Experiments with Alternate Currents of High Potential and High Frequency (1892) — part 5 of 8

certain that the structure is jarred. When the potential is very high, although the vibrations may be few — say 20,000 per second — the effect upon the structure may be consider¬ able. Suppose, for example, that a ruby is melted into a drop by a steady application of energy. When it forms a drop it will emit visible and invisible waves, which will be in a definite ratio, and to the eye the drop will appear to be of a certain brilliancy. Next, suppose we diminish to any degree we choose the energy steadily supplied, and, in¬ stead, supply energy which rises and falls according to a certain law. Now, when the drop is formed, there will be emitted from it three different kinds of vibrations — the or¬ dinary visible, and two kinds of invisible waves : that is, the ordinary dark waves of all lengths, and, in addition, waves of a well defined character. The latter would not exist by a steady supply of the energy ; still they help to jar and loosen the structure. If this really be the case, then the ruby drop will emit relatively less visible and more invisible waves than before. Thus it would seem that when a platinum wire, for instance, is fused by cur¬ rents alternating with extreme rapidity, it emits at the point of fusion less light and more invisible radiation than it does when melted by a steady current, though the total energy used up in the process of fusion is the same in both cases. Or, to cite another example, a lamp filament is not capable of withstanding as long with currents of extreme frequency as it does with steady currents, assuming that it be worked at the same luminous intensity. This means that for rapidly alternating currents the fila¬ ment should be shorter and thicker. The higher the fre-

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quency — that is, the greater the departure from the steady flow — the worse it would be for the filament. But if the truth of this remark were demonstrated, it would be erro¬ neous to conclude that such a refractory button as used in these bulbs would be deteriorated quicker by currents of extremely high frequency than by steady or low frequency currents. From experience I may say that just the oppo¬ site holds good: the button withstands the bombardment better with currents of very high frequency. But this is due to the fact that a high frequency discharge passes through a rarefied gas with much greater freedom than a steady or low frequency discharge, and this will say that with the former we can work with a lower potential or with a less violent impact. As long, then, as the gas is of no consequence, a steady or low frequency current is bet¬ ter; but as soon as the action of the gas is desired and im¬ portant, high frequencies are preferable.

In the course of these experiments a great many trials were made with all kinds of carbon buttons. Electrodes made of ordinary carbon buttons were decidedly more durable when the buttons were obtained by the application of enormous pressure. Electrodes prepared by depositing carbon in well known ways did not show up well ; they blackened the globe very quickly. From many experi¬ ences I conclude that lamp filaments obtained in this manner can be advantageously used only with low potentials and low frequency currents. Some kinds of carbon withstand so well that, in order to bring them to the point of fusion, it is necessary to employ very small buttons. In this case the observation is rendered very

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difficult on account of the intense heat produced. Never¬ theless there can be no doubt that all kinds of carbon are fused under the molecular bombardment, but the liquid state must be one of great instability. Of all the bodies tried there were two which withstood best — diamond and carborundum. These two showed up about equally, but the latter was preferable, for many reasons. As it is more than likely that this body is not yet generally known, I will venture to call your attention to it.

It has been recently produced by Mr. E. G. Aclieson, of Monongahela City, Pa., U. S. A. It is intended to replace ordinary diamond powder for polishing precious stones, etc. , and I have been informed that it accomplishes this object quite successfully. I do not know why the name “ carborundum ” has been given to it, unless there is something in the process of its manufacture which justifies this selection. Through the kindness of the inventor, I obtained a short while ago some samples which I desired to test in regard to their qualities of phosphorescence and capability of withstanding high degrees of heat.

Carborundum can be obtained in two forms — in the form of “crystals” and of powder. The former appear to the naked eye dark colored, but are very brilliant ; the latter is of nearly the same color as ordinary diamond powder, but very much finer. When viewed under a microscope the samples of crystals given to me did not appear to have any definite form, but rather resembled pieces of broken up egg coal of fine quality. The majority were opaque, but there were some which were transparent and colored. The crystals are a kind of carbon containing some impurities ; they are

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extremely hard, and withstand for a long time even an oxygen blast. When the blast is directed against them they at first form a cake of some compactness, probably in consequence of the fusion of impurities they contain. The mass withstands for a very long time the blast without further fusion ; but a slow carrying off, or burning, occurs, and, finally, a small quantity of a glass-like residue is left, which, I suppose, is melted alumina. When compressed strongly they conduct very well, but not as well as ordinary carbon. The powder, which is obtained from the crystals in some way, is practically non-conducting. It affords a magnificent polishing material for stones.

