On The Dissipation Of Electrical Energy Of The Hertz Resonator

Wednesday, December 21, 1892
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573 THE ELECTRIC ON THE DISSIPATION OF THE ELECTRICAL ENERGY OF THE HERTZ RESONATORJ BY V. BJERKNES. IN introducing conducting wires of different metals into his resonator Hertz asserted that neither the resistance nor the mag- netic properties of the metal involved a sensible change in the length of the secondary spark! We should, therefore, be tempted to conclude that the oscillations are completely imlcp<~n||o|\t of the nature of the mctnl. This imlopnmlence is, howovor, only ap- pirent. In my researches I employed a circular resonator having a diameter of 0.40m., and a complete period (double) of 14 x l0'“ second. The poles took the shape of vertical parallel discs 0.008m. apart. and 0.03m. in diameter. In the field constituted by the gap between these dics a piece of aluminum foil, forming an angle of 45 degrees with the azimuth of the discs, was suspended by a quartz thread as an electrometer needle. Instead of the second~ ary spark the deflection of this needle was observed. In this resonator I employed in succession geometrically ident- ical conducting wires of different metals, and I obtained very dif- ferent dellections. In the diagram Fig. 1, I give the results ob- tained with six wires, all of 0.5mm. diameter, but made of copper (Cu), brass (L), nickel (Ni), iron (Fe), German silver (BI), and plat- inum (Pt) respectively. The resistances of the wires are marked as sbscissae, and the ordinates show the deliections of the electro- mster needle. The results given in this Figure appear at first glance singularly opposed to those obtained by Hertz. I think, however, that this divergence is only due to* the difference of the methods. The oscillations of tho resonator conunence at zero and rapidly tend to a maximum, and gradually decrease afterwards. By the spark we evidently measure the height of this maximum; by the electro- meter we measure the sum of all the oscillations. Owingto damping, this sum may have very different values without materially altering the height of the maximum. Con- sequently, the observations of Hertz show that the resonator cu t M vi tn., ..=._ . 1 ~ .1 Fic. 1. always receives the same amount of electrical energy; the electro- magnetic measurements show that this energy is dissipated more or less quickly, according to the nature of the metal. The capital point is thus the dissipation. The laws which govern it are represented indirectly by the diagram; a small ordi- nate corresponding to a rapid dissipation. It will be seen that the four non-magnetic metals rangcd themselves on a curve having uniformly decreasing onlinatcs, On the other hand, the two magnetic metals, that is to say iron and nickel, are quitc outside the curve, and the most magnetic metal is that which is the fur- thest from the curve. I have, therefore, couie to the following conclusion: » The rapidity with which the electriwl energy nf the resonator is dissipated is increased by the resistance and the mog- netism ofthe conducting wire. This dissipation may take place in two different ways, by radiation across the dielectric medium, or by transformation into heat in the conducting wire. The con- siderable intluence possessed by the physical constants of the metals seems to indicate that the transformation into heat is the princ§pal cause. This transformation takes place in the thin super cial layer along which the electric currents llow. I en- deavored to determine the thickness of this layer with a view of getting to the bottom of the question. To ascertain this, I covered the iron wire with thicker and thicker electrolytic layers of copper, and I found that the deflec~ tions of the electrometer increased, approaching asymptotically to the value of the deflections which took place in the case of the solid copper wire; the difference between the wires disappeared when the layer had a thickness ol’ 0.0lmm. I next covered the copper wire with electrolytic layers of iron; even a layer 0.00U2mn1. thick had an appreciable eEect. The delleotions very quickly ap- proached the value which they had in the case of the solid iron wire, and all difference disappeared at the moment when the layer exceeded 0.003mm. in thickness. Iconclude, therefore that 1. From ths Compu: Rendus, Vol. CXV., No, 19. 2. Hartz Aiubrettung der Elektriachen Kraft, p. 50. AL ENGINEER. lfD°°-14,1392- the currents penetrate less deeply into the magnetic metals than into the non-magnetic metals. This result explains the part played by magnetism in the dis- sipation of electrical energy. The currents being confined to u thin layer, these would encounter a greater resistance, and cou- sequently there would be a greater generation of heat. This ex- planation agrees with the theory already put forward by Lord Rayleigh and Herr Stefan. I may remark in conclusion that the penetration of alternating currents into the metals is a phenom- enon of the same nature as the penetration of light into them. Tho results wl\icl\ I hnve obtnined uro sumclunt to prove that metals are more transparent to luminous uudulntions, which agrees perfectly with theory.