716 



PHYSICS, PROGRESS OF, IN 1890. 



size gave the node at different distances from a 

 reflecting sheet. The intensity of the spark in- 

 creased with the size of the resonator up to a 

 certain point, and then diminished. A vibrator, 

 therefore, sends out not a line spectrum but a 

 band spectrum, whose center is brightest, and 

 the period is that belonging to the middle of the 

 band. In like manner the same resonator indi- 

 cated the node at different places according to 

 the size of the vibrator that is, it responded to 

 the edge of one band for instance, and to the 

 middle of another. In the course of his experi- 

 ments on secondary waves, Trouton ("Philosoph- 

 ical Magazine." March) found that glass absorbs 

 Hertzian vibrations with comparative rapidity, 

 and that their velocity can be measured by plac- 

 ing a sheet of glass in front of a reflector. The 

 stationary waves obtained from a non-conductor 

 differ from those obtained by reflection from a 

 metal. Trowbridge and Sabine, at Harvard Col- 

 lege, by experiments on electric oscillations in 

 air, show that in quick oscillations there is a 

 marked periodicity in the vibrations. It is not 

 so marked in slower oscillations, and in glass it is 

 not noticed. They believe the cause of this pe- 

 riodicity to be analogous to the magnetic pheno- 

 menon of hysteresis (''Annual Cyclopaedia," 1889, 

 page 702). A certain amount of energy is spent in 

 overcoming the dielectric viscosity of air and in 

 straining it, which strain is not immediately re- 

 leased. The periodicity is most marked when the 

 capacity of the condenser bears a certain relation 

 to the time of oscillation that is, when they are 

 " in tune," as it were. The actual transference 

 of electric waves in air does not agree with 

 Hertz's theory, and this is probably the cause. 

 James Moser (Vienna "Berichte," Jan. 9) sur- 

 rounded the conductors in which the vibrations 

 occur with rarefied, spaces, and found that the 

 more nearly perfect vacua exerted no screening 

 action that is, they had become non-conducting. 

 J. Stefan (Vienna " Berichte, " Jan. 9) has inves- 

 tigated vibrations in a straight conductor. He 

 says that in solving questions of vibration we 

 may neglect the resistance, and deduces mathe- 

 matically the principle that the distribution of 

 variable current takes place in such a manner 

 that for any time its electro-dynamic or mag- 

 netic energy is a minimum. In a straight 

 current of circular section, the current can be 

 arranged only symmetrically about the axis. 

 In whatever manner the electric density may 

 vary from the axis toward the surface, the ex- 

 terior magnetic action is as if the entire current 

 were concentrated at the axis. As the energy 

 must have the smallest possible value the actual 

 distribution of the current in the wire is on an 

 infinitely thin surface layer. In the same way 

 it is proved that in a conductor of elliptical sec- 

 tion the density of the electric current of any 

 shot is proportional to the perpendicular dropped 

 from the center to the tangent at that spot. The 

 velocity of electric waves in a conductor depends 

 on the product of the coefficient of self-induction 

 and the capacity. Self-induction is independent 

 of magnetism when the current is thus distribut- 

 ed on the surface : so electric waves of high peri- 

 ods travel in iron wire just as fast as in copper. 

 Hertz explained this by assuming that the mag- 

 netism of iron could not follow such rapid vi- 

 brations. Stefan's explanation is simpler, be- 



ing that the iron is free from any magnetic ac- 

 tion. Hertz had already shown that electric vi- 

 brations of high frequency move only on the 

 surface of conductors, and that electric waves 

 are propagated in thin and thick wires with the 

 same velocity. This in a straight conductor is 

 independent not only of magnitude, but also of 

 the form of the section. The same author (Jan. 

 16) says that induced currents form an essen- 

 tial condition of the reflection of inducing actions, 

 " reflection " from a metal sheet being only the 

 action of currents induced in such sheet. Sev- 

 eral attempts have been made to construct elec- 

 tro-radiation meters. W. G. Gregory (London 

 Physical Society, Nov. 1, 1889) employs a long 

 fine platinum wire fastened to a delicate magnet- 

 ic spring in a tube of glass and brass. Between 

 wire and spring is a small mirror. When the 

 tube is placed parallel to a Hertz oscillator, the 

 mirror indicates an extension of the wire. An 

 elongation of 3iroWq millimetre has been detected, 

 and when the tube is placed four metres from the 

 oscillator the observed extension corresponds to 

 a change of temperature of 0'003 C. C. V. Boys 

 endeavored to construct a meter of two 

 wires, one straight and one shaped like 

 the full line in the figure, which was free 

 to turn on the dotted line as an axis. The 

 electro-dynamic attraction being greater 

 at the middle of the wires and the electro- 

 static repulsion greater at the ends, he 

 thought that the bent wire would be rotated, but 

 he found no result, which shows that the current 

 in each wire must have been less than ^^ ampere. 

 He then tried a Joule dynamic air thermometer, 

 which is a glass tube with a partition so arranged 

 that convection currents due to unequal heating 

 deflect an index. The tube was inclosed in a 

 larger tube that was rotated by clock-work to 

 equalize the temperature. A Hertz resonator 

 being placed in one compartment of the ther- 

 mometer, a large deflection was observed when 

 electro-radiation was directed toward it. The 

 theory of Boys's unsuccessful meter has been 

 worked out mathematically by Prof. O. J. Lodge, 

 who finds that there is a minute force between 

 the wires, and who also deduces other relations 

 between currents and magnets, which require ex- 

 perimental verification. Among these are the 

 following : The action of two given magnets 

 varies inversely as the permeability of the me- 

 dium. That of two currents varies directly as 

 the permeability, and that of a current and a 

 magnet is independent of it. The statical ac- 

 tion of two charges varies inversely as the di- 

 electric constant of the medium. That of a 

 charge that is moving at the speed of light and 

 a magnet is independent of the medium. The 

 dynamic action between two charges at light 

 speed is proportional to the permeability of the 

 medium. 



Photo-electricity. A. Stoletow (" Journal of the 

 Russian Physico-Chemical Society ") thus sums 

 up his researches in this direction : 1. When the 

 rays of a voltaic arc fall on a plate charged with 

 negative electricity it is discharged. 2. The ac- 

 tion is strictly unipolar, positive.electricity not be- 

 ing carried away. 3. The apparent charging of a 

 neutral body by light rays is probably due to 

 this cause. 4. The strongest action is due to 

 rays of the highest refrangibility those wanting 