The time has been too short to make a satisfactory study of the properties of this product, but enough experience has been gained in a few weeks I have experimented upon it to say that it does possess some remarkable properties in many respects. It withstands excessively high degrees of heat, it is little deteriorated by molecular bombardment, and it does not blacken the globe as ordinary carbon does. The only difficulty which I have found in its use in connec¬ tion with these experiments was to find some binding ma¬ terial which would resist the heat and the effect of the bombardment as successfully as carborundum itself does.

I have here a number of bulbs which 1 have provided with buttons of carborundum. To make such a button of carborundum crystals I proceed in the following manner : I take an ordinary lamp filament and dip its point in tar, or some other thick substance or paint which may be readi¬ ly carbonized. I next pass the point of the filament through the crystals, and then hold it vertically over a hot

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plate. The tar softens and forms a drop on the point of the filament, the crystals adhering to the surface of the drop. By regulating the distance from the plate the tar is slowly dried out and the button becomes solid. I then once more dip the button in tar and hold it again over a plate until the tar is evaporated, leaving only a hard mass which firmly binds the crystals. When a larger button is required I repeat the process several times, and I generally also cover the filament a certain distance below the button with crystals. The button being mounted in a bulb, when a good vacuum has been reached, first a weak and then a strong discharge is passed through the bulb to carbonize the tar and expel all gases, and later it is brought to a very intense incandescence.

When the powder is used I have found it best to proceed as follows: I make a thick paint of carborundum and tar, and pass a lamp filament through the paint. Taking then most of the paint off by rubbing the filament against a piece of chamois leather, I hold it over a hot plate until the tar evaporates and the coating becomes firm. I repeat this process as many times as it is necessary to obtain a certain thickness of coating. On the point of the coated filament 1 form a button in the same manner.

There is no doubt that such a button — properly prepared under great pressure — of carborundum, especially of powder of the best quality, will withstand the effect of the bombardment fully as well as anything we know. The difficulty is that the binding material gives way, and the carborundum is slowly thrown off after some time. As it does not seem to blacken the globe in the least, it might be

88

found useful for coating the filaments of ordinary incan¬ descent lamps, and I think that it is even possible to pro¬ duce thin threads or sticks of carborundum which will re¬ place the ordinary filaments in an incandescent lamp. A carborundum coating seems to be more durable than other coatings, not only because the carborundum can withstand high degrees of heat, but also because it seems to unite with the carbon better than any other material I have tried. A coating of zirconia or any other oxide, for instance, is far more quickly destroyed. I prepared buttons of dia¬ mond dust in the same manner as of carborundum, and these came in durability nearest to those prepared of car¬ borundum, but the binding paste gave way much more quickly in the diamond buttons : this, however, I attrib¬ uted to the size and irregularity of the grains of the dia¬ mond.

It was of interest to find whether carborundum possesses the quality of phosphorescence. One is, of course, prepared to encounter two difficulties: first, as regards the rough product, the “crystals,” they are good conducting, and it is a fact that conductors do not phosphoresce ; second, the powder, being exceedingly fine, would not be apt to exhibit very prominently this quality, since we know that when crystals, even such as diamond or ruby, are finely powdered, they lose the property of phosphorescence to a considerable degree.

The question presents itself here, can a conductor phos¬ phoresce ? What is there in such a body as a metal, for in¬ stance, that would deprive it of the quality of phosphores¬ cence, unless it is that property which characterizes it as a

80

conductor? for it is a fact that most of the phosphorescent bodies lose that quality when they are sufficiently heated to become more or less conducting. Then, if a metal be in a large measure, or perhaps entirely , deprived of that prop¬ erty, it should be capable of phosphorescence. Therefore it is quite possible that at some extremely high frequency, when behaving practically as a non-conductor, a metal or any other conductor might exhibit the quality of phos¬ phorescence, even though it be entirely incapable of phos¬ phorescing under the impact of a low-frequency discharge. There is, however, another possible way how a conductor might at least appear to phosphoresce.

