.  .  .  In all the last described experiments, tubes devoid of any electrodes may be used, and there is no difficulty in producing by their means sufficient light to read by.  The light effect is, however, considerably increased by the use of phosphorescent bodies such as yttria, uranium glass, etc.  A difficulty will be found when the phosphorescent material is used, for with these powerful effects, it is carried gradually away, and it is preferable to use material in the form of a solid.

Instead of depending on induction at a distance to light the tube, the same may be provided with an external--and, if desired, also with an internal--condenser coating, and it may then be suspended anywhere in the room from a conductor connected to one terminal of the coil, and in this manner a soft illumination may be provided.

The ideal way of lighting a hall or room would, however, be to produce such a condition in it that an illuminating device could be moved and put anywhere, and that it is lighted, no matter where it is put and without being electrically connected to anything.  I have been able to produce such a condition by creating in the room a powerful, rapidly alternating electrostatic field.  For this purpose I suspend a sheet of metal a distance from the ceiling on insulating cords and connect it to one terminal of the induction coil, the other terminal being preferably connected to the ground.  Or else I suspend two sheets as illustrated in Fig.  125, each sheet being connected with one of the terminals of the coil, and their size being carefully determined.  An exhausted tube may then be carried in the hand anywhere between the sheets or placed anywhere, even a certain distance beyond them; it remains always luminous.

EXCERPT:
From "On Light and Other High Frequency Phenomena"
ON ELECTRICAL RESONANCE
     The effects of resonance are being more and more noted by engineers and are becoming of great importance in the practical operation of apparatus of all kinds with alternating currents.  A few general remarks may therefore be made concerning these effects.  It is clear, that if we succeed in employing the effects of resonance practically in the operation of electric devices the return wire will, as a matter of course, become unnecessary, for the electric vibration may be conveyed with one wire just as well as, and sometimes even better than, with two.  The question first to answer is, then, whether pure resonance effects are producible.  Theory and experiment both show that such is impossible in Nature, for as the oscillation becomes more and more vigorous, the losses in the vibrating bodies and environing media rapidly increase and necessarily check the vibration which otherwise would go on increasing forever.  It is a fortunate circumstance that pure resonance is not producible, for if it were there is no telling what dangers might not lie in wait for the innocent experimenter.  But to a certain degree resonance is producible, the magnitude of the effects being limited by the imperfect conductivity and imperfect elasticity of the media or, generally stated, by frictional losses.  The smaller these losses, the more striking are the effects.  The same is the case in mechanical vibration.  A stout steel bar may be set in vibration by drops of water falling upon it at proper intervals; and with glass, which is more perfectly elastic, the resonance effect is still more remarkable, for a goblet may be burst by singing into it a note of the proper pitch.  The electrical resonance is the more perfectly attained, the smaller the resistance or the impedance of the conducting path and the more perfect the dielectric.  In a Leyden jar discharging through a short stranded cable of thin wires these requirements are probably best fulfilled, and the resonance effects are, therefore very prominent.  Such is not the case with dynamo machines, transformers and their circuits, or with commercial apparatus in general in which the presence of iron cores complicates the action or renders it impossible.  In regard to Leyden jars with which resonance effects are frequently demonstrated, I would say that the effects observed are often attributed but are seldom due to true resonance, for an error is quite easily made in this respect.  This may be undoubtedly demonstrated by the following experiment.  Take, for instance, two large insulated metallic plates or spheres which I shall designate A and B; place them at a certain small distance apart and charge them from a frictional or influence machine to a potential so high that just a slight increase of the difference of potential between them will cause the small air or insulating space to break down.  This is easily reached by making a few preliminary trials.  If now another plate—fastened on an insulating handle and connected by a wire to one of the terminals of a high tension secondary of an induction coil, which is maintained in action by an alternator (preferably high frequency)—is approached to one of the charged bodies A or B, so as to be nearer to either one of them, the discharge will invariably occur between them; at least it will, if the potential of the coil in connection with the plate is sufficiently high.  But the explanation of this will soon be found in the fact that the approached plate acts inductively upon the bodies A and B and causes a spark to pass between them.  When this spark occurs, the charges which were previously imparted to these bodies from the influence machine, must needs be lost, since the bodies are brought in electrical connection through the arc formed.  Now this arc is formed whether there be resonance or not.  But even if the spark would not be produced, still there is an alternating E. M. F. set up between the bodies when the plate is brought near one of them; therefore the approach of the plate, if it does not always actually, will, at any rate, tend to break down the air space by inductive action.  Instead of the spheres or plates A and B we may take the coatings of a Leyden jar with the same result, and in place of the machine,—which is a high frequency alternator preferably, because it is more suitable for the experiment and also for the argument,—we may take another Leyden jar or battery of jars.  When such jars are discharging through a circuit of low resistance the same is traversed by currents of very high frequency.  The plate may now be connected to one of the coatings of the second jar, and when it is brought near to the first jar just previously charged to a high potential from an influence machine, the result is the same as before, and the first jar will discharge through a small air space upon the second being caused to discharge.  But both jars and their circuits need not be tuned any closer than a basso profundo is to the note produced by a mosquito, as small sparks will be produced through the air space, or at least the latter will be considerably more strained owing to the setting up of an alternating E. M. F. by induction, which takes place when one of the jars begins to discharge.  Again another error of a similar nature is quite easily made.  If the circuits of the two jars are run parallel and close together, and the experiment has been performed of discharging one by the other, and now a coil of wire be added to one of the circuits whereupon the experiment does not succeed, the conclusion that this is due to the fact that the circuits are now not tuned, would be far from being safe.  For the two circuits act as condenser coatings and the addition of the coil to one of them is equivalent to bridging them, at the point where the coil is placed, by a small condenser, and the effect of the latter might be to prevent the spark from jumping through the discharge space by diminishing the alternating E. M. F. acting across the same.  All these remarks, and many more which might be added but for fear of wandering too far from the subject, are made with the pardonable intention of cautioning the unsuspecting student, who might gain an entirely unwarranted opinion of his skill at seeing every experiment succeed; but they are in no way thrust upon the experienced as novel observations.

