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EARTH CURRENTS
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See also:EARTH CURRENTS . After the invention of telegraphy it was soon found that See also:telegraph lines in which the See also:circuit is completed by the earth are traversed by natural electric currents
which occasionally interfere seriously with their use, and which are known as " earth currents."
1. Amongst the pioneers in investigating the subject were several See also:English telegraphists, e.g. W. H. See also:Barlow (1) and C. V. See also: At the same See also:time he noticed that whilst at any given instant the currents in parallel lines have with rare exceptions the same direction, some lines show normally stronger currents than others, and he suggested that See also:differences in the See also:geological structure of the intervening ground might be of importance. This is a point which seems still somewhat obscure. Our See also:present knowledge of the subject owes much to See also:practical men, but even in the See also:early days of telegraphy the fact that telegraph systems are commercial undertakings, and cannot allow h T. Albrecht, Resultate See also:des internat. Breitendienstes, i. and ii. (See also:Berlin, 1903 and 1906); F. See also:Klein and A. See also:Sommerfeld, Uber See also:die Theorie des Kreisels, iii. p. 672; R. Spitaler, " Die periodischen Luftmassenverschiebungen and ihr Einfluss auf die Lagenanderung der Erdaxe " (Petermanns Mitteilungen, Erganzungsheft, 137) ; S. New-See also:comb, " Statement of the Theoretical See also:Laws of the Polar See also:Motion " (Astronomical See also:Journal, 1898, xix. 158) ; F. R. Helmert, " Zur Erklarung der beobachteten Breitenanderungen " (See also:Asir. Nachr. No. 3014) ; J. Weeder, " The 14-monthly See also:period of the motion of the See also:Pole from determinations of the See also:azimuth of the See also:meridian marks of the See also:Leiden See also:observatory" (Kon. Ak. See also:van Wetenschappen to Amster-See also:dam, 1900) ; A. Sokolof, " Determination du mouvement du pole terr. au moyen des mires meridiennes de Poulkovo " (Mel. math. et astr. vii., 1894); J. Bonsdorff, " Beobachtungen von 3 Cassiopejae mit dem grossen Zenitteleskop " (Mitteilungen der Nikolai-Hauptsternwarte zu Pulkowo, 1907) ; J. Larmor and E. H. Hills, " The irregular See also:movement of the Earth's See also:axis of rotation : a contribution towards the See also:analysis of its causes " (Monthly Notices R.A.S., 1906, l(vii. 22) ; A. S. Cristie, " The See also:latitude variation See also:Tide " (Phil. See also:Soc. of.See also:Wash., 1895, See also:Bull. xiii, 103) ; H. G. van de Sande Bakhuysen, "Uber die Anderung der Polhohe " (Asir. Nachr. No. 3261) ; A. V. Backlund, " Zur Frage nach der Bewegung des Erdpoles " (Asir. Nachr. No. 3787) ; R. See also:Schumann, Uber die Polhi henschwankung " (Asir. Nachr. No. 3873) ; ' Numerische Untersuchung " (Erganzungshefte zu den Astr. Nachr. No. 11); Weitere Untersuchungen (No. 4142) ; Bull. astr., 1900, See also:June, See also:report of different theoretical See also:memoirs. , the public to wait the convenience of See also:science, was a serious obstacle to their employment for See also:research. Thus Walker feelingly says, when regretting his paucity of data during a notable earth current disturbance : " Our clerks were at their wits' end to clear off the telegrams. . . At a time when observations would have been very highly acceptable they were too much occupied with their See also:ordinary duties." Some valuable observations have, however, been made on See also:long telegraph lines where See also:special facilities have been given. Amongst these may be mentioned the observations on See also:French lines in 1883 described by E. E. Blavier (3), and those on two See also:German lines Berlin-See also:Thorn and Berlin-See also:Dresden during 1884 to 1888 discussed by B. Weinstein (4). 2. Of the experimental lines specially constructed perhaps the best known are the See also:Greenwich lines instituted by See also:Sir G. B. See also:Airy (5), the lines at Pawlowsk due to H. See also:Wild (6), and those at Parc See also:Saint Maur, near See also:Paris (7). Experimental Lines.—At Greenwich observations were commenced in 1865, but there have been serious disturbances due to artificial currents from electric See also:railways for many years. There are two lines, one to See also:Dartford distant about 10 m., in a direction somewhat south of See also:east, the other to See also:Croydon distant about 8 m., in a direction See also:west of south. See also:Information from a single line is incomplete, and unless this is clearly understood erroneous ideas may be derived. The times at which the current is largest and least, or when it vanishes, in an east-west line, tell nothing directly as to the See also:amplitude at the time of the resultant current. The lines laid down at Pawlowsk in 1883 See also:lay nearly in and perpendicular