Considerable doubt still exists as to what really is phos¬ phorescence, and as to whether the various phenomena comprised under this head are due to the same causes. Suppose that in an exhausted bulb, under the molecular impact, the surface of a piece of metal or other conductor is rendered strongly luminous, but at the same time it is found that it remains comparatively cool, would not this luminosity be called phosphorescence ? Now such a result, theoretically at least, is possible, for it is a mere question of potential or speed. Assume the potential of the elec¬ trode, and consequently the speed of the projected atoms, to be sufficiently high, the surface of the metal piece against which the atoms are projected would be rendered highly incandescent, since the process of heat generation would be incomparably faster than that of radiating or conducting away from the surface of the collision. In the eye of the observer a single impact of the atoms would cause an instantaneous flash, but if the impacts were re-

90

peated with sufficient rapidity they would produce a con¬ tinuous impression upon his retina. To him then the sur¬ face of the metal would appear continuously incandescent and of constant luminous intensity , while in reality the light would be either intermittent or at least changing periodically in intensity. The metal piece would rise in temperature until equilibrium was attained — that is, until the energy con¬ tinuously radiated would equal that intermittently sup¬ plied. But the supplied energy might under such condi¬ tions not be sufficient to bring the body to any more than a very moderate mean temperature, especially if the fre¬ quency of the atomic impacts be very low — just enough that the fluctuation of the intensity of the light emitted could not be detected by the eye. The body would now, owing to the manner in which the energy is supplied, emit a strong light, and yet be at a comparatively very low mean temperature. How could the observer call the lu¬ minosity thus produced ? Even if ilie analysis of the light would teach him something definite, still he would prob¬ ably rank it under the phenomena of phosphorescence. It is conceivable that in such a way both conducting and non¬ conducting bodies may be mainiained at a certain lumin¬ ous intensity, but the energy required would very greatly vary with the nature and properties of the bodies.

These and some foregoing remarks of a speculative na¬ ture were made merely to bring out curious features of alternate currents or electric impulses. By their help we may cause a body to emit more light, while at a certain mean temperature, than it would emit if brought to that temperature by a steady supply; and, again, we may bring

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a body to the point of fusion, and cause it to emit less light than when fused by the application of energy in ordinary ways. It all depends on how we supply the energy, and what kind of vibrations we set up: in one case the vibrations are more, in the other less, adapted to affect our sense of vision.

Some effects, which I had not observed before, obtained with carborundum in the first trials, I attributed to phos¬ phorescence, but in subsequent experiments it appeared that it was devoid of that quality. The crystals possess a noteworthy feature. In a bulb provided with a single electrode in the shape of a small circular metal disc, for instance, at a certain degree of exhaustion the electrode is covered with a milky film, which is separated by a dark space from the glow filling the bulb. When the metal disc is covered with carborundum crystals, the film is far more intense, and snow-white. This I found later to be merely an effect of the bright surface of the crystals, for when an aluminium electrode was highly polished it exhibited more or less the same phenomenon. I made a number of ex¬ periments with the samples of crystals obtained, princi¬ pally because it would have been of special interest to find that they are capable of phosphorescence, on account of their being conducting. I could not produce phosphores¬ cence distinctly, but I must remark that a decisive opinion cannot be formed until other experimenters have gone over the same ground.

The powder behaved in some experiments as though it contained alumina, but it did not exhibit with sufficient distinctness the red of the latter. Its dead color brightens

92

considerably under the molecular impact, but I am now convinced it does not phosphoresce. Still, the tests with the powder are not conclusive, because powdered carborundum probably does not behave like a phosphorescent sulphide, for example, which could be finely powdered without im¬ pairing the phosphorescence, but rather like powdered ruby or diamond, and therefore it would be necessary, in order to make a decisive test, to obtain it in a large lump and polish up the surface.

If the carborundum proves useful in connection with these and similar experiments, its chief value wall be found in the production of coatings, thin conductors, buttons, or other electrodes capable of withstanding extremely high degrees of heat.

The production of a small electrode capable of withstand¬ ing enormous temperatures I regard as of the greatest im¬ portance in the manufacture of light. It would enable us to obtain, by means of currents of very high frequencies, certainly 20 times, if not more, the quantity of light which is obtained in the present incandescent lamp by the same expenditure of energy. This estimate may appear to many exaggerated, but in reality I think it is far from being so. As this statement might be misunderstood I think it neces¬ sary to expose clearly the problem with which in this line of work -we are confronted, and the manner in which, in my opinion, a solution will be arrived at.

