622 



PHYSICS, PROGRESS OP, IN 1893. 



Thermo-electricity. Henri Bagard, of Nancy 

 (Paris Academy of Science, Jan. 2), has measured 

 the thermo-electric force between parts of the 

 same electrolyte in different stages of dilution, 

 the parts being separated by a diaphragm of 

 goldbeater's skin. With a 5-per-cent. solution 

 and one of 45 per cent, the difference of po- 

 tential varied from 78 at a temperature of 17'9 

 to 155 at 73-5, the unit being O0001 of a Daniell 

 cell. Three couples respectively of 5 and 25 

 per cent., 25 and 45, and 45 and 5 gave no de- 

 flection, showing that the law of intermediate 

 bodies is fulfilled. 



Alternating Currents and Discharqes. John 

 Trowbridge (" American Journal of Science." 

 September) has photographed the discharge of 

 a current having 300 to 400 alternations per sec- 

 ond, using a rotating mirror. The interval be- 

 tween the oscillations of the discharge was 

 0-00001 second. On each photograph 10 to 12 

 oscillations could be counted ; after that there 

 was a brush discharge. On intercalating a non- 

 inductive water resistance and a vacuum tube, 

 effects similar to those of the aurora were obr 

 served (see below). A. A. C. Swinton (" Philo- 

 sophical Magazine," September) attached to one 

 electrode of an alternating circuit of high fre- 

 quency and potential a flat tin dish containing 

 a half-inch layer of resin oil, and to the other 

 a wire with its extremity 3 or 4 inches above the 

 oil. The oil bubbled and foamed violently and 

 crept up the sides of the dish, probably from 

 -the repulsion of air particles. Water did not 

 act in this way, but showed an octopuslike 

 branched spark. I. Pupin (ibid., April) obtains 

 an alternating current of constant and easily de- 

 termined frequency by means of a transformer 

 whose principal circuit has for an interrupter a 

 vibrating wire stretched between the pole pieces 

 of two permanent horseshoe magnets, and having 

 at its center a short wire dipping in a mercury 

 cup. The tension is altered till its note is in 

 unison with that of a fork of known pitch. 



Brush Discharge. Harvey and Hirsch (" Phil- 

 osophical Magazine," July) find that in air and 

 in oxygen positive electricity passes in brush 

 discharge from a point to a* neighboring, con- 

 ductor more readily than negative, while in hy- 

 drogen the opposite is true. 



Electro-magnetic Radiation and Oscillation. 

 Augusto Righi (Academy of the Lincei) finds that 

 ebonite, paraffin, and rock salt are perfectly trans- 

 parent to electro-magnetic waves. Mica 1*7 mil- 

 limetre thick absorbs 10 per cent. ; glass 8 milli- 

 metres thick, 37 per cent. ; and quartz (normal 

 to the axis) 8 millimetres thick, 40 per cent. 

 Reflection from paraffin indicated that the plane 

 of polarization was at right angles to that of the 

 electric force ; from metal, that it was parallel. 

 The refractive index of a paraffin prism was 

 found to be 1-4. Bjerknes (Wiedemann's "An- 

 nalen," No. 9, 1892) finds that different metals 

 have different absorptive powers for electric 

 waves, increasing with electric resistance and 

 magnetization. Iron and nickel show a marked 

 damping effect. Sarasin and De la Rive (" Ar- 

 chives des Sciences Physiques et Naturelles") 

 find that a circular resonator has a constant 

 wave length to which it responds whatever be 

 the dimensions of the oscillation ; the strength 

 of the induced spark only varies, attaining a 



maximum for a certain length of oscillation. 

 One half the wave length of such a resonator is 

 equal to twice its diameter. They find also that 

 in normal reflection from a metal mirror the 

 first node is exactly on the surface. T. II. 

 Blakesley(" Philosophical Magazine," May) shows 

 that the ordinary equations o.f the discharge of 

 a Leyden jar do not account for radiation, the 

 assumption being that all the energy is expended 

 in heating the circuit. He finds that the work , 

 is divided between the circuit and the field in 

 the ratio R : \, where R is the resistance and \ 

 a quantity (constant or variable) depending, 

 among other things, on the frequency. Thus, 

 as the frequency increases, more and more en- 

 ergy is expended in the field. Janet ("Journal 

 de Physique," August) finds that in a condenser 

 with solid dielectrics, under the influence of 

 rapid electric oscillation there is a lagging of 

 the charges behind the differences of potential, 

 or, in other words, for equal differences of po- 

 tential the charges are smaller with increasing 

 than with decreasing potential. R. Colson 

 (I'aris Academy of Science, Nov. 14) has experi- 

 mented with a Ruhmkorff coil vibrating 130 

 times a second, with one terminal of which was 

 connected a copper wire ending in a hook to 

 which was suspended a linen thread 3 metres 

 long, soaked in copper chloride, the lower end 

 hanging free. A telephone connected with the 

 thread ceased to give any sound at certain 

 points. When both ends of the thread were put 

 in connection with the terminal by fine copper 

 wires two nodes appeared. When the thread 

 was shortened these coalesced, forming a zone 

 of extinction, which spread over the copper 

 wires as the thread approached zero. This was 

 apparently due to the interference of two waves 

 meeting from opposite directions. G. H. Barton 

 (British Association) finds that electro-magnetic 

 waves in wires can be made to exhibit phenome- 

 na analogous to those of Newton's rings. Lud- 

 wig Boltzmann (Wiedemann's " Annalen," No. 1) 

 removes an apparent contradiction between 

 Maxwell's theory and Hertz's observations that 

 even excessively thin metal plates do not trans- 

 mit waves a few decimetres long, by showing 

 that this is due to limiting conditions at the 

 surface, which are deducible from Maxwell's 

 formulae. Patterson and Arnold ("American 

 Journal of Science," November) call attention to 

 a common ambiguity in stating the frequency of 

 an alternating current, consisting in confusing 

 the frequency of impulse with the frequency of 

 oscillation. "The latter depends on self-induc- 

 tion and capacity, and while the number of im- 

 pulses per second is the same as in the primary 

 coil, the number of oscillations may be greater 

 or less. Nikola Tesla, in a lecture (Franklin Insti- 

 tute, Feb. 24) states that single- wire transmission 

 is now practicable for a high electro-motive force 

 and great frequency. Transmission without 

 wires was also discussed, and, among the con- 

 ditions, the possibility of ascertaining the earth's 

 capacity. This could be done, the lecturer said, 

 if we could find the period of oscillation of the 

 earth's charge. If the earth were perfectly in- 

 sulated its capacity would probably be less than 

 a farad. 



ITie Hall Effect. Basset ("Nature," May 11), 

 in a review of recent work, says that there seems 



