Experiments with Alternate Currents of High Potential and High Frequency (1892) — part 3 of 8

A peculiar property of oils, and liquid insulation in gen¬ eral, when subjected to rapidly changing electric stresses, is to disperse any gaseous bubbles which may be present, and diffuse them through its mass, generally long before any injurious break can occur. This feature may be easily observed with an ordinary induction coil by taking the primary out, plugging up the end of the tube upon which the secondary is wound, and filling it with some fairly transparent insulator, such as paraffine oil. A primary of a diameter something like six millimetres smaller than the inside of the tube may be inserted in the oil. When the coil is set to work one may see, looking from the top through the oil, many luminous points — air bubbles which are caught by inserting the primary, and which are ren¬ dered luminous in consequence of the violent bombard¬ ment. The occluded air, by its impact against the oil, heats it ; the oil begins to circulate, carrying some of the air along with it, until the bubbles are dispersed and the luminous points disappear. In this manner, unless large bubbles are occluded in such way that circulation is ren¬ dered impossible, a damaging break is averted, the only effect being a moderate warming up of the oil. If, instead of the liquid, a solid insulation, no matter how thick, were used, a breaking through and injury of the apparatus would be inevitable.

The exclusion of gaseous matter from any apparatus in

which the dielectric is subjected to more or less rapidly changing electric forces is, however, not only desirable in order to avoid a possible injury of the apparatus, but also on account of economy. In a condenser, for instance, as long as only a solid or only a liquid dielectric is used, the loss is small ; but if a gas under ordinary or small pressure be present the loss may be very great. Whatever the nature of the force acting in the dielectric may be, it seems that in a solid or liquid the molecular displacement produced by the force is small : hence the product of force and displace¬ ment is insignificant, unless the force be very great ; but in a gas the displacement, and therefore this product, is considerable ; the molecules are free to move, they reach high speeds, and the energy of their impact is lost in heat or otherwise. If the gas be strongly compressed, the dis¬ placement due to the force is made smaller, and the losses are reduced.

In most of the succeeding experiments I prefer, chiefly on account of the regular and positive action, to employ the alternator before referred to. This is one of the sev¬ eral machines constructed by me for the purposes of these investigations. It has 384 pole projections, and is capable of giving currents of a frequency of about 10, COO per sec¬ ond. This machine has been illustrated and briefly de¬ scribed in my first paper befoie the American Institute of Electrical Engineers, May 20, 1891, to which I have already referred. A more detailed description, sufficient to enable any engineer to build a similar machine, will be found in several electrical journals of that period.

The induction coils operated from the machine are rather

41

small, containing from 5,000 to 15,000 turns in the second¬ ary. They are immersed in boiled-out linseed oil, con¬ tained in wooden boxes covered with zinc sheet.

I have found it advantageous to reverse the usual posi¬ tion of the wires, and to wind, in these coils, the primaries on the top; this allowing the use of a much bigger primary, which, of course, reduces the danger of overheating and increases the output of the coil. I make the primary on each side at least one centimetre shorter than the secondary , to prevent the breaking through on the ends, which would surely occur unless the insulation on the top of the second¬ ary be very thick, and this, of course, would be disadvan¬ tageous.

When the primary is made movable, which is necessary in some experiments, and many times convenient for the purposes of adjustment, I cover the secondary with wax, and turn it off in a lathe to a diameter slightly smaller than the inside of the primary coil. The latter I provide with a handle peaching out of the oil, which serves to shift it in any pssition along the secondary.

I will now venture to make, in regard to the general manipulation of induction coils, a few observations bear¬ ing upon points which have not been fully appreciated in earlier experiments with such coils, and are even now often overlooked.

The secondary of the coil possesses usually such a high self-induction that the current through the wire is inap¬ preciable, and may be so even when the terminals are joined by a conductor of small resistance, If capacity is aided to the terminals, the self-induction is counteracted,

42

and a stronger current is made to flow through the second¬ ary, though its terminals are insulated from each other. To one entirely unacquainted with the properties of alter¬ nating currents nothing will look more puzzling. This feature was illustrated in the experiment performed at the beginning with the top plates of wire gauze attached to the terminals and the rubber plate. When the plates of wire gauze were close together, and a small arc passed between them, the arc pi evented a strong current from passing through the secondary, because it did away with the capacity on the terminals; when the rubber plate was inserted be¬ tween, the capacity of the condenser formed counteracted the self-induction of the secondary, a stronger current passed now, the coil performed more work, and the dis¬ charge was by far more powerful.