Any one who begins a study of the problem will be apt to think that what is wanted in a lamp with an electrode is a very high degree of incandescence of the electrode. There he will be mistaken. The high incandescence of

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the button is a necessary evil, but what is really wanted is the high incandescence of the gas surrounding the button. In other words, the problem in such a lamp is to bring a mass of gas to the highest possible incandescence. The higher the incandescence, the quicker the mean vibration, the greater is the economy of the light production. But to maintain a mass of gas at a high degree of incandes¬ cence in a glass vessel, it will al ways be necessary to keep the incandescent mass away from the glass ; that is, to confine it as much as possible to the central portion of the globe.

In one of the experiments this evening a brush was pro¬ duced at the end of a wire. This brush was a flame, a source of heat and light. It did not emit much perceptible heat, nor did it glow with an intense light; but is it the less a flame because it does not scorch my hand ? Is it the less a flame because it does not hurt my eye by its brilliancy ? The problem is precisely to produce in the bulb such a flame, much smaller in size, but incomparably more power¬ ful. Were there means at hand for producing electric im¬ pulses of a sufficiently high frequency, and for transmitting them, the bulb could be done away with, unless it were used to protect the electrode, or to economize the energy by confining the heat. But as such means are not at dis¬ posal, it becomes necessary to place the terminal in a bulb and rarefy the air in the same. This is done merely to en¬ able the apparatus to perform the work which it is not capable of performing at ordinary air pressure. In the bulb we are able to intensify the action to any degree — so far that the brush emits a powerful light.

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The intensity of the light emitted depends principally on the frequency and potential of the impulses, and on the electric density on the surface of the electrode. It is of the greatest importance to employ the smallest possible button, in order to push the density very far. Under the violent impact of the molecules of the gas surrounding it, the small electrode is of course brought to an extremely high temperature, but around it is a mass of highly incandescent gas, a flame photosphere, many hundred times the volume of the electrode. With a diamond, carborundum or zir- conia button the photosphere can be as much as one thou¬ sand times the volume of the button. Without much reflecting one would think that in pushing so far the in¬ candescence of the electrode it would be instantly volatil¬ ized. But after a careful consideration he would find that, theoretically, it should not occur, and in this fact — which, however, is experimentally demonstrated — lies principally the future value of such a lamp.

At first, when the bombardment begins, most of the work is performed on the surface of the button, but when a highly conducting photosphere is formed the button is comparatively relieved. The higher the incandescence of the photosphere the more it approaches in conductivity to that of the electrode, and the more, therefore, the solid and the gas form one conducting body. The consequence is that the further is forced the incandescence the more work, comparatively, is performed on the gas, and the less on the electrode. The formation of a powerful photo¬ sphere is consequently the very means for protecting the electrode. This protection, of course, is a relative one,

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and it should not be thought that by pushing the incandes¬ cence higher the electrode is ac:uallyless deteriorated, Still, theoretically, with extreme frequencies, this result must be reached, but probably at a temperature too high for most of the refractory bodies known. Given, then, an electrode which can withstand to a very high limit the effect of the bombardment and outward strain, it would be safe no matter how much it is forced beyond that limit. In an incandescent lamp quite different considera¬ tions apply. There the gas is not at all concerned: the whole of the work is performed on the filament; and the life of the lamp diminishes so rapidly with the increase of the degree of incandescence that economical reasons com¬ pel us to work it at a low incandescence. But if an incan¬ descent lamp is operated with currents of very high fre¬ quency, the action of the gas cannot be neglected, and the rules for the most economical working must be consider¬ ably modified.

In order to bring such a lamp with one or two electrodes to a great perfection, it is necessary to employ impulses of very high frequency. The high frequency secures, among others, two chief advantages, which have a most important bearing upon the economy of the light production. First, the deterioration of the electrode is reduced by reason of the fact that we employ a great many small impacts, in¬ stead of a few violent ones, which shatter quickly the structure; secondly, the formation of a large photosphere is facilitated.

In order to reduce the deterioration of the electrode to the minimum, it is desirable that the vibration be liar-

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monic, for any suddenness hastens the process of destruc¬ tion. An electrode lasts much longer when kept at incan¬ descence by currents, or impulses, obtained from a high- frequency alternator, which rise and fall more or less harmonically, than by impulses obtained from a disruptive discharge coil. In the latter case there is no doubt that most of the damage is done by the fundamental sudden discharges.

