The A B C of the X rays

Front matter

>AWH'MEADOWCROFT

AUTHOR OF

CORNELL UNIVERSITY

Preface

The following pages have been written witli a view of offering some assistance to two classes of readers, namely, those who desire to add to their stock of general information, and others who wish to pursue for themselves a line of investigation and experiment in the fascinating domain of the mysterious X rays.

Many of those who will desire to make a special study of the subject or make practical use of Prof. Roentgen's discovery will probably have had no occasion to acquire any special knowledge of electrical matters, and will, therefore, be at a loss to know what apparatus is needed, having due regard to the conditions by which they are surrounded.

The aim has been, therefore, to make all explanations as full as possible and as simple as is consistent with statements of technical matters. A careful endeavor has also been made to include in the following chapters suflicient information

8 Preface.

to enable tlie layman to choose tlie apparatus most suitable to bis needs as well as to operate it and obtain successful results tberefrom.

Chapter I. Introductory

Although the main object of this book is to present to the reader a practical explanation of apparatus and metbods employed in producing and utilizing tbe X rays, the present chapter will be treated as a general introduction of the subject. The object in so doing is to set forth a general outline of Professor Roentgen's discovery and, incidentally, to correct certain popular misconceptions that have arisen as to some of the phenomena and the methods of their utilization.

Looking back along the years of the present century, we see within their bounds the greatest development and progress that has blessed the world since the beginning of time. A mere enumeration of the discoveries and inventions contributing to comfort, safety, and convenience that have been made since the year 1800 would fill many large volumes. If we should single out from this host only a few of those that are conspicuous by their greatness, such as the IntrolO The A B C of the X Rays.

duction of Illuminating Gas, the Steam Engine, the Railroad, the Steamboat, the use of Anaesthetics, the Electric Light, (Arc and Incandescent), the Electric Railroad, the Electric Transmission of Power, and the X rays, we cannot but be powerfully impressed with the wonderful progress made within the past years of the nineteenth century.

It seems fitting that the discovery of the wonderful phenomena of the X rays should be made as the hands upon the clock of the century are completing their round, as it was the logical result of research and experiment founded upon the most advanced kind of intellectual and scientific study and attainment.

How far reaching this discovery may ultimately prove to be in the arts and sciences it is impossible at this time to predict, but within the few months in which it has become known and practiced, the wonderful effects have been practically applied to the alleviation of much suffering, and we have at this moment at our command a mysterious power for good that it would have seemed sheer folly to even hope for only one short year ago.

The discovery of the X rays was made towards the end of the year 1895 by Professor William

Konrad Roentgen, professor of Physics at the Royal University of Wurzburg, in Germany. He had for many years made a study of phenomena arising from the action of currents of electricity in glass tubes more or less exhausted of air, known as Geissler, Crookes, or Vacuum tubes. In the" year 1894 some new phenomena in this line of experiment had been noted by Professor Lenard, an assistant of Hertz.

In the course of a study of these phenornena by Professor Roentgen, he discovered that an effect was produced by the rays emanating from a Crookes tube, which was being excited by an electric current, similar in many respects to the effect that would be produced by rays. of light, but with the wonderful diflference that the new rays would penetrate flesh, blood, and muscle and many thicknesses of paper, cloth, leather, wood, rubber and other substances which are opaque to ordinary light rays.

The first announcement of this great discovery was received by the world with incredulity and some amusement, but this was quickly turned to astonishment and wonder by subsequent announcements of practical results having been obtained by other scientists on following t,he methods employed by Professor Roentgen.

It is not deemed necessary to enter into a more detailed account of the history of this discovery than the outline above given, as the newspapers, magazines, and technical journals have already given much space to the subject. The following pages will therefore be devoted to a brief explanation of the X ray phenomena, followed by a description of the various items of apparatus employed and of the methods of using the same to produce the effects which are now becoming so generally known. <

The generation of the X rays is at the present time wholly electrical. It is possible that there may later be discovered some other means of producing them, but at this day there seems to be no well authenticated fact to prove that there has been any other source of generation discovered.

Like the electric current itself, the mysterious X rays are invisible ; their presence being known only by the effects produced under proper conditions. There has been a popular misconception that the X rays proceed from a glass vacuumtube giving a light of dazzling brilliancy, but such is not the fact, for very frequently the vacuumtube under excitation will glow, or fluoresce, so faintly that the whole illumination would not be sufficient to enable one to tell the time by a watcli and yet there might be rays of such strength as to affect a photographic plate in a few seconds through a piece of wood, cardboard, or rubber of considerable thickness.