     In order to make reliable observations of electric resonance effects it is very desirable, if not necessary, to employ an alternator giving currents which rise and fall harmonically, as in working with make and break currents the observations are not always trustworthy, since many phenomena, which depend on the rate of change, may be produced with widely different frequencies.  Even when making such observations with an alternator one is apt to be mistaken.  When a circuit is connected to an alternator there are an indefinite number of values for capacity and self-induction which, in conjunction, will satisfy the condition of resonance.  So there are in mechanics an infinite number of tuning forks which will respond to a note of a certain pitch, or loaded springs which have a definite period of vibration.  But the resonance will be most perfectly attained in that case in which the motion is effected with the greatest freedom.  Now in mechanics, considering the vibration in the common medium—that is, air—it is of comparatively little importance whether one tuning fork be somewhat larger than another, because the losses in the air are not very considerable.  One may, of course, enclose a tuning fork in an exhausted vessel and by thus reducing the air resistance to a minimum obtain better resonant action.  Still the difference would not be very great.  But it would make a great difference if the tuning fork were immersed in mercury.  In the electrical vibration it is of enormous importance to arrange the conditions so that the vibration is effected with the greatest freedom.  The magnitude of the resonance effect depends, under otherwise equal conditions, on the quantity of electricity set in motion or on the strength of the current driven through the circuit.  But the circuit opposes the passage of the currents by reason of its impedance and therefore, to secure the best action it is necessary to reduce the impedance to a minimum.  It is impossible to overcome it entirely, but merely in part, for the ohmic resistance cannot be overcome.  But when the frequency of the impulses is very great, the flow of the current is practically determined by self-induction.  Now self-induction can be overcome by combining it with capacity.  If the relation between these is such, that at the frequency used they annul each other, that is, have such values as to satisfy the condition of resonance, and the greatest quantity of electricity is made to flow through the external circuit, then the best result is obtained.  It is simpler and safer to join the condenser in series with the self-induction.  It is clear that in such combinations there will be, for a given frequency, and considering only the fundamental vibration, values which will give the best result, with the condenser in shunt to the self-induction coil; of course more such values than with the condenser in series.  But practical conditions determine the selection.  In the latter case in performing the experiments one may take a small self-induction and a large capacity or a small capacity and a large self-induction, but the latter is preferable, because it is inconvenient to adjust a large capacity by small steps.  By taking a coil with a very large self-induction the critical capacity is reduced to a very small value, and the capacity of the coil itself may be sufficient.  It is easy, especially by observing certain artifices, to wind a coil through which the impedance will be reduced to the value of the ohmic resistance only; and for any coil there is, of course, a frequency at which the maximum current will be made to pass through the coil.  The observation of the relation between self-induction, capacity and frequency is becoming important in the operation of alternate current apparatus, such as transformers or motors, because by a judicious determination of the elements the employment of an expensive condenser becomes unnecessary.  Thus it is possible to pass through the coils of an alternating current motor under the normal working conditions the required current with a low E. M. F. and do away entirely with the false current, and the larger the motor, the easier such a plan becomes practicable; but it is necessary for this to employ currents of very high potential or high frequency.