One of the elements of loss in such a lamp is the bom¬ bardment of the globe. As the potential is very high, the molecules are projected with great speed ; they strike the glass, and usually excite a strong phosphorescence. The effect produced is very pretty, but for economical reasons it would be perhaps preferable to prevent, or at least re¬ duce to the minimum, the bombardment against the globe, as in such case it is, as a rule, not the object to excite phos¬ phorescence, and as some loss of energy results from the bombardment. This loss in the bulb is principally depend¬ ent on the potential of the impulses and on the electric density on the surface of the electrode. In employing very high frequencies the loss of energy by the bombardment is greatly reduced, for, first, the potential needed to per¬ form a given amount of work is much smaller; and, sec ondly, by producing a highly conducting photosphere around the electrode, the same result is obtained as though the electrode were much larger, which is equivalent to a smaller electric density. But be it by the diminution of the maximum potential or of the density, the gain is ef¬ fected in the same manner, namely, by avoiding violent shocks, which strain the glass much beyond its limit of

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elasticity. If the frequency could be brought high enough, the loss due to the imperfect elasticity of the glass would be entirely negligible. The loss due to bombardment of the globe may, however, be reduced by using two elec¬ trodes instead of one. In such case each of the electrodes may be connected to one of the terminals; or else, if it is preferable to use only one wire, one electrode may be con¬ nected to one terminal and the other to the ground or to an insulated body of some surface, as, for instance, a shade on the lamp. In the latter case, unless some judgment is used, one of the electrodes might glow more intensely than the other.

But on the whole I find it* preferable when using such high frequencies to employ only one electrode and one con¬ necting wire. I am convinced that the illuminating device of the near future will not require for its operation more than one lead, and, at any rate, it will have no leading-in wire, since the energy required can be as well transmitted through the glass. In experimental bulbs the leading-in wire is most generally used on account of convenience, as in employing condenser coatings in the manner indicated in Fig. 22, for example, there is some difficulty in fitting the parts, but these difficulties would not exist if a great many bulbs were manufactured; otherwise the energy can be conveyed through the glass as well as through a wire, and with these high frequencies the losses are very small. Such illuminating devices will necessarily involve the use of very high potentials, and this, in the eyes of practical men, might be an objectionable feature. Yet, in reality, high potentials are not objectionable — certainly not

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in the least as far as the safety of the devices is con¬ cerned.

There are two ways of rendering an electric appliance safe. One is to use low potentials, the other is to deter¬ mine the dimensions of the apparatus so that it is safe no matter how high a potential is used. Of the two the latter seems to me the better way, for then the safety is absolute, un¬ affected by any possible combination of circumstances which might render even a low-potential appliance dangerous lo life and property. But the practical conditions require not only the judicious determination of the dimensions of the apparatus ; they likewise necessitate the employment of energy of the proper kind. It is easy, for instance, to con¬ struct a transformer capable of giving, when operated from an ordinary alternate current machine of low tension, say 50,000 volts, which might be required to light a highly ex¬ hausted phosphorescent tube, so that, in spite of the high potential, it is perfectly safe, the shock from it producing no inconvenience. Still, such a transformer would be ex¬ pensive, and in itself inefficient; and, besides, what energy was obtained from it would not be economically used for the production of light. The economy demands the em¬ ployment of energy in the form of extremely rapid vibrations. The problem of producing light has been likened to that of maintaining a certain high- pitch note by means of a bell. It should be said a barely audible note; and even these words would not express it, so wonderful is the sensitiveness of the eye. We may deliver powerful blows at long intervals, waste a good deal of energy, and still not get what we want; or we may keep up the note

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by delivering frequent gentle taps, and get nearer to the object sought by the expenditure of much less energy. In the production of light, as far as the illuminating device is concerned, there can be only one rule — that is, to use as high frequencies as can be obtained; but the means for the pro¬ duction and conveyance of impulses of such character im¬ pose, at present at least, great limitations. Once it is decided to use very high frequencies, the return wire becomes un¬ necessary, and all the appliances are simplified. By the use of obvious means the same result is obtained as though the return wire were used. It is sufficient for this purpose to bring in contact with the bulb, or merely in the vicinity of the same, an insulated body of some surface. The surface need, of course, be the smaller, the higher the frequency and potential used, and necessarily, also, the higher the economy of the lamp or other device.

