSION OF ELECTRICITY THROUGH GASES The conduction of electricity through gases is not governed by Ohm's law that the current is equal to the electromotive force divided by the resistance. It is not true, for instance, that multiplying the electromotive force or the voltage increases the current flow in the same proportion. Steam arisin~ from electrified water is not electrified. Vapor arising from boilmg mercury is not electrified, no matter how strongly the liquid mercury may be charged. When a jet of hydrogen is burned in the air the unburned hydrogen in the jet is negatively charged. Lavoissier and Laplace as long ago as 1782 noted the fact that hydrogen rapidly liberated by the action ofsulphuric acid upon iron possesses a strong positive charge. According to J. J. Thompson's observation, the presence of an electric charge upon a drop of water tends to prevent the evaporation of the water. Crookes, on the other hand, has found that evaporation takes place more rapidly from the surface of water which is negatively electrified than when the water is not electrified. Mascaret's observation is that either positively or negatively charged water evaporates faster than water which is uncharged. The possibility of error in these observations lies in the lack of uniform conditions as to the humidity and the temperature of the surrounding air and as to mechanic currents in it and in the lack of uniformity in the condi~ions which would ionize the air and influence its electric conductivity. If the air in contact with the surface of the water were ionized it would receive a charge of electricity from the water and be repelled from it, giving place to a fresh portion of air, which in its turn would be charged and repelled. Each portion of air woutd, of course, absorb more or less water and the result woula oe a more rapid evaporation than the normal, just as if a current were produced in the air in any other way. There are many ways in which the air might become accidentally ionized to a sufficient extent to affect the result in an experiment of this kind. THE PASSAGE OF ELECTRICITY THROUGH A VACUUM If the air or any other gas in a glass tube be partially exhausted by means of an air-pump, and there are two wires leading into it, the phenomena observed on connecting it with the source of high potential electricity may vary with the degree of exhaustion. Before the tube has been exhausted a discharge will take place through it as a zi~zag spark passing through it from one wire to the other, and the same 18 true of a tube in which the gas has been exhausted, but into which air has entered in consequence of a leak or a puncture. Such a state of things sometimes occurs with an x-ray tube, and it indicates the presence of so large a leak that no amount of regulation of the vacuum will be effective until the opening has been found and sealed up. The discharge through a tube in this condition does n9t differ materially from that which takes place through the open air, and as in the latter case the distance across which the discharge will pass is strictly limited to the number of inches which corresponds to the voltage or the difference of potential of the two poles. The spark length which certain voltages will produce is variously estimated and depends partly upon the shape and material of the discharging surfaces. A spark 1 inch long through the open air, or in a tube filled with air, requires at the least a potential of 10,000 volts. 624 MEDICAL ELECTRICITY AND RONTGEN RAYS A vacuum tube exhausted to the Geissler degree of Ti~ atmosphere does not become luminous on the passage of a continuous current, no matter of what tension. The moment the current is made intermittent or alternating the tube lights up. The illumination also takes place if the tube contains mercury vapor or certain other gases. A tube exhausted to this slight degree becomes filled with reddish, bluish, or violet light when the high-tension interrupted current is turned on, there being no visible or audible spark passing through it, and this light is more or less stratified, seeming to pass through the tube in waves. Geissler tubes (Fig. 385) of different shapes were favorite laboratory toys and were the forerunners of the Crookes tube and of a modern focus x-ray tube. A tube which becomes filled with bluish or reddish light allows the cathode stream to pass directly from the cathode to the anode and does not present as great resistance to the passage of electricity as one with a higher degree of vacuum, i, e., from which the gas has been more completely removed, and it does not generate a useful quantity of x-rays. The study of all the phenomena caused by the cathode rays has been more successfully performed upon the tubes with a higher vacuum. A Crookes' tube is a glass tube containing an almost complete vacuum of about ~ atmosphere. Crookes' theory ill regard