The first thing, then, in operating the induction coil is to combine capacity with the secondary to overcome the self-induction. If the frequencies and potentials are very high gaseous matter should be carefully kept away from the charged surfaces. If Leyden jars are used, they should be immersed in oil, as otherwise considerable dissipation may occur if the jars are greatly strained. When high frequencies are used, it is of equal importance to combine a condenser with the primary. One may use a condenser connected to the ends-of the primary or to the terminals of the alternator, but the latter is not to be recommended, as the machine might be injured. The best way is undoubt¬ edly to use the condenser in series with the primary and with the alternator, and to adjust its capacity so as to an¬ nul the self-induction of both the latter. The condenser

43

should be adjustable by very small steps, and for a finer adjustment a small oil condenser with movable plates may be used conveniently.

I think it best at this juncture to bring before you a phe¬ nomenon, observed by me some time ago, which to the purely scientific investigator may perhaps appear more in¬ teresting 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 phe¬ nomena — in fact, it is a brush, formed at, or near, a single terminal in high vacuum.

In bulbs provided with a conducting terminal, though it be of aluminium, the brush has but an ephemeral existence, and cannot, unfortunately, be indefinitely preserved in its most sensitive state, even in a bulb devoid of any conduct¬ ing electrode. In studying the phenomenon, by all means a bulb having no leading-in wire should be used. I have found it best to use bulbs constructed as indicated in Figs. 12 and 13.

In Fig. 12 the bulb comprises an incandescent lamp globe L, in the neck of which is sealed a barometer tube b, the end of which is blown out to form a small sphere s. This sphere should be sealed as closely as possible in the centre of the large globe. Before sealing, a thin tube t, of alu¬ minium sheet, may be slipped in the barometer tube, but it is not important to employ it.

The small hollow sphere s is filled with some conducting powder, and a wire w is cemented in the neck for the pur¬ pose of connecting the conducting powder with the gen¬ erator,

44

The construction shown in Fig. 13 was chosen in order to remove from the brush any conducting body which might possibly affect it. The bulb consists in this case of a lamp globe L, which has a neck n, provided with a tube band

Bulbs for Producing Rotating Brush.

small sphere s, sealed to it, so that two entirely independ¬ ent compartments are formed, as indicated in the drawing. When the bulb is in use, the neck n is provided with a tin- foil coating, which is connected to the generator and acts

45

inductively. upon the moderately rarefied and highly con¬ ducting gas inclosed in the neck. From there the current passes through the tube b into the small sphere s, to act by induction upon the gas contained in the globe L.

It is of advantage to make the tube t very thick, the hole through it very small, and to blow the sphere s very tliin. It is of the greatest importance that the sphere s be placed in the centre of the globe L.

Fig. 14.— Forms and Phases of the Rotating Brush.

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 ; 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. It is ob¬ served under the following conditions :

When the globe L (Figs. 12 and 13) is exhausted to a

46

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 ex¬ cite it, it is usually sufficient to grasp the globe L with the

hand. An intense phosphorescence then spreads at first over the globe, but soon gives place to a white, misty light. Shortly afterward one may notice that the luminosity is un¬ evenly distributed in the globe, and after passing the cur-

47

rent for some time the bulb appears as in Fig. 15. From this stage the phenomenon will gradually pass to that in¬ dicated 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.

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 th 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 aho before, it is affected by a magnet, and at a certain stage it is susceptible to mag¬ netic influence to an astonishing degree. A small perma¬ nent 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 accord¬ ing to how it is held relatively to the brush. 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 rota¬ tion, 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 rota¬ tion is always clockwise. In the southern hemisphere it would occur in the opposite direction and on the equator

48

the brush should not turn at all. 1 he rotation may be re¬ versed by a magnet kept at some distance. The brush ro¬ tates best, seemingly, when it is at right angles to the lines of force of the earth. 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. Chang¬ ing 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 alternat¬ ing currents of very high frequency the positive and nega¬ tive impulses are not equal, but that one always preponder¬ ates over the other.

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 iu a series motor, without nec¬ essarily one impulse being stronger than the other. The fact that the brush turns, as far as I could observe, in any

49

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 pre¬ ponderance of impulses of one kind.

