On Dissipation Of The Electrical Energy Of The Hertz Resonator

Friday, January 6, 1893
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me ELEClRlClA_li_JANUAR_&6, 1593. 2'Z}_ ON THE DISSIPATION OF THE ELECTRICAL ENERGY OF THE HERTZ RESONATORJ BY NIKQLA TESLA. Anyone who, like myself, has had the pleusure of witnessing the beautiful demonstrations with vibrating diaplirsgms which Prof. Bjerlmes exhibited in person at the Paris Exposition in 1889, mush have admired his ability and painstaking care to such a degree as to have an almost implicit faith in the cvrreetness of observations made by him. His experiments “On the Dissipation of the Elec- trical Energy of the Hertz Resonator," which are described in the issue of December 14th of the Eluutricul .Engineeigi are prepared in thu same ingenious and skilful manner, and the conclu- sions drawn from them are all the more interesting as they agree with the theories put forth by the must mlvsliced thinkers, There cannot he the slightest doubt as to the truth of these conclusions, yet the statements which follow may serve to explain in part the ' cmnpm zemfim, September 19, 1892, vel. cxv., Ne. 12. see also The lfleezffeimr, September so, 1892, p. svs, vel. xxlx, + 1-‘rem the fled,-:eel B/»gi11c¢rof New York. x See 1'/ie Eluclricirm, November Ja, 1892, P. 69.

272 THE ELECTRICIAN, JAN UARY 6, 1893. results arrived atin different manner ; and with this object in view I venture to call attention to zsconditien with which, in investigations such as those of Prof. Bjerknes, the experimenter is confronted. | »The npparzitus, oscillator and resonator, being immersed in air, or other discontinuous medium, there occurs-as I have pointed out in the description of my recent experiments before the English and French scientific sncitiesfrlissipatinu of energy by what I think might be appropriately called electricso1z1ul-wares orsoml/I-'raves of flerfri/ind air. In Prof, Bjerknesls experiments principally this dissipation in the resonator need be considered, though the sound- waves-if this term be permitted-which emnnatc from the surfaces at the oscillator may considerably affect the observations made at some distance from the latter. Owing to this dissipation the period uf vibration of nn niccmidensor cannot be accurately determined, and I have already drawn :ittentiou to this important fact. These waves are propagated at right angles from the charged sur- faces when their charges are alternated, and dissipation occurs, even if the surfaces arc covered with thick und excellent insulation. Assuming that the “ charge " imparted to n molecule or atom either by direct contact or inductively is pmportinnte to the electric density of the surface, the dissipation should he proportinate to the square of the density and io the number of ivnvcs ` per second. Tho above assumption, it should be stated, does not agree with some observations from which it appears that an atom cannot take but a certain maximum charge ; hence, the charge imparted may be pracvicslly independent of the density of the surface, but this is immaterial for the present consideration. This and other points will be decided when accurate qimntitntivo determinations, which are as yet wanting, shall be made. At present it appears certain from experiments with liiglrfreqiiency currents, and this dissipa- tion of energy from a wire, for instance, is not very far from being proporriunate to the frequency of the alternations, and -in~ creases very rapidly when the diameter of the wire is made exceed ingly snmll. On the latter point the recently published results of Prof. Ayrton mid Il. Kilgour on “The Thermal Einissivity of Thin Wires in Air ” throw u uuriouslight. Execedingly thin wires arc capable of clissipnting Ji coniparntivcly very great amount of energy by the agitation of the surrounding air, when they are, con- nected to a source of rapidly alternating potential. So in the ex- periment cited, s. thin hot wire is found to be capable of, emitting an extraordinarily great amount of heat, especially at elevated temperatures. In the case of a hot wire it must, of course, be assumed that the increased einissivity is due to the more rapid con- vection, hnd not, to any appreciable degree, to nn increased radia- tion. \Vere the latter demonstrated, it would show thatawire, made hot by vhs application 4 f heat in ordinary ways, behaves in some respects like one the charge of which is rapidly alternated, the dissipation of energy per unit of surface kept at a certain temperature depending on the curvature of the surface. I do not recall any record of experiments intended to demonstrate thi , yet this effect, though probably very small, should certainly be looked for. A number of observations showing the peculiarily of very thin, wires were made in the course of my experiments. I noted, for instance, th:\t in the well»known Crookes instrument the mica vanes are repelled with comparatively greater force when the incandes- cent platinum wire is exceedingly thin. This observation enabled me to produce the spin oi such vancs mounted in a vacuum tube when the latter was placed in nn alternating electrostatic field. This, however, does not prove anything in regard toradiation, as in :i highly exhausted vessel the phenomena are principally due to molecular b0mb:\rxl|nent or convection. ` \Vlicn l first undertook to produce the incsndescence of s. wire enclosed in a. bulb, by connecting it to only une of the terminals of a high tension transformer, I could not succeed for along time. On one occasion I had mounted in a bulb A thin platinum wire, but my apparatus was not adequate to produce the incandescence. I made other bulbs, reducing the length of the wire to a small fraction ; still, I did not succ»ed. It then occurred to me that it would be desirable to have the surface of the wire as large cs possible, yet the bulk smull, and I provided a bulb with an exceedingly thin wire of xi bulk about equal to that of the short hut much thicker wire. On turning the current on the bulb the wire was instantly fused. A series of subsequent experiments showed that when the diameter of the wire was exceedingly small, considerably more energy would bo dissipated per unit