TESLA?S SENSITIVE BRUSH BULB:
A SPHERICAL GLASS LENARD TUBE?
Nikola Tesla, Royal Society Lecture, February 1892

GENERAL NOTES:
- a spark-gap Tesla Coil cannot operate this device, so is wasn't practical to replicate in 1892
- this is a hard-vacuum bulb driven by hundreds of KV (extreme x-ray hazard, unknown in 1892)

"I think it best at this juncture to bring before you a phenomenon, observed by me some time ago, which to the purely scientific investigator may perhaps appear more interesting than any of the results which I have the privilege to present to you this evening.

"It may be quite properly ranked among the brush phenomena ? in fact, it is a brush, formed at, or near, a single terminal in high vacuum.

[construction details of bulb]

"Figs. 14, 15 and '16 indicate different forms, or stages, of the brush. Fig. 14 shows the brush as it first appears in a bulb provided with a conducting terminal:

[Note: in fig. 14 the brush is depicted as the familiar curved plasma streamer.]

"but, as in such a bulb it very soon disappears ? often after a few minutes ? I will confine myself to the description of the phenomenon as seen in a bulb without conducting electrode.

[Note: metal electrodes form an ion pump which causes the vacuum to become higher and higher. If the phenomenon relies on the physics of cold-cathode Crookes x-ray tubes, then it suffers from the same problem as those tubes: the gas molecules become embedded in the metal electrodes during operation. The pressure drops too low, causing the tube to go dark when the required operating voltage rises higher than the HV power supply.]

"It is observed under the following conditions:

"When the globe L (Figs. 12 and 13) is exhausted to a very high degree, generally the bulb is not excited upon connecting the wire w (Fig. 12) or the tinfoil coating of the bulb (Fig. 13) to the terminal of the induction coil. To excite it, it is usually sufficient to gasp the globe L with the hand.

[Note: in common with many high-vacuum tubes, the discharge would only start at ?trigger voltage? higher than the drive voltage. Placing a grounded object near the tube will compress the e-field and increase it past the triggering threshold.]

"An intense phosphorescence then spreads at first over the globe

[Note: phosphorescing glass appears during 10KV operating voltage or higher, and is a symptom of significant x-ray output. This in 1892, years before Roentgen. A discovery waiting to happen.]

"but soon gives place to a white, misty light.

[Note: what does "misty light" here mean? It?s a different term than "phosphorescing glass." I predict that the "mist" is appearing outside the globe. In other parts of this lecture, Tesla mentions "misty" glow-discharges in air.]

"Shortly afterward one may notice that the luminosity is unevenly distributed in the globe, and after passing the current for some time the bulb appears as in Fig. 15.

[Note: in fig. 15, the rays are depicted within the bulb, but also passing through the glass barrier and out into the air of the room.]

"From this stage the phenomenon will gradually pass to that indicated in Fig. 16, after some minutes, hours, days or weeks, according as the bulb is worked. Warming the bulb or increasing the potential hastens the transit.

[Note: in fig. 16 the ?rays? are more concentrated into a fan or cone-shape aiming to the side, and are very clearly depicted as *not* remaining in the vacuum, but as exiting through the side of the globe. Most important: Tesla unknowingly is describing the process of ion-pumping, where the pressure within the tube is slowly falling to very ?hard? vacuum values, and where prodigious x-ray emission is expected.]

"When the brush assumes the form indicated in Fig. 16, it may be brought to a state of extreme sensitiveness to electrostatic and magnetic influence. The bulb hanging straight down from a wire, and all objects being remote from it, the approach of the observer at a few paces from the bulb will cause the brush to fly to the opposite side, and if he walks around the bulb it will always keep on the opposite side. It may begin to spin around the terminal long before it reaches that sensitive stage. When it begins to turn around principally, but also before, it is affected by a magnet, and at a certain stage it is susceptible to magnetic influence to an astonishing degree.

[Note: simple neon-filled Plasma Globes will exhibit this DC effect, where the orange streamers inside the globe are deflected and will spin around when approached by an extremely strong (50,000 gauss) permanent magnet. But Tesla is describing an effect involving ion-pumping to hard vacuum where no plasma streamers are possible. ]

"A small permanent magnet, with its poles at a distance of no more than two centimetres, will affect it visibly at a distance of two metres, slowing down or accelerating the rotation according to how it is held relatively to the brush.

[Note: since the only 1892 permanent magnets were steel, we can today construct a 20mm steel bar-magnet and measure the field strength at 2M distance. It should be perhaps roughly 0.01 gauss.]

"I think I have observed that at the stage when it is most sensitive to magnetic, it is not most sensitive to electrostatic, influence. My explanation is, that the electrostatic attraction between the brush and the glass of the bulb, which retards the rotation, grows much quicker than the magnetic influence when the intensity of the stream is increased.

"When the bulb hangs with the globe L down, the rotation is always clockwise.

[Note: we could determine the polarity of current carriers from this. . . but we don?t know if it?s clockwise as observed from above or below.]

"In the southern hemisphere it would occur in the opposite direction and on the equator the brush should not turn at all. The rotation may be reversed by a magnet kept at some distance. The brush rotates best, seemingly, when it is at right angles to the lines of force of the earth.

[Note: this sounds just like a description of a rotating conductive rod in a magnetic field, illustrating the Left-Hand rule for motors.]