Although the X rays in their effects closely approximate ordinary light rays, there are such differences that scientists have not as yet been able to properly classify them. Their exact nature is at present unknown. Professor Roentgen himself gave the new rays the name of X, (or unknown), rays, by which name they have since been generally designated. Research and experiment will, however, possibly result in ultimately determining the question of their proper classification. The most recent investigations as to the nature of the rays tend to leave little doubt that they are transverse vibrations in the ether, like those which constitute light and radiant heat but of very much higher pitch or excessively short length of waves.

One instance will suffice to make the above differences clear to the reader. - It is well known that rays 'of light may be deflected, refracted or condensed, but not so with the X rays. They proceed only in straight lines normal to the point at which they are produced. Take, for instance, an ordinary magnifying glass whicli is simply a glass lens of convex shape as shown in Fig. i. The rays of light proceeding from A strike the lens and are condensed- and brought to a focns, or point of meeting, on the other side of the lens. . This is one of the principles applied in the ordinary photographic lens, making it possible to condense the light rays proceeding from a large area and project them in greatly reduced form upon a sensitized plate in the camera. If, on the other hand, such a lens should be placed in the path of the X rays, they would proceed through it in straight lines (if indeed they could penetrate the glass at all, glass being more or less opaque to the rays as we shall presently learn), and would neither converge nor diverge.

Thus far we have found :

(i) that the generation of X rays is from electrical sources.

(2) That the X rays are invisible.

(3) That they proceed in straight lines and, so far as present discovery goes, cannot be reflected, refracted, or condensed, but can be diffused.

Let us now take a preliminary glance at one of the methods employed in producing the rays, reserving more complete details for a further chapter.

The appliances used consist of (i) some form of apparatus which will give a current of electricity of very high potential, as for instance, a Ruhmkorff induction coil ; (2) a mechanical contrivance called a " Contact breaker " for interrupting with great- frequency the continuity of the circuit ; (3) a condenser for accumulating and rapidly discharging accumulations of electric energy ; (4) a Crookes vacuum tube ; and (5) a battery, or other source of electric current. More particular explanation of these various items of apparatus will be found on other pages of this book.

If the electric current be applied to the coil with contact breaker and condenser attached, and the secondary terminals of the coil be separated within certain distances, leaving only the air between, bluish-colored, snappy, lightning-like sparks will jump across from one terminal to the other at such frequent intervals as to appear like a continuous stream. The length of these sparks will depend largely upon the amount of wire upon the coil. In ordinary coils for this work the sparking capacity will vary from one to twelve or more inches in length. These sparks are of the character just described only when they occur in the open air, and their curious zigzag form is said to be due to the current taking a path through the particles of air having the least resistance.

Suppose, however, that we take these two secondary terminal wires from the coil and seal them into a glass bulb or cylinder so that the ends of such wires will project therein. These ends would thus be known as " electrodes." If the bulb were not exhausted of air, the same character of discharge will take place between the two electrodes when the coil is energized by an electric currrent.

Now, if the air be partially exhausted from the bulb so as to make a moderately low vacuum, say a pressure of three thousand one-millionths of an atmosphere (an atmosphere being equal to a pressure of 15 pounds to the square inch), a very different appearance will be noted when the coil is once more energized. Instead of a stream of sparks jumping across from one electrode to the other, the whole bulb will become filled with a purplish-colored cloud having a rapidly vibrating or travelling movement. This occurs by reason of the molecules of air remaining in the bulb being driven off from one electrode towards the other, and coming into violent contact with each other, the force of the collision of these molecules produces light.

If now the tube be exhausted to a high degree of vacuum, say, to one one-millionth of an atmosphere, a new set of phenomena is developed on discharging the current from the coil into the tube. The purplish cloud has disappeared and the interior of the tube is clear, but the glass itself has assumed a beautiful fluorescence caused by the bombardment against it of the remaining molecules of air. This is now a Crookes vacuum tube.

The fluorescence will be most noticeable and brilliant at that part of the glass tube that is opposite the negative, or cathode, electrode, and it is from this part of such a tube that the X rays proceed in greatest abundance. We shall see in a later chapter that modifications of this simple form of Crookes vacuum tube have recently been made, providing for a special bombardment surface, within the tube, with the result of obtaining i8 The A B C of the X Rays.

a mucli greater abundance of the rays, together with less diffusion and, consequently, a greater degree of sharpness of definition, in the shadows produced by them.