to a tube exhausted to thIS degree was that the molecules were so few and far between that they could move from one wall of the tube to the other without encountering other molecules. This is the ultragaseous state of matter. It was formerly believed that the cathode ray consisted of molecules of the , residual gas, but it is now thought to con~ist \ of particles of matter, perhaps only onethousandth the size of an atom torn from the atoms and thrown from the surface of the cathode. \ Such a tube offers great resistance to the passage of electricity, and even with a source of very high potential will transmit a current of only a few milliamperes. If its two terminals are simple wires leading into the tube not much change may be noted on turning on the r ~ current. There may be a little fluorescence -of the glass around the negative pole and this. may be of the greenish-yellow tint which is excited in ordinary glass by the cathode ray. This is apt to be greatest around the cathode -or negative wire. The rest of the tube may present little or no color, there will be some : heating of the glass near the two wires, and --0--. this is often greatest near the cathode. While a simple Crookes' tube of this construction does not look very unusual, tne phenomena taking place in it are of great interest and their study led to the discovery of the x-ray. The Cathode Ray.-The most important phenomena produced by the passage of electricity through a Crookes tube; are res.ults of the cathode ray. This is probably a stream of matena~ partIcles mu~h smaller than atoms driven from the cathode at arIght an,gle to ItS \ \ I ry \ \ ~ -~ Fig. 385.-Geissler tube Cathode stream attracted b) f1n""'.- . 625 TRANSMISSION OF ELECTRICITY THROUGH GASES rface and carrying a negative charge of electricity. The other hypoths that the cathode ray consists of vibrations in the luminiferous ler does not explain manyof the phenomena as well as this corpUSlar theory.. According to the theory adopted in the present work the cathode ream of material particles proceeds from every part of the cathode a right angle to its surface, and without regard to the position of e anode. The cathode stream is invisible, but its presence can be adily demonstrated to the eye by the fluorescence which it will excite various gaseous, liquid, or solid substances placed inside the tube Id by the mechanic motion which it will produce. Sometimes in an ,ray tube if the vacuum becomes very low the v~sible fluorescence roduced by the passage of the cathode ray through the gas can be seen I a bluish streak passing from the cathode to the anticathode. If le cathode consists of a straight rod or wire pointing toward the anode, le cathode stream will proceed chiefly from the lateral surface of the )d, since that is of greater extent than its end. It will cause motion I a little wheel made up of several vanes or fan-like disks revolving on n axle in the same way that a current of air or water produces motion .1 a wind-mill or 'a water-mill. The best arrangement is to have one urface of each vane covered with polished metal foil and the other urface roughened, or a shield may be placed so that the cathode stream an strike only the vanes on one side of the wheel. The unopposed mpact causes the wheel to rotate. The same motor effect can be proluced in a tube in which the cathode is formed by a disk or a concave :ircle of metal, its is the case in an x-ray tube. This directs the cathode Itream toward the particular spot desired to influence, The luminous effects of the cathode ray are seen in the fluorescence ,t excites in the glass wall of the tube, and this is of different colors with glass of different composition; for instance, glass containing a large I)ercentage of lead changes to a beautiful blue, while the ordinary glass J.ssumes a yellowish-green tint. If a solid object such as a glass or metal disk or cross is placed in the path of the cathode stream, a distinct shadow is cast upon the wall of the tube. Beautiful effects are seen when various substances are introduced into such a tube. Under the influence of the cathode ray the following substances show phosphorescence with the specified colors: Phosphorescent Colors Produced by the Cathode Ray (J. J. Thomson). CuSO , , Faintorange. CuSO. + MnSO. Bright green. SrSO None. SrSO. + MnSO Brightred. BaSO Faintdarkviolet. BaSO. +MnSO. Darkblue. MgSO. Red. MgSO. + 1 per cent. MnSO. Intense dark red. ~~~6~. +.0:5 p~~'~~n:t...M:n:so~ : : : : : : : : : : : :~~~:~ brownish yellow. CdSO Yellow. CaFla Faint blue. CaFI. + MnHa Intense blue. The most striking effects are produced upon what Thomson calls 'solid solutions. A great deal of our knowledge of the transmission 40 626 MEDICAL ELECTRICITY AND RONTGEN RAYS 1) " / of electricity is due to the published works of Professor J. J. Thomson of Cambridge University, England (The Discharge of