

PHYSICS, PROGRESS OP, IN 1900. 



575 



d between condenser plates, at which electricity If a charged conductor is caused to approach an- 

 first passes. He finds that hydrogen is a worse other conductor, or if the first conductor is 

 insulator than air or carbon dioxide at high charged or increased in potential, the second 

 pressures, but a better one at low pressures, conductor becomes charged with electricity of the 

 Water vapor in itself (apart from deposited films same kind, and with reverse motion the charge is 

 of moisture) insulates perfectly. As between dif- of opposite kind. A conductor set in motion in 

 ferent gases, the critical value of the potential the neighborhood of a second conductor tends to 

 gradient has an absolute term that is nearly the displace itself in the inverse sense of the corn- 

 same for all, and a term proportional to the municated motion. Thus an oscillation of poten- 

 pressure whose coefficient varies from gas to gas; tial transmits itself to a distance very probably 



but it does not seem to be related either to the by inductance and not by influence, 

 mean free path or to the specific inductive ca- Luminous Effects on Conducting Wires. Borg- 

 pacity. The author attributes this discrepancy man (Comptes Rendus, April 30) finds that a 

 to the fact that other experimenters used systems noninsulated metallic wire becomes covered with 

 of rigid recording levers which interpreted mere a luminous aureole when it is inserted in the cir- 

 distortions as alterations in length. In his own cuit of an induction coil containing a spark gap 

 observations he used optical methods. or a vacuum tube. When the wire traverses a 

 Coherence. Harden (Elektrotechnische Zeit- gradually exhausted tube, the luminous phenom- 

 schrift, April 5) has constructed a coherer capable ena show a peculiar development. The aureole de- 

 of observation under the microscope and consist- creases in intensity, and is gradually replaced by 

 ing of two fine adjustable wires. When steel a series of equidistant stars, around which lumi- 

 wires were approached to 0.006 millimetre from nous disks are formed with their planes at right 

 each other, while sparking under the action of angles to the wire. The number of stars and disks 

 electric waves, a dark bridge sprang across be- increases until the latter touch each other and 

 tween them, and then the galvanometer indicated fill the whole tube with a slightly stratified light, 

 a current. Little particles of metal could be ob- The approach of a magnet has the effect of in- 

 served passing from the one wire to the other pre- dining the plane of the rings. Tommasina 

 viously to the formation of the bridge, which (Comptes Rendus, May 28) notes that rhythmical 

 was destroyed by shaking. Guthe and Trowbridge crepitations may be heard along the " antenna " 

 (Physical Review, July), from experiments on or radiating prolongation of one of the branches 

 ball coherers, conclude that the high initial resist- of a Hertzian primary. In the dark, mobile lu- 

 ance of a coherer may be attributed to the pres- minous aigrettes may be seen around the radiat- 

 ence on the surface of the metallic particles of a ing wire, and appear to vibrate synchronously, 

 film (possibly of condensed water vapor) through not with the sparks of the oscillator, but with 

 which electrolytic action occurs. Balls of 3 to 10 the movements of the interrupter of the induction 

 millimetres diameter were used, of steel, lead, coil. 



phosphor-bronze, and copper-plated steel. Bose Photo-electricity. Buisson (Comptes Rendus, 

 (London Royal Society, Aug. 4) finds that con- May 14) finds that sunlight causes a diminution 

 tact sensitiveness to electric radiation differs with in rapidity of discharge from an amalgamated 

 different chemical substances, and is a periodic zinc plate, and that this effect is not permanent, 

 function of the atomic weight. Continued radia- The potential of a plate also varies under the 

 tion produces such a molecular change in the action of sunlight, a large number of metals be- 

 substance that it becomes less sensitive, whether coining more negative after illumination, while 

 the sensitiveness is in the direction of increase or platinum becomes more positive. In general a 

 diminution of resistance. In certain cases this plate has two definite potentials, one in dark- 

 diminution may go so far as to reverse the direc- ness and one in light, and there is an inversion 

 tion of sensitiveness. So-called " fatigue " ap- wave length, such that more refrangible waves 

 pears to be due to the presence of radiation prod- cause a negative variation of potential, and less 



ucts. 



Contact Electro-motive Force. Lodge (Philo- 

 sophical Magazine, May) maintains that contact 



refrangible ones a positive variation. For zinc 

 this wave length is 0.310 ju. 



Electro-photography. Nipher ( Electrical Re- 



force is due to the action of oxygen on the two view, Aug. 10), after exposing a photographic 



metals. It is not necessary, he says, to assume plate to the light of an ordinary room for sev- 



any actual chemical combination, but only an ex- eral days, places it on a glass plate covering a 



tremely minute approach of the oxygen atoms metal plate that is connected to one electrode of 



toward the zinc, and a recession from the copper, an influence machine. A coin is connected to the 



Hall Effect. Moretto (Nuovo Cimento, April) other electrode, and in four to ten minutes an 

 finds that for strengths of field from 3,200 to electro-photograph of the coin is made. Develop- 

 9,600 C. G. S. units the Hall effect is proportional ment is carried out by a moderate light. Schaf- 

 to the strength of field, but that it diminishes fers (Comptes Rendus. April 2) has shown that 

 proportionally to that strength as the strength images may be produced on a sensitive plate by 

 further increases. The effect is approximately an induction coil, using two needles as poles, 

 proportional to the current down to 0.04 am- The positive needle touches the plate, while the 

 |>erc, but is relatively greater for lower intensi- negative is about half a millimetre off the plate, 

 ties. The effect is of the same order of mag- and a sheet of metal below facilitates the action, 

 nitude with discharge currents as with continu- The image consists of small black lines directed 

 ous currents. Marx (Annalen der Physik, Au- from the positive to the negative pole, and ap- 

 gust) has succeeded in finding an appreciable pears to be due at first to the fusion of the sil- 

 Hall effect in gases, which is contrary to the mod- ver bromide and of the medium, and after pro- 

 em ionic theory. He used a flat Bunsen flame, longed action to the reduction of the bromide. 

 The effect in electrolytes he believes will remain Magnetic Field prndiK-ed bii Motion of tin Elec- 

 too small for measurement. trifled Body. Cremieu (Comptes Rendus, June 



Electrostatic Inductance. De Heen (Comptes 5) has repeated Rowland's experiments that indi- 



Rendus, April 17) states that in certain cases a cate that an electrified body in rapid motion 



phenomenon of inductance comparable to that produces a magnetic field in its neighborhood, 



of electro-dynamics appears to be superposed on He has examined the inductive action of a revolv- 



tJie ordinary phenomena of electrostatic influence, ing charged disk upon a neighboring circuit, and 