As to the causes of the formation of the brush or stream, I thinx 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. 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 ex¬ tremely 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 an elec-

50

trostatic or magnetic field. 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 telegra¬ phy. With such a brush it would be possible to send dispatches across the Atlantic, for instance, with any speed, sin< e 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. 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, 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.

In operating an induction coil with rapidly alternating currents, wre realize with astonishment, for the first time,

51

the great importance of the relation of capacity, self-in¬ duction and frequency as regards the general result. The effects of capacity are the most striking, for in these exper¬ iments, since the self-induction and frequency both are high, the critical capacity is very small, and need be but slightly varied to produce a very considerable change. The experimenter may bring his body in contact with the ter¬ minals of the secondary of the coil, or attach to one or both terminals insulated bodies of very small bulk, such as bulbs, and he may produce a considerable rise or fall of potential, and greatly affect the flow of the current through the pri¬ mary. In the experiment before shown, in which a brush appears at a wire attached to one terminal, and the wire is vibrated when the experimenter brings his insulated body in contact with the other terminal of the coil, the sudden rise of potential was made evident.

I may show you the behavior of the coil in another man¬ ner which possesses a feature of some interest. I have here a little light fan of aluminium sheet, fastened to a needle and arranged to rotate freely in a metal piece screwed to one of the terminals of the coil. When the coil is set to work, the molecules of the air are rhythmically attracted and repelled. As the force with which they are repelled is greater than that with which they are attracted, it results that there is a repulsion exerted on the surfaces of the fan. If the fan were made simply of a metal sheet, the repulsion would be equal on the opposite sides, and would produce no effect. But if one of the opposing surfaces is screened, or if, generally speaking, the bombardment on this side is weakened in some way or other, there remains the repul-

sion exerted upon the other, and the fan is set in rotation . The screening is best effected by fastening upon one of the op¬ posing sides of the fan insulated conducting coatings, or, if the fan is made in the shape of an ordinary propeller screw, by fastening on one side, and close to it, an insulated metal plate. The static screen may, however, be omitted, and simply a thickness of insulating material fastened to one of the sides of the fan.

To show the behavior of the coil, the fan may be placed upon the terminal and it will readily rotate when the coil is operated by currents of very high frequency. With a steady potential, of course, and even with alternating cur¬ rents of very low frequency, it would not turn, because of the very slow exchange of air and, consequently, smaller bombardment; but in the latter case it might turn if the potential were excessive. With a pin wheel, quite the op¬ posite rule holds good ; it rotates best with a steady poten¬ tial, and the effort is the smaller the higher the frequency. Now, it is very easy to adjust the conditions so that the potential is normally not sufficient to turn the fan, but that by connecting the other terminal of the coil with an insulated body it rises to a much greater value, so as to rotate the fan, and it is likewise possible to stop the rota¬ tion by connecting to the terminal a body of different size, thereby diminishing the potential.

Instead of using the fan in this experiment, we may use

the “electric” radiometer with similar effect. But in this

«

case it will be found that the vanes will rotate only at high exhaustion or at ordinary pressures; they will not rotate at moderate pressures, when the air is highly conducting.

M

Tins curious observation was made conjointly by Professor Crookes and myself. I attribute tlie result to the high con¬ ductivity of the air, the molecules of which then do not act as independent carriers of electric charges, but act all together as a single conducting body. In such case, of course, if there is any repulsion at all of the molecules from the vanes, it must be very small. It is possible, how¬ ever, that the result is in part due to the fact that the greater part of the discharge passes from the leading-in wire through die highly conducting gas, instead of pass¬ ing off from the conducting vanes.

In trying the preceding experiment widi the electric radiometer the potential should not exceed a certain limit, as then the electrostatic attraction between the vanes and the glass of the bulb may be so great as to stop the rota- tation.

. ' \

A most curious feature of alternate currents of high fre¬ quencies and potentials is that they enable us to perform many experiments by the use of one wire only. In many respects this feature is of great interest.