Electricity through Gases) .These " solid solutions " are formed when two salts, one greatly in excess of the other, are precipitated simultaneously from a liquid in which both are held in solution, the familiar barium platinocyanid of the fluoroscopic screen for x-ray work is an example of a " solid solution." The cathode stream travels at the rate of about 20,000 miles a second and in a straight line, from which, however, it may be deflected in a variety of ways. It is arrested by the glass wall of the tube, and a thin sheet of glass placed within the tube and across the path of the cathode stream casts a very dark shadow contrasting with the fluorescence of the wall of the tube. Gold-leaf is less opaque. A sheet of aluminum 0.00265 millimeters thick forming a window in the wall of the tube will allow the cathode ray to pass through it in sufficient amount to produce visible light and to cause phosphorescence in bodies outside of the tube. Experiments with a great variety of substances have shown that the most phosphorescent substance is apiece of tissue paper soaked in a solution of pentadekylparatoleketon. The cathode rays suffer diffuse reflectioh when: they fall upon a surface, whether it be of an insulator or of a conductor. Cathode rays start in all directions from such a surface, especially if the rays have struck it obliquely. And the substance struck generally becomes itself a cathode and emits cathode rays, principally normal or perpendicular to its surface. These reflected or secondary cathode rays occur independently of the existence of x-rays, the latter are ethereal vibrations, while the cathode rays consist of particles of matter. The cathode stream undergoes no regular refraction, but it may be deflected from its straight path by a magnet. Deflection of the Cathode Stream by a M agnet.- The cathode stream is deflected toward a magnet (Fig. 386) . and this is the case with the positive or negative pole or Fig. 386.-'.-cathode stream deflected by a magnet. both poles, as in the case of a horseshoe magnet. Birkelandl discovered a magnetic spectrum in the cathode stream, some particles being more deflected than others, and the result being a broad band of phosphorescence instead of a single spot. Strutt2 showed that this was due to inequalities in voltage in successive discharges from an induction-coil, and that it does not occur with the discharge from a large static machine or from a large battery of storage-cells. Deflection by Another Cathode.-In a tube with two cathodes so arranged that the cathode streams are parallel when they leave the surface of the cathodes, the two strelJ,ms will be found to repel each other and to form somewhat divergent curves. This seems almost 1 Comptes Rendus de la societe Fran~aise des Sciences,cxxiii, p. 92, 1897. 2Phila. Ma£r.. vol. v. No.. 48. D. 478. 1899. 627 TRANSMISSION OF ELECTRICITY THROUGH GASES conclusive evidence that the cathode stream consists of particles df matter charged with negative ele'ctricity, The particles in both streams ~re similarly charged and consequently repel each other. This property is taken into account in the construction of an x-ray tube, the platinum disk or anticathode upon which the cathode stream is to be focused is not placed at the center of curvature of the cathode mirror, but at a point considerably further away, the repulsion between the particles of the cathode stream deflects them so that they meet at a point beyond what would be the focus if each particle proceeded in a straight line at a right angle to the part of the surface of the cathode from which it started. Lenard Rays.-The cathode rays which have passed through an aluminum window and so have escaped from the vacuum tube present very similar characteristics to the cathode rays inside the tube, but are given the distinguishing name of Lenard rays, after their ( discoverer , They spread out very diffusely and cast shadows of solid objects which are larger than the geometric ones or larger than would "result from rays traveling in a perfectly straight line from a single point. They cause photographic effects, but since the x-ray is also present under these conditions it is hard to say just what part the Lenard rays play in this. These rays are arrested by quartz crystal, but pass through alum, They discharge bodies charged with either positive or negative electricity as do the x-rays. These rays and all th2 properties attributed to them were discovered before the x-ray, and some of these properties may be due to the latter; still, "the discovery of the x-ray has not thrown any doubt upon the existence of Lenard rays. There is, however, some doubt as to whether the Lenard rays consist of material particles or of vibrations in the luminiferous ether. Their passage through solid bodies gives some ground for the latter theory , but J. J. Thomson believes that they are corpuscular in nature. The cathode rays lose about 10 per cent. in velocity in passing through an aluminum window and emerging from a Crookes tube as Lenard rays. Channel Rays (Kanalstrahlen) .