ELSTERAND GEITEL'S R•SUNI• OF RECENTPAPERS OAr ATMOSPHERIC ELECTRICITY. 1

BY ALEXANDER I•[CADIE.

In this supplement to the Year-book of the Ducal Gymnasium at Wolfenbfittel for •897, ProfessorsElster and Geitel sum up in twenty-four closely printed pages the chief results of the more recent investigations iu atmospheric electricity. Few are more competent to undertake such a task. •Vhile there has been con- siderableactivity displayed in many parts of the world lately in connectionwith atmosphericelectricity, probably nowhere has this activity been more marked than at Wolfenbiittel. We do not for- get the work of ProfessorF. Exner, the occasionalpapers of Lord Kelvin and his co-workers, and the contributions of Schuster, Chree, Andrde, and others. Yet for systematic and comprehensive con- trib.utions to our knowledge of atmospheric electricity, we must give the larger measureof praise to ProfessorsFAster and Geitel. For someyears they have pointed out the proper lines of inquiry, and at the same time have described new methods of observation. Previousto their publication,in •893, of a Report embracing the period trom •86o (when Lord Kelvin gave the subject a great im- petus)to 1892, it was not easyto find a good compendiumof the subject. ATewcontributions were scattered, naturally enough, in differentjournals and reports of scientific bodies,and as for man- uals of Physicsand Meteorolgy, it was but too true, as our authors state, for one to find in these, references only to the work of Frank- lin, Volta, and perhapsDellman. Referringto their publicationof •893, the authors state that many views then thought to be well established,are now in doubt. In the presentpaper they seek to present the newest results, lay- ing stressupon facts rather than theories. The whole subject is treated under three heads' First, the static phenomena connected with watervapor; second, electrical disturbances accompanying precipitation;and third, discharges.Naturally, the divisionsare not sharplymarked. At the end of the paper, the known i•cts in atmosphericelectricity are classifiedas follows' First, thosedeal- • Zusammenstellun der Er ebnisse nearer Arbeiten fiber atmosphiirische Elec- trieitiit. Von J. ]•LSTERund H. GEITEL. I47issenschaftlzcheZiezlage :urn jrahres- bericht des tarerzoglichenGymnasiums zu •k'olfenb•i.llel, •897. Progr. No. 7•6. Wolfenbiittel, •897. =xx=5 cm. PP. =4. •8 .-I Tz•.œOSPHER•rC •L27 CTRICIT •' x29 ing with the electricalfield abovethe precipitationlevel and its variations; second,those associatedwith precipitation;and third, auroraldisplays and similarappearances. The importantquestion of how the earth and atmosphereacquire an electricalcharge is discussedat length; but it must still remain a matter of conjecture whether the earth as a planet has a given negative charge, or whether the free positive electricity of the atmospherebelongs to the lower air strata. Some hold that the seat of the complementary charge of the earth is at a very great distancefrom us. But furtherexperimentation is necessaryto accountfor the greatvaria- bility of the potential,not alonein the irregularperturbations, but alsoin the daily and yearly periods. Otnitting days of rain and snow,it would appearthat in the temperatezones the meandaily value of the potentialis higher in winter than in summer;and there is also a plainly marked diurnal variation. There are no simultaneouschanges as in magneticdisturbances. If observations were made in calm, clear weather, beyond the influence of smoky, dustycities, with stationsnot morethan xoo metres apart, it would be tound that the mean values ot observationsof about five minutes durationwould agree closely. Our authorsinter that the causeof the variation is to be found in the lower air strata. To determine the causeof the daily and yearly period,observations should be madeon the highestmountain peaks. Our .authorsrefer to the observationsmade for them by Peter Lechner at the Sonnblick, 3,ooometres above sea level, from which it wouldseem that the dailyand yearlyvariability is lessin clearweather at thisheight than in the lowlands. The daily mean appearsto be independent of the time of the year. Observationsmade at the summitof Dodabetta,in India, andon the Eiffel Towerconfirm this viewthat the potentialbecomes less variable with increasing height. Obser- vationsof the potentialin the freeair are of the greatestimpor- tance. Referenceis madeto the experimentsof Exnerand Tuma, nearVienna, and the contradictoryexperiments with the balloon Phoenix at Charlottenburg. The views of Ekholm and Arrhenius that the moon and earth arenegatively charged, and that there is a twenty-fivehour period, but not well-defined,in the potentialvalues, are thoughtto be largelyspeculative. Sohncke's experiments showing' that the fric- tionof watervapor and ice particles can cause a markedelectrifica-

ß Zionare treatedwith more consideration,though it is arguedthat meteorologicalconditions frequently occur which would disprove z3o ,4. •œC,/IDI27 [voL.•, Xo.4.]