We find, then, that the X rays are projected from a place or point in a highly exhausted glass tube that is directly opposite the negative, or cathode, electrode, and that this place or point may, but need not necessarily be, the glass of the tube itself.

Having brought the reader to the point where the X rays are produced, let us now see in what manner and by what methods we can ascertain definitely that they are present.

It requires no special methods to ascertain when ordinary light rays (sun or artificial) are present, for it involves only a radical difference which is apparent to every one who is possessed of eyesight, namely, the difference between darkness and light. The fact of the X rays being invisible but producing some effects similar to, and in some aspects more powerful than, effects produced by light rays, presents apparently a somewhat paradoxical state of things.

Comparisons with known standards may in many cases be advantageously made in explaining new phenomena, but in this case there are no standards by wliicli the new set of phenomena incident to the X rays may be strictly comparable.

We shall endeavor, therefore, to make the explanation in the simplest possible manner.

There are at present only three ways in which the presence of the X rays can be ascertained. One of the strongest characteristics of X rays is their power of discharging electrified bodies or surfaces exposed to them. This, in fact, is the most sensitive way of detecting them. Another way is by means of the ordinary sensitized photographic plate, and the other is through the fluorescence of certain crystalline chemical salts when brought within the influence of the rays.

It will be necessary only to discuss the latter two, and taking these in their order, we will first deal with the photographic plate, and at the start it will be well to .correct the popular error that an ordinary photographic camera is used in obtaining pictures with the X rays. A camera is hot necessary, in fact no use could be made of it in this connection, as the rays cannot be condensed or refracted.

A picture of any object obtained by means of the X rays is not a photograph, strictly speaking, although it is made upon a sensitized plate and developed and printed in the manner usually followed in making photographs. As defined in the dictionaries, photography is the " fixing of an object or objects by -chemical rays of light upon certain salts sensitive to light," and as ~l ordinarily practiced, includes the use of lenses by which the objects photographed may be focussed on the sensitized plate on a smaller or greater scale.

An X ray picture of any object, Photography. (or " radiograph " as it is generally termed), cannot in the first instance be made any larger or smaller than the object itself, because lenses cannot be successfully used to condense the rays, and therefore, a radiograph is practically a \\ie.-size. shadow picture of the object. A radiograph, once taken, can, however, h&photographed by rays of light in the ordinary way ; and thus a new picture of the object, increased or diminished in size, may be obtained.

If we should take a sensitized photographic plate and place an opaque object upon it, as, for instance, a hand, and expose it for a few seconds in daylight, the rays of light would not reach that part of the plate covered by the hand. Upon developing the plate there would be seen clear the shape, or shadow, of the hand, while all

The A B C of the X Rays. 2i

around it the plate would be dark owing to the rays of light having affected this portion. No part of the bones, muscles, or other interior portions of the hand would be visible upon the plate, as the rays of light would not penetrate the flesh and blood, all portions of the hand being to a very great extent opaque to light rays. Such a picture as this would be termed 2^. photography having been obtained through the agency of light rays. A print from this photograph made upon" paper in the regular way would show 'simply a black shadow of a hand, while all around it would be clear white, just as a hand might be seen outlined upon a white window shade if a light were placed behind it. That part of the photographic plate which in prints from it is white is the part that has been directly acted upon by the rays of light.

The X rays allowed to fall directly upon a sensitized photographic plate will affect it in the same way as it would be affected by light rays. A plate so exposed would, after development, yield prints of a clear white.

Now, for comparison, let us expose two sensitized plates, one to ordinary light rays and the other to the X rays, first placing over each plate, so as to cover it entirely, a piece of wood, a book, a sheet of rubber, or a tbin sheet of the metal aluminium. On developing and printing these two plates we find that the one exposed to the light rays will print black, thus showing that these rays have not penetrated the substance interposed. The development and printing of the one which was exposed to the X rays will, however, result in a clear white print, proving thereby that the plate was acted upon and that the substances above named vsrere no obstacles to the passage of the X rays.

Although the X rays will penetrate the above named articles, and many others, it is to.be noted that they do not readily penetrate all substances, and that among those that are fairly opaque are bone, glass and almost all the metals. By reason of that fact we are enabled to make examination of the bones in a living body and we may also locate foreign opaque substances which may by accident have entered the human frame.