-Goldsteinl discovered the presence of these rays in a vacuum tube. They are produced with a perforated cathode, are found only near the cathode and behind it, and are not deflected by a magnet, and their only known property is that of being accompanied by luminosity. Possibly they are jets of phosphorescent gas emitted from the perforations in the cathode by a sort of . explosion. They are made up of positively charged particles of matter. Lodge's Theory of the Transmission of Electricity Through Gases.2- Lodge considers that the principal part is played by positive ions passing from the anode to the cathode along the path of least resistance. The r6le of the negative electrons projected from the cathode normally to its surface he regards as subsidiary , not contributing directly to the transportation of electricity. The electrons, however, are emitted with great force and velocity and, according to Lodge's theory, they may coltide with the positive ions and so interfere with their access to the cathode, and under certain circumstances prevent the flow of the current. For instance, in a valve tube the size and position of the electrodes is such that with an alternating potential only currents in one instance can easily get through. The current can easily pass in such a direction that 1 Berliner Sitzungsbericht.el. 39, p, 691, 1886. I Sir Oliver Lodge, Phil. Mag., 22, 1911. 628 MEDICAL ELECTRICITY AND RONTGEN RAYS the large electrode, with free access to its surface, is the cathode, and is greatly impeded when the small electrode in a narrow part of the tube is the cathode. Magnetic Rays or Magnetocathodic Rays.-Righil distinguishes these from ordinary cathode rays by the fact that in the former some of the electrons unite wi th positive ions, forming systems analogous to a planet and its satellite the rotation of which is controlled by the magnetizing current. These rays result from the action of a magnetic field .--J' ~ Fig. 387.-Lodge's valve tube.' This acts as an efficient rectifier when the stop-cock S is open, affording an unobstructed path for the positive ions from the anode A to the cathode B, avoiding collision with the cathode particles. upon a cathode stream. They are repelled to a portion of the tube where the magnetic field is weakest; there they become dissociated and an accumulation of positive ions is demonstrable. The x-Ray.-Whenthe cathode ray asa stream of material particles traveling at the rate of 20,000 miles a second strikes any solid object, such as the glass wall of the original Crookes tube or the platinum disk in the modem .focus x-ray tube, the impact gives rise to the ethereal vibrations known as the x-ray. Were particles as large as pebbles to bombard any hard surface at a tremendous velocity the effect would be vibrations in the air which would be perceptible as a deafening noise. In a vacuum tube the moving bodies are only one-thousandth the size of an atom and the speed at which they strike is inconceivably great. The result is equally beyond the range of the human senses; vibrations in the luminiferous ether five or ten times as rapid as the most rapid vibrations of visible light, and millions of times as rapid as the highest pitched audible sound-waves. A special part of the present woJik is devoted to the consideration of the x-ray. It is mentioned here only as one of the phenomena produced by the passage of electricity through a vacuum tube and for the purpose of detailing the differences between the x-ray and the cathode ray. Differences Between the Cathode and x-Rays.-The cathode rays differ chiefly in the facts that they carry a charge of negative electricity and that they are deflected from their straight path by the influence of another cathode or of a magnet. Cathode rays consist of particles of matter, while the x-ray is a form of motion like light and heat. The cathode ray is. essentially a phenomenon occurring inside a Crookes tube and has very little penetrating power, while the x-ray is chiefly known by its effects outside of the tube and has great penetrative power . Similarities Between Cathode and x-Rays.-They both ionize the air, 1 Le Radium, 9, August, 1912, 300. 2 Ibid.. 1912. D. 5.,. 629 TRANSMISSION OF ELECTRICITY THROUGH GASES rendering it a conductor of electricity, act on photographic plates, produce fluorescence, and are incapable of regular reflection, refraction, or polarization. They both give rise to cathode rays and x-rays when they strike a solid substance. Passage of Electricity Through a Practically Perfect Vacuum.