surface at all degrees of exhaustion than was to be expected, even err the assumption that the energy given off was in proportion to the square of the electric density. There is likewise evidence which, though not possessing the certainty of nn accurate quantitative determination, is nevertheless reliable because it is the result of a great many observations, namely, that with the increase of the density the dissipation is more rapid for thin than for thick wires. The effects notedin exhausted vcsselswith high-frequency currents are merely diminished in degree when the air _is at ordinary pres- sure ; but heating and dissipation occurs, as I have demonstrated, under the ordinary atmospheric conditions. Two very thin wires, attached to the terminals of a high-frequency coil are capable 'of givuig off an appreciable amount of energy. \Vhen the density id very_great, the temperature of the wires may be perceptibly raised, and_1n such case probably the greater portion of the energy which is dissipated owing to the presence of a dticorttiniiaiu medium is transformed into heat at the surface or in close proximity to the wires. Such heating could not occur in a medium assessing either of the two qualities, namely, perfect i.ncompress§\ility or perfect elasticity. In fluid insulators, such as oils, though they are far from being perfectly incompressible or elastic to electric displacement, the heating is much smaller because of the continuity of the fluid. When thc electric density of the wire surfaces is small, there is no appreciable local heating ; nevertheless, energy is dissipated in air by waves, which differ from ordinary sound-waves only because the sir is electrified. _These waves are especially conspicuous when the ,discharges of a powerful battery are directed through a short and thick metal bar, the number of discharges per second being very small. The experimenter may feel the impact of the air at dis- tances of six feet or more from the bar, especially if he takes the precaution to sprinkle the face or hands with cther. These waves cannot be entirely stopped by the interposition of an insulated metal plate. Most of the striking henoniena of mechanical displacement, sound, heat and light wliich have been observed, imply the pre- sence of a medium of a gaseous structure-that is, one consisting of independent carriers capable of free motion. When`a glass (plate is placed near a condenser the charge of which is alternate , the plate emits a sound. This sound is due to the rythmical impact of the air against the plate, I have also found that the ringing of a condenser, first noted by Sir William Thom- son, is'due to the presence of the air between or near the charged surfaces. ` " " ¢ ` When a disruptive discharge coil is immersed in oil contained in a tank, it is observed that the surface of the oil is agitated, This may be thought to be due to the displacements produced in the oil by the chan ing stresses, but such is not the case. It is the air above the oil which is agitated and causes the motion of the latter; the oil itself would remain at rest. The displacements produced i.n it by changing electrostatic stresses are insignificant ; to such stresses it may be said to be compressible to but a very small degree. The action of the air is shown in a curious manner, for if a pointed metal bar is taken in the hand and held with the point close to the oil, a hole two inches deep is formed in the oil by the molecules of the air, which are violently projected from the point. ' The preceding statements may have a general bearing upon investigations in' which currents of high frequency and potential are made use of, but they also have A more directbearing upon the experiments of Prof. Bjerknes which are here considered, namely, lthe -“skin efect," is increased by the action of the sir. Imaginea wire immersed in s medium, the conductivity of which would he some function of the frequency and potential difference, but such that the conductivity increases when either or both of these ele- ,mente are increased. In such a medium, the higher the frequency and potential difference the greater will be the current which will ‘find its way through the surrounding medium, and the smaller the ipart which will pass through ‘the central portion of the wire. In ithe case of a wire immersed in air and traversed by a high-frequency _current, the facility with which the energy is dissipated ma be jconsidered as the equivalent of the conductivity ; and the analogy would be quite complete, were it not that besides the air another _medium is present, the total dissipation being merely modified by ihe presence of the air to an extent as yet not ascertained. Never- theless, I have sufficient evidence to draw the conclusion that the results obtainedby Prof. Bjerknes are effected by the resenoe of air in the following manner :-1. The dissipation of) energy is more rapid when the resonator is immersed in air than it would be ina practically continuous medium, for instance. oil. 2. The dissipation owing to the presence of air renders the diderence between magnetic and non-magnetic metals more striking. The first conclusion follows dieectly`from the preceding remarks; the second follows from the two facts that the resonator receives always the same amount of energy, independent of the nature of the metal, and that the magnetism of the metal increases the im~ pedance of the circuit. A resonator of magnetic metal believes virtually as though its circuit were longer. There is' a greater potential difference set up per unit of length, although this may not show itself in the dedection of the electrometerowing to the lateral dissipation. The effect of the increased' impedance is strikingly illustratedjn the two experiments of Prof. Bjerkneswhen copper-is deposited upon an iron wire, and next iron upon a copper wire. T Considerable thickness of copper deposit was required in the former experiment, but very little thickness of iron in the latter, as should be expected. Taking the above views, I believe, that in the experiments of Prof. Bjerknes, which led him to undoubtedly correct conclusions, the air is a factor fully as important, if not more eo, than the resistance of the metals.