I,et us now expose one more sensitized plate by way of illustration ; this time to the X rays. We will wrap it up in black paper, or enclose it in a plate-holder, sliding a cardboard or rubber cover over the latter Now we will place it on a table, lay a hand upon the covering of the plate and produce the X rays by means of a suspended

Radiography.

Crookes tube. In a few minutes tlie excitation of the tube is stopped and tbe plate taken away and developed. Upon printing this we find that that part of the plate not covered by the hand is a clear white. We also find that the X rays have penetrated entirely, the muscles, and to a greatextent the flesh, blood, and nails, leaving only a faint shadow of the outline of the hand. The rays have not, however, penetrated the bones, consequently the plate immediately beneath them has not been affected (or, if at all, very slightly), and therefore, as they show in dark relief, we have a strong picture of the bones of the hand.

Had there been a piece of metal sucJb, for instance, as a bullet or a needle, embedded in tbe hand, it would also have shown, these substances baing also opaque to the X rays.

Thus we can to-day obtain pictures of the bony structure of any part of the body and show the presence not only of foreign substances of a metallic nature, but also of calcareous deposits or other substances that are opaque to the X rays. Broken bones, or those improperly set after being broken, as well as dislocations, can also be disclosed as clearly as if the flesh were stripped from that part of the body in which the3' are located.

A great many experiments have been tried and much thought has already been expended in the direction of obtaining clearly defined radiographs of the heart and other organs, but as these are not sufl&ciently opaque to the X rays it cannot be said that entire success has yet been achieved in this direction.

A recent cable from abroad announces that a radiograph of the stomach and intestines of a person has been successfully obtained by having the person first drink a quantity of a harmless solution of a chemical nature impervious to the X rays and then making the usual exposure.

Made by the author with Thomson Double-focus Tube and

Inductorium.

Time of Exposure z minutes 50 seconds.

Distance from Tube 12 inches.

Inductorium working on 5 inch spark.

Thus far we have shown only one way of ascertaining the presence of X rays, namely, by their effect upon a sensitized plate. There is another and quicker way, however, which arises by reason of the fact that certain chemical salts possess the peculiar property of fluorescing when brought within the influence of the X rays. This fluorescence is not brilliant, but merely presents to the vision a moderate glow when the eyes are shielded from other light. It is not an easy matter to find an apt comparison with something familiar to every one, but perhaps this fluorescence is more nearly comparable to a ground-glass window pane at night, having behind it, at some little distance away, an artificial light of moderate candle power.

The idea of the X rays, which are themselves invisible, having the property of causing this fluorescence is somewhat of an abstruse conception for the layman, but probably a further simple comparison will tend to an easily comprehended and reasonable view of this.

It is well known that a diamond is a crystal and that in a perfectly dark room it will not show any light nor the beautiful colors for which it is esteemed. Exposed to rays of light, however, it at once becomes a sparkling gem. The chemical salts which exhibit fluorescence in the X rays are all crystalline, but so far as their fluorescent property is concerned, they may be said to be always in the dark until they are brought into the presence of the X rays, when they at once light up, as the diamond does in light rays, although not with the same brilliancy or color effects.

It must be remembered that this comparison is not put forth as an explanation of the phenomena in question, but, as stated above, merely to assist the reader in forming some sort of an idea which may lead to a fair appreciation of fluorescence as observed in the Fluoroscope.

This peculiar property of these crystalline chemical salts has been availed of for the purpose of practical and instantaneous observations by means of a device called the Fluoroscope,. which will be described in detail in a later chapter.

This, fluorescent property was known three years ago and a screen covered with the crystals was used by Lenard in his investigations. Roentgen also matje use of a fluorescent screen about a year ago. Similar screens were also used by Professor Blihu Thomson in the United States very early in the present year, very soon after the announcement of Roentgen's discovery. The knowledge of the fluorescent screen became general some months ago, and subsequently Mr. Bdison put it in the convenient form, now known as the Fluoroscope.

It will be sufiicient to note at this point that opaque objects placed between the Crookes tube and the fluorescent crystals will prevent the X rays from striking that part of the crystals covered by such objects and, consequently, the crystals will not exhibit fluorescence at that spot. For instance, if a fluorescent screen be brought within the influence of the X rays and a hand be placed between the source of rays and the screen, the X rays pass right through the flesh, blood, veins, and muscles but do not penetrate the bones. Inasmuch as the crystals reached by the X rays become fluorescent, the screen glows except in the places not affected by the rays, and we therefore see in dark shadowy outlines the shape of the bones in the hand as they are at that present moment. When the generation of the X rays is stopped, or the fluorescent screen removed from the sphere of their influence, the fluoresence of the crystals ceases, not to appear again until once more exposed to the rays.