This plan of working has been resorted to on several oc¬ casions this evening. So, for instance, when the incandes¬ cence of a button was produced by grasping the bulb with the hand, the body of the experimenter merely served to intensify the action. The bulb used was similar to that il¬ lustrated in Fig. 19, and the coil was excited to a small po¬ tential, not sufficient to bring the button to incandescence when the bulb was hanging from the wire ; and incident¬ ally, in order to perform the experiment in a more suitable manner, the button was taken so large that a perceptible time had to elapse before, upon grasping the bulb, it could be rendered incandescent. The contact with the bulb was, of course, quite unnecessary. It is easy, by using a rather large bulb with an exceedingly small electrode, to adjust

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the conditions so that the latter is brought to bright incan¬ descence by the mere approach of the experimenter within

Fig. 24.— Bulb Without Leading-In Wire, Showing Effect of Projected Matter.

a few feet of the bulb, and that the incandescence subsides upon his receding.

101

In another experiment, when phosphorescence was ex¬ cited, a similar bulb was used. Here again, originally, the potential was not sufficient to excite phosphorescence until the action was intensi¬ fied — in this case, however? to present a different feature, by touching the socket with a metallic object held in the hand. The electrode in the bulb was a carbon button so large that it could not be brought to incandescence, and thereby spoil the effect produced by phosphores¬ cence.

Again, in another of the early experiments, a bulb was used as illustrated in Fig. 12. In this instance, by touching the bulb with one or two fingers, one or two shadows of the stem inside were projected against the glass, the touch of the fin¬ ger producing the same result as the application Fig. 25.— Improved Experi- of an external negative mental Bulb.

electrode under ordinary circumstances.

In all these experiments the action was intensified by

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• •

augmenting the capacity at the end of the lead connected to the terminal. As a rule, it is not necessary to resort to such means, and would be quite unnecessary with still higher frequencies ; but when it is desired, the bulb, or tube, can be easily adapted to the purpose.

Iu Fig. 24, for example, an experimental bulb L is shown, which is provided with a neck n on the top for the appli¬ cation of an external tinfoil coating, which may be con¬ nected to a body of larger surface. Such a lamp as illus-

Fig. 26.— Improved Bulb with Intensifying Reflector.

trated in Fig. 25 may also be lighted by connecting the tin- foil coating on the neck n to the terminal, and the leading- in wire w to an insulated plate. If the bulb stands in a socket upright, as shown in the cut, a shade of conducting material may be slipped in the neck n, and the action thus magnified.

A more perfected arrangement used in some of these bulbs is illustrated in Fig. 26. In this case the construction

IDS

0

of the bulb is as shown and described before, when refer¬ ence was made to Fig. 19. A zinc sheet Z, with a tubular extension T, is slipped over the metallic socket S. The bulb hangs downward from the teiminal t, the zinc sheet

Fig. 27.— Phosphorescent Tube with Intensifying

Reflector.

Z, performing the double office of intensifier and reflector. The reflector is separated from the terminal t by an exten¬ sion of the insulating plug P.

A similar disposition with a phosphorescent tube is illus-

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strated in Fig. 27. The tube T is prepared from two short tubes of a different diameter, which are sealed on the ends. On the lower end is placed an outside conducting coating C, which connects to the wire w. The wire has a hook on the upper end for suspension, and passes through the centre of the inside tube, which is filled with some good and tightly packed insulator. On the outside of the upper end of the tube Tis another conducting coating Cu upon which is slipped a metallic reflector Z, which should be separated by a thick insulation from the end of wire w.

The economical use of such a reflector or intensifier would require that all energy supplied to an air condenser should be recoverable, or, in other words, that there should not be any losses, neither in the gaseous medium nor through its action elsewhere. This is far from being so, but, fortu¬ nately, the losses may be reduced to anything desired. A few remarks are necessary on this subject, in order to make the experiences gathered in the course of these investigations perfectly clear.

Suppose a small helix with many well insulated turns, as in experiment Fig. 17, has one of its ends connected to one of the terminals of the induction coil, and the other to a metal plate, or, for the sake of simplicity, a sphere, insu¬ lated in space. When the coil is set to work, the potential of the sphere is alternated, and the small helix now be¬ haves as though its free end were connected to the other terminal of the induction coil. If an iron rod be held within the small helix it is quickly brought to a high tem¬ perature, indicating the passage of a strong current through the helix. How does the insulated sphere act in this case ?

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