PHYSICS IN 1899. 



711 



transparency is due to chinks or to the waves 

 getting round the metal. With no chinks a thick- 

 ness of Y^ millimetre is as opaque as one of 2 

 millimetres. A chink is much more effective than 

 a series of aperture, so that metallic gauze is 

 nearly as opaque as a plate. The waves turn 

 round obstacles with great facility, owing to their 

 length. This accounts for the apparent partial 

 transparence of metallic mirrors. Nonmetallic 

 substances have been considered to be perfectly 

 transparent, but the author believes that the 

 waves do not come through a hill but around it. 

 He finds that 12 centimetres of Portland cement 

 are partially opaque; 30 centimetres wholly so. 

 Dry sand is almost completely transparent; wet 

 sand much less so. Freestone is much less opaque 

 than cement, but it becomes more so when wet. 

 In all these cases the transparency is greater 

 for Hertzian waves than for light, but it varies 

 from one body to another, decreases as the thick- 

 ness increases, and increases when the bodies are 

 wet. Sommerfield (Wiedemann's Annalen, LXVII, 

 2, p. 233) finds from a mathematical investiga- 

 tion that in metallic conductors, especially heavy 

 ones, the wave near the wire has always very 

 nearly the velocity of light, and the damping is 

 small. But if the wire is very thin or the oscil- 

 lation slow both divergence from light velocity 

 and damping may be considerable. In the first 

 case the process is confined to a surface layer, 

 and the electric force lines are nearly tangential 

 inside the wire and nearly normal without. In 

 the second the lines outside are not even approxi- 

 mately normal. Maclean (American Journal of 

 Science, July), using an elementary type of co- 

 herer suitable for the determination of wave 

 lengths from nodes produced by metallic reflec- 

 tion, finds that the velocity of Hertzian waves is 

 2.991 X 10 10 centimetres per second, or practically 

 the same as along wires. 



Coherence. Sundorph (Wiedemann's Annalen, 

 LXVIII, 3, p. 594) and Tommasina (Complies 

 Rendus, CXXIX, p. 40) report experiments which 

 they regard as supporting the theory that " co- 

 herence " consists of the formation of actual con- 

 ducting chains of particles. The latter finds that 

 the chains are formed less easily in air than in 

 liquid dielectrics. In distilled water he has made 

 a chain 20 centimetres long, the action requiring 

 a little less than one second. The former sup- 

 ports his conclusion by sparking across the gap 

 between two metallic blocks on a glass plate, 

 having between them a layer of nickel or iron 

 filings. Augustus Trowbridge (American Asso- 

 ciation) finds that a certain minimum electro- 

 motive force of from 8 to 10 volts is necessary to 

 break down the resistance of any particular co- 

 herer, and that electrical impulses after the first 

 have practically no beneficial effect. It thus ap- 

 pears unnecessary that the disturbance should be 

 oscillatory, as the first wave would produce all 

 the effect. This observation explains much of 

 the erratic behavior of coherers in experimental 

 use. Branly (Comptes Rendus, CXXVIII, p. 

 1089) finds that a row of balls 1 centimetre in 

 diameter forms a sensitive coherer, steel balls 

 being equal to gold filings in this respect. To 

 obtain uniform effects the shock used for de- 

 cohering must be carefully graduated. Tomma- 

 sina (ibid., p. 1092) has made an electrolytic 

 coherer by inclosing a drop of mercury between 

 cylindrical brass electrodes in a glass tube. Con- 

 ductors coated with electrolytic copper and im- 

 mersed in water act similarly, a chain of particles, 

 probably cupric oxide, joining the two conductors. 

 This chain is often invisible. This same investi- 

 gator (ibid., .p. 1225) has discovered that the 



presence of a magnet immediately destroys the 

 conductivity of the coherer after impact of the 

 electro-magnetic wave. Bose (Proceedings of the 

 Royal Society, LXV, p. IG) finds that all metals 

 show contact sensitiveness toward electric radia- 

 tion, the resistance generally diminishing. Potas- 

 sium, however, and also the allied metals show 

 an increase. Fine silver threads coated electro- 

 lytically with cobalt make very sensitive co- 

 herers. Neugschwender (Wiedemann's Annalen, 

 LXVIII, 1, p. 92) publishes an explanation of the 

 action of a gap between metallic deposits on glass 

 bridged by a film of moisture. This device, which 

 has been called an " anticoherer," shows an in- 

 crease of resistance under the influence of elec- 

 tric waves, but the author observes that this 

 occurs only when the moisture contains a salt 

 that is capable on electrolysis of depositing pure 

 metal. The crystalline deposits so formed are 

 broken up by the sparks that pass when electric 

 waves impinge, thus diminishing the conductiv- 

 ity. On the cessation of the wave the crystals 

 partially cohere again, so that there is a so-called 

 " self-righting " action. 



Contact Breaker. Wehnelt (Elektrotechnisches 

 Zeitschrift, XX, p. 76) finds that a cell consisting 

 of lead plate and a platinum wire inserted into 

 a beaker of dilute sulphuric acid (the wire being 

 inclosed in a glass tube of mercury) acts as an 

 interrupter, with a frequency of about 1,000 per 

 second, giving sparks 40 centimetres long with 6 

 amperes and 12 volts. A condenser is superfluous. 

 D'Arsonval (Comptes Rendus, CXXVIII, p. 529) 

 extends these results. With 70 volts he obtained 

 sparks forming a continuous stream of fire 25 

 centimetres long and as thick as a lead pencil, 

 the frequency being 1,700 per second. The new 

 interrupter gave results ten times as strong as 

 the Foucault interrupter, and acted also as a 

 current separator. It gives regular and powerful 

 Hertzian waves, and can therefore be used in 

 wireless telegraphy. This Wehnelt interrupter 

 has been discussed by a large number of writers. 

 Kallir and Eichberg (Zeitschrift fiir Elektrotech- 

 nologie, XVII, p. 184) find from stroboscopic 

 methods that the time interval of interruption 

 is of the order of 0.0005 second. By using the 

 platinum point first as anode and then as cathode 

 the time interval with a point anode is found 

 to be about double that corresponding to a point 

 cathode. Simon (Wiedemann's Annalen, LXVIII, 

 2, p. 273) publishes a complete theory of the in- 

 terrupter, according to which it is not electro- 

 lytic at all but "Joulean," there being no elec- 

 trolysis, though possibly disassociation by heat. 

 Spies and Wehnelt (German Physical Society, I, 

 p. 53) find that if the applied volts are increased 

 the current increases, the rate of interruption in- 

 creases, and the current density increases; while 

 if the anode surface is increased the correspond- 

 ing quantities are respectively increased, dimin- 

 ished, and diminished; and if the self-induction 

 of the circuit is increased they are all diminished. 

 Humphreys (Physical Review, July) suggests 

 that the action of the Wehnelt interrupter depends 

 upon electrical discharges. The current first in- 

 creases, on account of self-induction, until the 

 electrode is sheathed by a layer of gas, chiefly 

 oxygen, which abruptly breaks the circuit. Owing 

 to the self-induction, there is now an electrical 

 discharge across the layer, giving rise to the 

 glow, and decomposing the liquid. The sudden 

 freeing of these gases and the heat of the dis- 

 charge together produce an explosive effect; when 

 this is over the liquid falls back to its place, 

 and the action is continued. Voller and Walter 

 (Wiedemann's Annalen, LXVIII, 3, p. 526) find 