-Experiments by Coolidge with x-ray tubes, exhausted far beyond the ordinary limit of ~ atmosphere, show that no current passes under any voltage while the electrodes are cold, but that it will do so if the cathode consists of tungsten wire and is heated by a current passing through it. In that case even as low a voltage as 220 may send a current tl:trough this vacuum. A Rontgen ray tube constructed upon this principle is described on page 745. Special ForD1s of Geissler Tubes.-Vacuum tubes which are not exhausted to the x-ray degree have already been spoken of. In the original type there were two leading-in wires and the whole bulb became filled with colored light which was more or less stratified. This light could be concentrated at one spot if the finger was applied to the side ~ Fig. 388.-X-ray tube. / Fig. 389.-Geissler tube, showing cathode stream. of the bulb (Fig. 389) and the finger then received a slight discharge of electricity from the surface of the glass. From this early type have been evolved the vacuum electrodes which form such an important part of high-frequency apparatus. A glass bulb with a suitable stem (fig. e Fig. 390.-Geissler tube for use as a vacuum electrode. Insulated handle. 390) and exhausted to the proper degree may be excited by connection with one pole of a static machine, x-ray coil, or high-frequency apparatus. This does not require the presence of any wire at all leading into the tube, and if there is none, the electrification of the enclosed gas must take place by a sort of condenser action. The metal handle is charged from the static machine, we will say, and induces in the gaseous contents a 630 MEDICAL ELECTRICITY AND RONTGEN RAYS charge through the glass wall of the tube. The gas becomes luminous with a violet light and with a certain degree of vacuum such a tube will be found to give out light which contains an appreciable amount of the ultraviolet rays, the invisible actinic rays beyond the violet end of the solar spectrum. The presence of the ultraviolet ray is most readily demonstrated by the fluorescence it excites in apiece of Willemite held near the tube. The activity of the tube is greatly increased by making some additional connection, for instance, by touching the other end with the hand. When there is a leading-in wire passing through the glass wall of the tube the visible effect is the same, but it does not take so strong a charge of electricity to excite it. The color of such a tube varies with the degree of exhaustion, the kind of gas contained in it, and the composition of the glass. Such a tube may be made long and curved into a flat spiral (Fig. 391) with leading-in wires connected with the two poles of an x-ray coil, and gives a beautiful. violet and ultraviolet radiance with very little discharge of electricity to the patient. , Fig. ;:!91 -High-frequency vacuum electrode with two leading-in wires. Vacuum Tubes for Electric l11umination.~The first electric light on record was reported by Hawksbee two hundred years ago. It was a vacuum tube which when connected with one pole of a frictional static machine gave sufficient light to read large print by. The practicable vacuum tube lights at the present day all depend upon the fluorescence excited in the residual gas by the passage of an electric current through it. In the Cooper Hewitt lamp the current is of the direct 110 volts. In the Moore lamp an alternating current of 5000 volts is used. In the Tesla light the voltage has been raised by a high-frequency transformer. The Cooper Hewitt lamp (Fig. 392) consists of a vacuum tube about 1 inch in diameter and from 2 to 4 feet long. It contains a certain quantity of metallic mercury and,' of course, is filled with mercury vapor. The latter is a very poor conductor of electricity when cold, and to start the current it is necessaty either to tip the tube and make a complete connection of liquid mercury from pole to pole, or el~ to pass a high-tension current of at least 1000 volts through it from an induction-coil. In either case the 110-volt continuous current is thereafter transmitted through the gas and causes brilliant fluorescence. The smaller size tube gives 300 and the larger size 700 candle-power . The tube does become hot, but not nearly as much of the power is consumed in this way as in the incandescent electric lamp. Only 3~ amperes of current are used. Its efficiency is correspondingly high, in fact, the claim is made that it requires only i Watt per candle-power, while a 16-candle-power incandescent lamp requires! ampere and 110 volts, making 55 Watts; or 3! Watts per candle-power. The cathode terminal should be liquid mercury at the lower end of the tube. The positive terminal is ,usually of iron. The Cooper Hewitt lamp has about the same efficiency as the electric arc lamp. The light from this lamp pre- 631 TRANSMISSION OF ELECTRICITY THROUGH GASES Bents the spectrum of incandescent mercury vapor, it is rich in violet rays, and almost entirely lacking in red rays. It is not especially rich in ultraviolet rays, as tested by Willemite. A very interesting observation may be made with the spectroscope in connection with this lamp described in the next paragraph. Nature of Fluorescence.