to the See also:geographical meridian, a distinct desideratum, but were only about 1 km. long. The See also:installation at Parc Saint Maur, discussed by T. Moureaux, calls for See also:fuller description. There are three lines, one having terminal earth plates 14.8 km. apart in the geographical meridian, a second having its earth plates due east and west of one another, also 14.8 km. apart, and the third forming a closed circuit wholly insulated from the ground. In each of the three lines is a Deprez d'See also:Arsonval See also:galvanometer. See also:Light reflected from the galvanometer mirrors falls on photographic See also:paper See also:wound See also:round a See also:drum turned by clockwork, and a continuous See also:record is thus obtained. 3. Each galvanometer has a resistance of about 200 ohms, but is shunted by a resistance of only 2 ohms. The See also:total effective resistances in the N.-S. and E.-W. lines are 225 and 348 ohms respectively. If i is the current recorded, L, g and s the resistances of the line, galvanometer and shunt respectively, then E, the difference of potential between the two earth plates, is given by E=i(1+g/s) {L+gs/(g+s)}. To' calibrate the record, a See also:Daniell See also:cell is put in a circuit including l000 ohms and the three galvanometers as shunted. If i' be the current recorded, e the E.M.F. of the cell, then e =a (1+g/s) { t000+3gs/(g+s) }. Under the conditions at Parc Saint Maur we may write 2 for gs/(g + s), and 1.072 for e, and thence we have approximately E=o.24o(i/i') for the N.-S. line, and E=0.371(i/i') for the E.-W. line.
The method of standardization assumes a potential difference between earth plates which varies slowly enough to produce a practically steady current. There are several causes producing currents in a telegraph See also:wire which do not satisfy this See also:limitation. During thunderstorms surgings may arise, at least in overhead wires, without these being actually struck. Again, if the circuit includes a variable magnetic See also: Naturally one employs similar plates buried to the same See also:depth at the two ends, but See also:absolute identity and invariability of conditions can hardly be secured. In some cases, in See also:short lines (8), there is See also:reason to fear that plate E.M.F.'s have been responsible for a See also:good See also:deal that has been ascribed to true earth currents. With deep earth plates, in dry ground, this source of uncertainty can, however, enter but little into the diurnal inequality. 4. Another difficulty is the question of the resistance in the earth itself. A given E.M.F. between plates to m. apart may mean very different currents travelling through the earth, according to the chemical constitution and See also:condition of the See also:surface strata. According to See also:Professor A. Schuster (9), if p and p' be the specific resistances of the material of the wire and of the soil, the current i which would pass along an underground See also:cable formed of actual soil, equal in See also:diameter to the wire connecting the plates, is given by i= i'p/p', where i' is the observed current in the wire. As p' will vary with the depth, and be different at different places along the route, while discontinuities may arise from geological faults, See also:water channels and so on, it is clear that even the most careful observations convey but a general See also:idea as to the absolute intensity of the currents in the earth itself. In Schuster's See also:formula, as in the formulae deduced for Parc Saint Maur, it is regarded as immaterial whether the wire connecting the plates is above or below ground. This view is in accordance with records obtained by Blavier (3) from two lines between Paris and See also:Nancy, the one an See also:air line, the other underground. 5. The earliest quantitative results for the regular diurnal changes in earth currents are probably those deduced by Airy (5) from the records at Greenwich between 1865 and 1867. Airy resolved the observed currents from the two Greenwich lines in and perpendicular to the magnetic meridian (then about 21° to the west of astronomical See also:north). The information given by Airy as. to the precise meaning of the quantities he terms " magnetic tendency " to north and to west is somewhat scanty, but we are unlikely to be much wrong in accepting his figures as proportional to the earth currents from magnetic east to west and from magnetic north to south respectively. Airy gives mean hourly values for each See also:month of the See also:year. The corresponding mean diurnal inequality for the whole year appears in Table I., the unit being arbitrary. In every month the algebraic mean of the 24 hourly values represented a current from north to south in the magnetic meridian, and from east to west in the perpendicular direction; in the same arbitrary See also:units used in Table I. the mean values of these two " See also:constant " currents were respectively 777 and 559• 6. Diurnal Variation.