In a type cf alternate current motor invented by me some years ago I produced rotation by inducing, by means of a single alternating current passed through a motor circuit, in the mass or other circuits of the motor, secondary cur¬ rents, which, jointly with the primary or inducing current, created a moving field of force. A simple but crude form of such a motor is obtained by winding upon an iron core a primary, and close to it a secondary coil, joining the ends of the latter and placing a freely movabA metal disc within the influence of the field produced by both. The

54

iron core is employed for obvious reasons, but it is not es¬ sential to the operation. To improve the motor, the iron core is made to encircle the armature. Again to improve, the secondary coil is made to overlap partly the primary, so that it cannot free itself from a strong inductive action of the latter, repel its lines as it may. Once more to im¬ prove, the proper difference of phase is obtained between the primary and secondary currents by a condenser, self- induction ^resistance or equivalent windings.

I had discovered, however, that rotation is produced by means of a single coil and core ; my explanation of the phenomenon, and leading thought in trying the experi¬ ment, being that there must be a true time lag in the mag¬ netization of the core. I remember the pleasure I had when, in the writings of Professor Ayrton, which came later to iny hand, I found the idea of the time lag advocated. Whether there is a true time lag, or whether the retarda¬ tion is due to eddy currents circulating in minute paths, must remain an open question, but the fact is that a coil wound upon an iron core and traversed by an alternating current creates a moving field of force, capable of setting an armature in rotation. It is of some interest, in conjunc¬ tion with the historical Arago experiment, to mention that in lag or phase motors I have produced rotation in the oppo¬ site direction to the moving field, which means that in that experiment the magnet may not rotate, or may even rotate in the opposite direction to the moving disc. Here, then, is a motor (diagrammatically illustrated in Fig. 17), compris¬ ing a coil and iron core, and a freely movable copper disc in proximity to the latter.

55

To demonstrate a novel and interesting feature, I have,

for a reason which I will explain, selected this type of

»

motor. When the ends of the coil are connected to the terminals of an alternator the disc is set in rotation. But it is not this experiment, now well known, which I desire

to perform. What I wish to show you is that this motor rotates with one single connection between it and the gen¬ erator; that is to say, one terminal of the motor is connected to one terminal of the generator — in this case the secondary of a high-tension induction coil — the other terminals of

56

motor and generator being insulated in space. To produce rotation it is generally (but not absolutely) necessary to connect the free end of the motor coif to an insulated body of some size. The experimenter’s body is more than suffi¬ cient. If he touches the free terminal with an object held in the hand, a current passes through the coil and the copper disc is set in rotation. If an exhausted tube is put in series with the coil, the tube lights brilliantly, showing the passage of a strong cur¬ rent. Instead of the experimenter’s body, a small metal sheet suspended on a cord may be used with the same result. In this case the plate acts as a condenser in series with the coil. It counteracts the self-induction of the latter and allows a strong current to pass. In such *' combination, the greater the self-induction of the coil the smaller need be the plate, and this means that a lower fre¬ quency, or eventually a lower potential, is required to operate the motor. A single coil wound upon a core has a high self-induction; for this reason principally, this type of motor was chosen to perform the experiment. Were a sec¬ ondary closed coil wound upon the core, it would tend to diminish the self-induction, and then it would be necessary to employ a much higher frequency and potential. Neither would be advisable, for a higher potential would endanger the insulation of the small primary coil, and a higher fre¬ quency would result in a materially diminished torque.

It should be remarked that when such a motor with a . closed secondary is used, it is not at all easy to obtain rota¬ tion with excessive frequencies, as the secondary cuts off almost completely the lines of the primary — and this, of

57

course, the more, the higher the frequency — and allows the passage of but a minute current. In such a case, unless the secondary is closed through a condenser, it is almost essen¬ tial, in order to produce rotation, to make the primary and secondary coils overlap each other more or less.

But there is an additional feature of interest about this motor, namely, it is, not necessary to have even a single connection between the motor and generator, except, per¬ haps, through the ground; for not only is an insulated plate capable of giving off energy into space, but it is likewise capable of deriving it from an alternating electrostatic field, though in the latter case the available energy is much smaller. In this instance one of the motor terminals is con¬ nected to the insulated plate or body located within the al¬ ternating electrostatic field, and the other terminal prefer¬ ably to the ground.