-Fluorescent substances have the property of intense absorption of light at their surfaces and of slowing the rate of vibration of light falling upon them. In the case of the mercury vapor light apiece of cloth saturated with a solution of a fluorescent substance like rhodamin, and dried, may be wrapped around the luminous tube. Red lines and others not in the mercury spectrum will immediately be seen with the spectroscope. It is in the same way that Willemite slows up the vibrations of invisible ultraviolet light and changes it to a brilliant green. Something of an analogous nature must take place in connection with the ionization of the air by the ultraviolet ray. It will be remembered that the ultraviolet ray ionizes a gas and renders it a con- Fig. 392.-Cooper Hewitt lamp. ductor of electricity and capable of discharging a charged body only when the light is reflected from a fluorescent substance or from a metal immersed in the gas. The Cooper Hewitt light, of course, does not give the natural color to objects illuminated by it. Red objects appear blue or purple and every little capillary in the skin and the entire mucous surface of the lips appears bluish. The visible effect is as if the person were dead and decomposition had begun. While it is not suitable for general illumination, excellent photographs may be made by it, either originals or reproductions from others.. It is made up almost exclusi,vely' of the most actinic rays of visible light and on this account has seemed of value to the present author in the treatment of tuberculosis by light baths. The uviol lamp is made of glass which transmits a greater percentage of ultraviolet rays. The eyes, however, should be protected from a light so rich in ultraviolet rays. Moore's Vacuum-tube Light.-Tubes of any length may b(}used and Dassed from room to room, distributing the light just likj. thesteam- 632 MEDICAL ELECTRICITY AND RONTGEN RAYS or gas-pipes. They are connected at a c~ntral box in the ee'llar or elsewhere with the alternating current of 110 volts, or with the 110-volt direct current modified by the use of a vacuum-tube rotator, producing extra currents by its sudden breaks in passing through an electromagnetic coil. Either of these currents is passed through a step-up transformer, raising it to 5000 volts. Any kind of vapor may be used in the tubes and light of any desired color and spectrum may ..be produced. Daylight may be imitated very closely. The light is accompanied by very little heat. The Nikola Tesla Vacuum-tube Light.-This is produced in a vacuum tube of any length by charging and discharging a condenser and passing the discharge through the primary of an induction-coil. The secondary current thus obtained is of very high voltage and frequency and can be used with tubes with or without leading-in wires. Disruptive Nature of Vacuum-tube Transmission.-A discharge of electricity through a gas which has been ionized can, it is true, take place by simple conduction, as in the apparatus employed for testing the radio-activity of radium salts, or in testing the quantity of the x-ray by the rf!.pidity with which a charged electroscope becomes discharged, but such a transmission of the current is as free from any special phenomena as i( the charge were conducted by an equal length of copper wire. The discharges through the vacuum-tubeswhich have just been described are essentially of the nature of sparks or disruptive discharges breaking through t4e gas, not carried by it. Still the same ionized gas will also transmit electricity in the silent and invisible manner characteristic of true conduction. A Cooper Hewitt lamp, for experiment, may have a couple of leading-in wires at opposite sides near the middle of the length of the tube, and these two opposite wires may be connected with wires leading from a galvanic battery of one or two cells. A galvanometer placed in the circuit will show that no current passes through the battery circuit until the Cooper Hewitt light is turned on, and the rarefied gas between the two wires coming from the battery and leading into the tube is ionized and becomes a conductor of electricity. A chemically active; form of nitrogen is produced when pure nitrogen gas is used in a Geissler tube and an electric discharge takes place thrpugh it. -r The explosive distance in vacuum tubes is increased in a magnetic field parallel With the space, and there is a best strength for the field.l A tube may light up in a field of 1400 gauss and become dark in a field of 3000 gauss (electromagnet with a current of 16 amperes). 1 nmlv (; R- rl" I", .qn~- rl" Rinl. 1.';0. 1910. 15.';2. and 151. 1910. 1320.
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