     In Fig. 184 I is shown a plan which has been followed in the study of the resonance effects by means of a high frequency alternator.  C₁ is a coil of many turns, which is divided into small separate sections for the purpose of adjustment.  The final adjustment was made sometimes with a few thin iron wires (though this is not always advisable) or with a closed secondary.  The coil C₁ is connected with one of its ends to the line L from the alternator G and with the other end to one of the plates C of a condenser C C₁, the plate (C₁) of the latter being connected to a much larger plate P₁.  In this manner both capacity and self-induction were adjusted to suit the dynamo frequency.

     As regards the rise of potential through resonant action, of course, theoretically, it may amount to anything since it depends on self-induction and resistance and since these may have any value.  But in practice one is limited in the selection of these values and besides these, there are other limiting causes.  One may start with, say, 1,000 volts and raise the E. M. F. to 50 times that value, but one cannot start with 100,000 and raise it to ten times that value because of the losses in the media which are great, especially if the frequency is high.  It should be possible to start with, for instance, two volts from a high or low frequency circuit of a dynamo and raise the E. M. F. to many hundred times that value.  Thus coils of the proper dimensions might be connected each with only one of its ends to the mains from a machine of low E. M. F., and though the circuit of the machine would not be closed in the ordinary acceptance of the term, yet the machine might be burned out if a proper resonance effect would be obtained.  I have not been able to produce, nor have I observed with currents from a dynamo machine, such great rises of potential.  It is possible, if not probable, that with currents obtained from apparatus containing iron the disturbing influence of the latter is the cause that these theoretical possibilities cannot be realized.  But if such is the case I attribute it solely to the hysteresis and Foucault current losses in the core.  Generally it was necessary to transform upward, when the E. M. F. was very low, and usually an ordinary form of induction coil was employed, but sometimes the arrangement illustrated in Fig. 184 II., has been found to be convenient.  In this case a coil C is made in a great many sections, a few of these being used as a primary.  In this manner both primary and secondary are adjustable.  One end of the coil is connected to the line L1 from the alternator, and the other line L is connected to the intermediate point of the coil.  Such a coil with adjustable primary and secondary will be found also convenient in experiments with the disruptive discharge.  When true resonance is obtained the top of the wave must of course be on the free end of the coil as, for instance, at the terminal of the phosphorescence bulb B.  This is easily recognized by observing the potential of a point on the wire w near to the coil.

     In connection with resonance effects and the problems of transmission of energy over a single conductor, which was previously considered, I would say a few words on a subject which constantly fills my thoughts, and which concerns the welfare of all.  I mean the transmission of intelligible signals, or, perhaps, even power, to any distance without the use of wires.  I am becoming more convinced of the practicability of the scheme; and though I know full well that the great majority of scientific men will not believe that such results can be practically and immediately realized, yet I think that all consider the developments in recent years by a number of workers to have been such as to encourage thought and experiment in this direction.  My conviction has grown so strong that I no longer look upon the plan of energy or intelligence transmission as a mere theoretical possibility, but as a serious problem in electrical engineering, which must be carried out some day.  The idea of transmitting intelligence without wires is the natural outcome of the most recent results of electrical investigations.  Some enthusiasts have expressed their belief that telephony to any distance by induction through air is possible.  I cannot stretch my imagination so far, but I do firmly believe that it is practical to disturb, by means of powerful machines, the electrostatic conditions of the earth, and thus transmit intelligible signals, and, perhaps, power.  In fact, what is there against carrying out such a scheme?  We now know that electrical vibrations may be transmitted through a single conductor.  Why then not try to avail ourselves of the earth for this purpose?  We need not be frightened by the idea of distance.  To the weary wanderer counting the mileposts, the earth may appear very large; but to the happiest of all men, the astronomer, who gazes at the heavens, and by their standards judges the magnitude of our globe, it appears very small.  And so I think it must seem to the electrician; for when he considers the speed with which an electrical disturbance is propagated through the earth, all his ideas of distance must completely vanish.