Sohncke'sviews. Exner's formula showilaga definite relation betweenwater vaporand electrificationis noticedwith the criticism that watervapor, so far as known,does not play the part in the dis- sipationof an electricalcharge which the theoryrequires. Finally, the x-Jewof •rrhenius, modified by the authors themselves,is men- rioned. In this, the active dischargingagency is the ultra-violet radiation. .& formulamay be deducedshowing the relationbetween electrificationand ultra-violet light. If the electrificationof the air is positive,however, the theory falls. It is plain that there cau be no satisfactorytheory as long as the fundamentalquestion whether the air containsfree positive or free negativeelectricity or both in divided strata, remainsin doubt. It is questionableif continued observations at the earth's surface will ever lead to an answer. Precipitationcauses a disturbanceof the electricalpotential, and the constant positive values, found in clear weather, and also in the daily and yearly periods,disappear. Positiveand negative values far in excessof the mean follow one another rapidly, and observa- tions at two neighboring stations show no agreement. lL•ith con- tinued rain or snow, the potential is more even. Maximum values occur during thunderstorms. It must be remembered that the flash of lightning indicates a potential difference between cloud and earth or cloud and cloud, exceeding a certain limiting value. The differencesin electrometer curves during thunder, rain, hail, or snow storms,are differences of degree only. Our authors ad- vance the view tentatively, that generally the front side of a storm just before precipitation occurs,gives high positive values, and.the rear of a storm high negative values, while in between occur irreg- ular fluctuations. The tall of fine snowis frequently accompanied with positivevalues, while large flakes give negative results. With lightning flashesthere are rapid and marked disturbances,doubt- lessconnected xvith inductive effects in the collecting apparatus. Despitethe fluctuations,there is a gradualassertion of the new con- dition. It will sometimeshappen that the electrification of the air and that of the precipitation,will differ. In such a casethere are evidently two differentlycharged fields. Raindrops are in general negativelyelectrified, and large snowflakesare so strongly electri- fied that with a Thomson quadrant electrometer one can plainly note the variation when a flake falls on the collector. Systematic observationsof the electrificationof precipitation promisemany dis- closures which will be of value in future theories as to the elec- tricity of clouds. As to the origin of the electricity of thunder- ..4 T.•'IIOSPHtœRIC ELE C7"]?ICIT :}' storms,our authors refer to the paper by Kollert in Electrotechnische Zeilschrz.'/tfor September,• 889. Referenceis alsomade to Kelvin's recentpapers on the electrificationof air, where it is statedthat a uniformlyelectrified globe of a metre diameterproduces a differ- enceof potential of thirty-eight volts betweenits surfaceand cen- ter; and a globe of a kilometer diameterelectrified to the sameelec- trical density would give a difference of thirty-eight million volts between surface and center. Palmieri's views on the developmentof electricity by the con- densationof water vapor remain as yet unsupportedby experiment. The experimentsof Blake and Kalischerare referredto, andwhile unquestionablygood experiments, it mustbe notedthat the condi- tions of the laboratory are not exactly those of nature. Faraday's experiments,modified by Sohncke,wherein a jet 9f saturatedair underpressure plays upon a pieceof ice belowthe freezingpoint, is' next touchedupon. Lenard has shown that ice will be posi- tively electrifiedby frictionwith water. It is necessary,however, to provethat ice crystalsand waterexist in sufficientquantity. The next questionis that of the electrificationof the air by falling drops. Lenard'sobservation is significantthat the potentialof the air is negativein the neighborhoodof waterfalls,and the more markedthe purerthe water. It is saidthat sparks may be obtained by elevatingrapidly a flame collectorin the vicinityof. a cascade. The seat of the free electricity is not in th'e drops,but in the air which moves. It is evident that the electricity developed in the neighborhoodof waterfallshas not its originin the earth'sfield. The authors mention certain cataractswhich are screenedfrom the earth'sfield, yet act like unscreenedfails in producinga negative electrification. The electrifyingof the air by the sprayfrom ocean wavesis touchedupon, and the experimentsof Kelvinand Exner are cited. Theseways of developingelectricity seem hardly ade- quatehowever. All difficultieswould vanish if we couldprove in the thunderstorm a direct transformationof the mechanicalenergy into electrical energy. Pellat'shypothesis of a cloudcharged in theearth's field, posi- tivelyon its undersurface and negatively on its uppersurface, and the subsequentdivision of the cloud,overlooks the factthat the assumeddevelopment could only occtxr on a conductor,and clouds composedof separate drops do not permit this assumption. (This theorywas in largepart disproven, it seems to us,by Rowlandand Morrillin a paperwhich is probablyunknown to the authors.) Doescloud formationapart from precipitationinfluence the poten- tial? Little canbe saiddefinitely as yet. Oftenthe valuesobserved under a sky covered with cumulus and stratus clouds do not differ from thoseof a clear sky. Ground fog at freezing temperature or low, generally causes a marked increase in the values. Herr Baschenfound, while passingover a cloud in a balloon at a height of 3,7oo metres,a distinct positive indication stronger than while in the free air. This observationdeserves special notice. Concerningthe relation between cosmic phenomena,such as sun spots or solar periods, we can yet consider only that these act indi- rectly. The last division of the paper dealswith St. Elmo's fire and discharges of the brush type. Peter Lechner has for several years recordedthese appearanceson the Sonnblick. It is said that thesedisplays are never noticedunder a clear sky. With regard to ball lightning our authors do not agree with the views of Plant•, but consider the experiments of Righi of more weight. Heat lightning is held to be, for temperate climates, the reflectionof distant lightning so far away that no sound of thunder is audible. With regard to auroras,the observation of the Swedish Polar Expedition is noticed,where a fall in potential even to a negative value was observedduring an aurora. The authors made some ex- perimentson March 3o, •894, at Wolfenbiittel,during magneticdis- turbances, but without success. But even had the weather been favorable,it is doubtful if the electrostaticand electromagnetic fields could have been experimentally correlated. Lemstr•Sm'sexperi- ments are referred to, but we think that these experiments are not now generally accepted. Paulsen'sviews andthe general questions of the relation of aurorasto magnetismare touched upon briefly.