We believe that the reader will now have gathered a general idea of the phenomena involved in Professor Roentgen's wonderful discovery, and we shall now proceed to describe more in detail the various items of apparatus used to produce the effects above noted and endeavor to give some hints as to its manipulation.

Chapter II. The Exciting Apparatus

The electric current necessary to produce tlie proper excitation in a Crookes vacuum tube should be possessed of two important qualifications. First, it should be of very high potential, and, second, it should preferably be an alternating, interrupted or intermittent current.

If we were to attach a Crookes tube direct to two of the conductors forming part of an electric lighting circuit no effect would be perceptible. We might take away these conductors and connect to them ten or twenty incandescent lamps of from 16 to 100 candle power each, and the current flowing in the wires would light up all these lamps to full candle power, although it would not excite one Crookes tube which would require under proper conditions less than one five-hundredth of the actual electrical energy needed to illuminate the lamps.

There are two reasons for this. First, the current (let us say, continuous, or direct, at no volts) did not fulfil the qualifications referred to in the first paragraph ; and second, that in the incandescent lamps the current has continuous paths of comparatively low resistance in which to travel, while in the Crookes tube the path in which the current may pass is exceedingly meagre, consisting merely of the molecules of air that are left in the tube after exhausting it to a degree where it contains only about one-millionth of an atmosphere. In this latter case it might be imagined that the current on entering the tube by one conductor is obliged to make a path for itself by driving the molecules of air towards each other and a second conductor and thus using them as its path.

The ordinary electric lighting current employed for incandescent lamps is not suflSciently powerful in pressure to jump across an air space, even if very small, although sometimes by accident some conducting substance may come in the gap forming an air space separating two conductors, and thus help the current across. In such a case, however, a continuous arc would be formed which diifers from a stream of sparks jumping from one conductor to another when these two are connected to an apparatus which is producing currents of very high potential with extremely rapid interruptions.

Klectric lighting circuits for incandescent lamps and ordinary power are usually of comparatively low pressure, or voltage, generally between 50 to 120 volts, and in some cases as high as 240 volts. In all these systems, however, the rate of flow, or amperes, of current is usually large on account of the nature of the effects to be produced. The voltage in any such system is kept constant and steady and regulated within a few points, but the strength in amperes of current will vary very considerably according to the requirements of the consumers.

The electric current 'necessary to properly excite a Crookes vacuum tube to the degree at which the X rays are produced is enormously high in voltage, as compared with that of an electric light circuit. Such a current may vary in pressure or potential from ten thousand to hundreds of thousands of volts. The current in amperes, of such a circuit will, however, be exceedingly low, being only a very small fraction of one ampere in all cases.

It will be seen, therefore, that the actual electrical energy required to produce the X rays is comparatively small ; indeed, the actual amount of energy needed to light up an ordinary i6 candle power lamp to incandescence will be sufficient, when employed with the proper apparatus, to cause excitation enough to produce abundant and powerful X rays.

The reader will naturally inquire how this can be. To this we answer that it is by a transformation of the electrical energy from one form of potentia;l to another. This is done by means of certain kinds of apparatus which will presently be described. The construction of this apparatus is based upon certain electrical phenomena, the laws of which have been well defined and availed of for the perfecting of many useful inventions.

The best known form of apparatus and the one most generally used for transforming electrical energy of comparatively low voltage and high amperes into a current of very high voltage and low amperes is the Induction Coil ; frequently called the Ruhmkorff Coil. Before proceeding to describe this, we shall, for the better understanding of the reader, devote a chapter to a brief explanation of the phenomena to which it owes its existence. In this explanation will also be involved the philosophy of another type of exciting apparatus, known as the High Frequency

The A B C of the X Rays. TiZ

Transformer, now coming into general use in connection witli X ray work. This device is also one of tlie class which, like the Ruhmkorff Coil, depends upon electrical induction for its usefulness.

A third and entirely distinct type of exciting apparatus embraces what are known as Static Machines, which, while they rely upon induction for the "effects produced, do not require to be continuously connected with a source of electrical energy, such as a' battery or an electric lighting circuit, but from an initial charge biiild up, produce and discharge electric current as long as they are kept in rotation. Such machines are generally known as Holtz Machines and Wimshurst Influence Machines.