"It very likely rotates, when at its maximum speed, in synchronism with the alternations, say 10,000 times a second. The rotation can be slowed down or accelerated by the approach or receding of the observer, or any conducting body, but it cannot be reversed by putting the bulb in any position. When it is in the state of the highest sensitiveness and the potential or frequency be varied the sensitiveness is rapidly diminished. Changing either of these but little will generally stop the rotation. The sensitiveness is likewise affected by the variations of temperature. To attain great sensitiveness it is necessary to have the small sphere s in the centre of the globe L, as otherwise the electrostatic action of the glass of the globe will tend to stop the rotation. The sphere s should be small and of uniform thickness; any dissymmetry of course has the effect to diminish the sensitiveness.

"The fact that the brush rotates in a definite direction in a permanent magnetic field seems to show that in alternating currents of very high frequency the positive and negative impulses are not equal, but that one always preponderates over the other.

[Note: Decades later Lenard discovered that, in glow-discharge tubes provided with a thin foil windows, electrons exit the vacuum and create a glowing ?beam? in the air outside. If Tesla?s drive voltage was high enough, this tube might emit electrons of high enough energy to penetrate the glass and produce a glowing ?brush? effect in the air.]

"Of course, this rotation in one direction may be due to the action of two elements of the same current upon each other, or to the action of the field produced by one of the elements upon the other, as in a series motor, without necessarily one impulse being stronger than the other. The fact that the brush turns, as far as I could observe, in any position, would speak for this view. In such case it would turn at any point of the earth's surface. But, on the other hand, it is then hard to explain why a permanent magnet should reverse the rotation, and one must assume the preponderance of impulses of one kind.

"As to the causes of the formation of the brush or stream, I think it is due to the electrostatic action of the globe and the dissymmetry of the parts. If the small bulb s and the globe L were perfect concentric spheres, and the glass throughout of the same thickness and quality, I think the brush would not form, as the tendency to pass would, be equal on all sides.

[Note: gas discharge has nonlinear character, and usually two parallel discharges from the same electrodes are unstable: the stronger one grows while the weaker one vanishes. If this effect is part of ?sensitive brush tube,? then a stream-like structure is stable, but a fan-like structure is unstable and would collapse to form a stream.]

"That the formation of the stream is due to an irregularity is apparent from the fact that it has the tendency to remain in one position, and rotation occurs most generally only when it is brought out of this position by electrostatic or magnetic influence. When in an extremely sensitive state it rests in one position, most curious experiments may be performed with it. For instance, the experimenter may, by selecting a proper position, approach the hand at a certain considerable distance to the bulb, and he may cause the brush to pass off 'by merely stiffening the muscles of the arm. When it begins to rotate slowly, and the hands are held at a proper distance, it is impossible to make even the slightest motion without producing a visible effect upon the brush. A metal plate connected to the other terminal of the coil affects it at a great distance, slowing down the rotation often to one turn a second.

"I am firmly convinced that such a brush, when we learn how to produce it properly, will prove a valuable aid in the investigation of the nature of the forces acting in 1n electrostatic or magnetic field.

[Note: if this brush is composed of 100KV electrons and gamma rays, then it will cause bright illumination of any fluorescent bulb or panel or paint, and will ruin any glass photographic plates in its path. A discovery waiting to happen. If it lights up a fluorescent sheet, it?s forming a fluoroscope, and if a human hand casts a shadow, only the shadow of the bones will be seen.]

"If there is any motion which is measurable going on in the space, such a brush ought to reveal it. It is, so to speak, a beam of light, frictionless, devoid of inertia.

"I think that it may find practical applications in telegraphy. With such a brush it would be possible to send dispatches across the Atlantic, for instance, with any speed, since its sensitiveness may be so great that the slightest changes will affect it. If it were possible to make the stream more intense and very narrow, its deflections could be easily photographed.

"I have been interested to find whether there is a rotation of the stream itself, or whether there is simply a stress traveling around in the bulb. For this purpose I mounted a light mica fan so that its vanes were in the path of the brush.

[Note: is the phenomenon confined to the globe, or does it extend out into the air? Here Tesla offhandedly describes a mica fan, but without describing the significant work needed to place such a device *inside* the glass globe. This is one more thing suggesting that the glowing beam is not confined to the bulb, but extends into the air.]

"If the stream itself was rotating the fan would be spun around. I could produce no distinct rotation of the fan, although I tried the experiment repeatedly; but as the fan exerted a noticeable influence on the stream, and the apparent rotation of the latter was, in this case, never quite satisfactory, the experiment did not appear to be conclusive.

'I have been unable to produce the phenomenon with the disruptive discharge coil,

[Note: HUGE ISSUE! SPARK-GAP TESLA COILS DO NOT PRODUCE THE EFFECT! Until vacuum tube TCs and later SSTCs appear, the people able to replicate this phenomenon were limited to those in possession of a high-frequency sine wave source; a multi-pole high speed alternator (20KHz?) Only Tesla was working with these. The so-called ?Alexanderson Alternator? only appeared decades later.]

"although every other of these phenomena can be well produced by it ? many, in fact, much better than with coils operated from an alternator.

"It may be possible to produce the brush by impulses of one direction, or even by a steady potential, in which case it would be still more sensitive to magnetic influence.

[Note: yes, build a metal-foil Lenard tube operated by a very large Wimshurst machine, then use a small magnet to deflect the glowing beam of electrons/gammas. But no physicist realized that a spherical glass version of Lenard?s tube would be possible.]