It is quite possible, however, that such “no-wire” motors, as they might be called, could be operated by conduction through the rarefied air at considerable distances. Alter¬ nate currents, especially of high frequencies, pass with as¬ tonishing freedom through even sligh tly rarefied gases. The upper strata of the air are rarefied. To reach a number of miles out into space requires the overcoming of difficul¬ ties of a merely mechanical nature. There is no doubt that with the enormous potentials obtainable by the use of high frequencies and oil insulation luminous discharges might be passed through many miles of rarefied air, and that, by thus directing the energy of many hundreds or thousands of liorse-power, motors or lamps might be operated at con¬ siderable distances from stationary sources. But such

J * k

58

schemes are mentioned merely as possibilities. We shall have no need to transmit power in tliis way. We shall have ncTneetl' to tfrOTlSwwTpower at all. Ere many genera¬ tions pass, our machinery will be driven by a power ob¬ tainable at any point of the^. iiniverse. This idea is not novel. Men have been led to it long ago by instinct or reason. It has been expressed in many ways, and in many places, in the history of old and new. We find it in the delightful myth of Antheus, who derives power from the earth; we find it among the subtile speculations of one of your splendid mathematicians, and in many hints and statements of thinkers of the present time. Throughout space there is energy. Is this energy static or kinetic? If static our hopes are in vain; if kinetic- -and this we know it is, for certain — then it is a mere ques¬ tion of time when men will succeed in attaching their machinery to the very wheelwork of nature. Of all, liv¬ ing or dead, Crookes came nearest to doing it. His radi¬ ometer will turn in the light of day and in the darkness of the night; it will turn everywhere where there is heat, and heat is everywhere. But, unfortunately, this beauti¬ ful little machine, while it goes down to posterity as the most interesting, must likewise be put on record as the most inefficient machine ever invented !

The preceding experiment is only one of many equally interesting experiments which may be performed by the use of only one wire with alternate currents of high poten¬ tial and frequency. We may connect an insulated line to a source of such currents, we may pass an inappreciable current over the line, and on any point of the same we are

59

able to obtain a heavy current, capable of fusing a thick copper wire. Or we may, by the help of some artifice, de¬ compose a solution in any electrolytic cell by connecting only one pole of the cell to the line or source of energy. Or we may, by attaching to the line, or only bringing into its vicinity, light up an incandescent lamp, an exhausted tube, or a phosphorescent bulb.

However impracticable this plan of working may appear in many cases, it certainly seems practicable, and even recommendable, in the production of light. A perfected lamp would^ require but little energy, and if wires were used at all we ought to be able to supply tbat energy with- out a return wire.

It is now a fact that a body may be rendered incandes¬ cent or phosphorescent by bringing it either in single con¬ tact or merely in the vicinity of a source of electric im¬ pulses of the proper character, and that in this manner a quantity of light sufficient to afford a practical illuminant may be produced. It is, therefore, to say the least, worth while to attempt to determine the best conditions and to invent the best appliances for attaining this object.

Some experiences have already been gained in this direc¬ tion, and I will dwell on them briefly, in the hope that they might prove useful.

The heating of a conducting body inclosed in a bulb, and connected to a source of rapidly alternating electric im¬ pulses, is dependent on so many things of a different nature, that it would be difficult to give a generally applicable rule under wThich the maximum heating occurs. As re¬ gards the size of the vessel, I have lately found that at or-

60

dinary or only slightly differing atmospheric pressures, when air is a good insulator, and hence practically the same amount of energy by a certain potential and fre¬ quency is given off from the body, whether the bulb be small or large, the body is brought to a higher temperature if inclosed in a small bulb, because of the better confine¬ ment of heat in this case.

At lower pressures, when air becomes more or less con¬ ducting, or if the air be sufficiently warmed as to become conducting, the body is rendered more intensely incandes¬ cent in a large bulb, obviously because, under otherwise equal conditions of test, more energy may be given off from the body when the bulb is large.

At very high degrees of exhaustion, when the matter in the bulb becomes “radiant,” a large bulb has still an ad¬ vantage, but a comparatively slight one, over the small bulb.

Finally, at excessively high degrees of exhaustion, which cannot be reached except by the employment of special means, there seems to be, beyond a certain and rather small size of vessel, no perceptible difference in the heating.

These observations were the result of a number of ex¬ periments, of which one, showing the effect of the size of the bulb at a high degree of exhaustion, may be described and shown here, as it presents a feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4 inches diameter were taken, and in the centre of each was mounted an equal length of an ordinary incandescent lamp filament of uniform thickness. In each bulb the piece of filament was fastened to the leading-in wire of platinum, con-

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