     A point of great importance would be first to know what is the capacity of the earth? and what charge does it contain if electrified?  Though we have no positive evidence of a charged body existing in space without other oppositely electrified bodies being near, there is a fair probability that the earth is such a body, for by whatever process it was separated--and this is the accepted view of its origin--it must have retained a charge, as occurs in all processes of mechanical separation. . . .  If ever we can ascertain at what period the earth's charge, when disturbed, oscillates, with respect to an oppositely charged system or known circuit, we shall know a fact possibly of the greatest importance to the welfare of the human race.  I propose to seek for the period by means of an electrical oscillator or a source of alternating currents.  One of the terminals of this source would be connected to the earth, as, for instance, to the city water mains, the other to an insulated body of large surface.  It is possible that the outer conducting air strata or free space contains an opposite charge, and that, together with the earth, they form a condenser of large capacity.  In such case the period of vibration may be very low and an alternating dynamo machine might serve for the purpose of the experiment.  I would then transform the current to a potential as high as it would be found possible, and connect the ends of the high tension secondary to the ground and to the insulated body.  By varying the frequency of the currents and carefully observing the potential of the insulated body, and watching for the disturbance at various neighboring points of the earth's surface, resonance might be detected.  Should, as the majority of scientific men in all probability believe, the period be extremely small, then a dynamo machine would not do, and a proper electrical oscillator would have to be produced, and perhaps it might not be possible to obtain such rapid vibrations.  But whether this be possible or not, and whether the earth contains a charge or not, and whatever may be its period of vibration, it is certainly possible--for of this we have daily evidence--to produce some electrical disturbance sufficiently powerful to be perceptible by suitable instruments at any point on the earth's surface.

     Assume that a source of alternating currents be connected, as in Fig.  185, with one of its terminals to earth (conveniently to the water mains) and with the other to a body of large surface P.  When the electric oscillation is set up there will be a movement of electricity in and out of P, and alternating currents will pass through the earth, converging to, or diverging from, the point C where the ground connection is made.  In this manner neighboring points on the earth's surface within a certain radius will be disturbed.  But the disturbance will diminish with the distance, and the distance at which the effect will still be perceptible will depend on the quantity of electricity set in motion.  Since the body P is insulated, in order to displace a considerable quantity, the potential of the source must be excessive, since there would be limitations as to the surface of P.  The conditions might be adjusted so that the generator or source S will set up the same electrical movement as though its circuit were closed.  Thus it is certainly practicable to impress an electric vibration at least of a certain low period upon the earth by means of proper machinery.  At what distance such a vibration might be made perceptible can only be conjectured.  I have on another occasion, considered the question how the earth might behave to electric disturbances.  There is no doubt that, since in such an experiment the electrical density at the surface could be but extremely small considering the size of the earth, the air would not act as a very disturbing factor, and there would be not much energy lost through the action of the air, which would be the case if the density were great.  Theoretically, then, it could not require a great amount of energy to produce a disturbance perceptible at a great distance, or even all over the surface of the globe.  Now, it is quite certain that at any point within a certain radius of the sources, a properly adjusted self induction and capacity device can be set in action by resonance.  But not only this can be done, but another source, S1, similar to S, or any number of such sources, can be set to work in synchronism with the latter, and the vibration thus intensified and spread over a large area, or a flow of electricity produced to or from source S1, if the same or of opposite phase to the source S.  I think that, beyond doubt, it is possible to operate electrical devices in a city, through the ground or pipe system, by resonance from an electrical oscillator located at a central point.  But the practical solution of this problem would be of incomparably smaller benefit to man than the realization of the scheme of transmitting intelligence, or, perhaps, power, to any distance through the earth or environing medium.  If this is at all possible, distance does not mean anything.  Proper apparatus must first be produced, by means of which the problem can be attacked, and I have devoted much thought to this subject.  I am firmly convinced it can be done, and I hope we shall live to see it done.

EXCERPT:
From Mechanical and Electrical Oscillators
     On the evening of Friday, August 25,1893, Mr.  Tesla delivered a lecture on his mechanical and electrical oscillators, before the members of the Electrical Congress, in the hall adjoining the Agricultural Building, at the World's Fair, Chicago.  Besides the apparatus in the room, he employed an air compressor, which was driven by au electric motor.

Mr.  Tesla was introduced by Dr.  Elisha Gray, and began by stating that the problem he had set out to solve was to construct first, a mechanism which would produce oscillations of a perfectly constant period independent of the pressure of steam or air applied, within the widest limits, and also independent of frictional losses and load.  Secondly, to produce electric currents of a perfectly constant period independently of the working conditions, and to produce these currents with mechanism which should be reliable and positive in its action without resorting to spark gaps and breaks.  This he successfully accomplished in his apparatus, and with this apparatus, now, scientific men will be provided with the necessities for carrying on investigations with alternating currents with great precision.  These two inventions Mr.  Tesla called, quite appropriately, a mechanical and an electrical oscillator, respectively.

The former is substantially constructed in the following way.  There is a piston in a cylinder made to reciprocate automatically by proper dispositions of parts, similar to a reciprocating tool.  Mr.  Tesla pointed out that he had done a great deal of work in perfecting his apparatus so that it would work efficiently at such high frequency of reciprocation as he contemplated, but he did not dwell on the many difficulties encountered.  He exhibited, however, the pieces of a steel arbor which had been actually torn apart while vibrating against a minute air cushion. .  .  .