—Probably the most See also:complete records of diurnal variation are those discussed by Weinstein (4), which depend on several years' records on lines from Berlin to Dresden and to Thorn. Relative to Berlin the geographical co-ordinates of the other two places are: Thorn . . o° 29' N. See also:lat. 5° 12' E. long. Dresden . . 1 ° 28' S. lat. 0° 21' E. long. Thus the Berlin-Dresden line was directed about 82° east of south, and the Berlin-Thorn line somewhat more to the north of east. The latter line had a length about 2.18 times that of the former. The resistances in the two lines were made the same, so if we suppose the difference of potential between earth plates along a given direction to vary as their distance apart, the current observed in the Thorn-Berlin line has to be divided by 2.18 to be comparable with the other. In this way, resolving along and perpendicular to the geographical meridian, Weinstein gives as proportional to the earth currents from east to west and from south to north respectively J=o•147i'+0.4351, and J'=o•989i'—o• too i, 2 where i and i' are the observed currents in the Thorn-Berlin and Dresden-Berlin lines respectively, both being counted See also:positive when flowing towards Berlin. It. is tacitly assumed that the See also:average earth conductivity is the same between Berlin and Thorn as between Berlin and Dresden. It should also be noticed that See also:local time at Berlin and Thorn differs by fully 20 minutes,while the crests of the diurnal See also:variations in short lines at the two places would probably occur about the same local time. The result is probably a less See also:sharp occurrence of See also:maxima and minima, and a relatively smaller range, than in a short line having the same See also:orientation. It was found that the average current derived from a number of undisturbed days on either line might be regarded as made up of a " constant part " plus a regular diurnal inequality, the constant part representing the algebraic mean value of the 24 hourly readings. In both lines the constant part showed a decided alteration during the third year—changing sign in one line—in consequence, it is believed, of alterations made in the earth plates. The constant part was regarded as a plate effect, and was omitted from further See also:consideration. Table I. shows in terms of an arbitrary unit—whose relation to that employed for Greenwich data is unknown—the diurnal inequality in the currents along the two lines, and the inequalities thence calculated for ideal lines in and perpendicular to the geographical meridian. Currents are regarded as positive when directed from Berlin to Dresden and from north to south, the opposite point of view to that adopted by Weinstein. The table also shows the mean numerical value of the resultant current (the " constant " part being omitted) for each See also:hour of the See also:day, for the year as a whole, and for See also:winter (See also:November to See also:February), See also:equinox (See also: Nov. Dec. 93 328 313 314 337 300 258 235 165 132 There is thus a conspicuous minimum at See also:mid-winter, and but little difference between the monthly means from April to August. This is closely analogous to what is seen in the daily range of the magnetic elements in similar latitudes (see See also:MAGNETISM, TERRESTRIAL). There is also considerable resemblance between the See also:curve whose ordinates represent the diurnal inequality in the current passing from north to south, and the curve showing the hourly change in the See also:westerly component of the See also:horizontal magnetic force in similar See also:European latitudes. 8. Relations with See also:Sun-spots, Auroras and Magnetic Storms.—Weinstein gives curves representing the mean diurnal inequality for See also:separate years. In both lines the diurnal amplitudes were notably smaller in the later years which were near sun-spot minimum. This raises a presumption that the regular diurnal earth currents, like the ranges of the magnetic elements, follow the 11-year sun-spot period. When we pass to the large and irregular earth currents, which are of practical See also:interest in telegraphy, there is every reason to suppose that the sun-spot period applies. These currents are always accompanied by magnetic disturbances, and when specially striking by brilliant See also:aurora. One most conspicuous example of this occurred in the end of August and beginning of September 1859. The magnetic disturbances recorded were of almost unexampled See also:size and rapidity, the accompanying aurora was extra-ordinarily brilliant, and E.M.F.'s of 700 and Boo volts are said to have been reached on telegraph lines 500 to 600 km. long. It is doubtful whether. the disturbances of 1859 have been equalled since, but earth current voltages of the See also:order of o.5 volts per mile have been recorded by various authorities, e.g. Sir W. H. Preece (10). It was the practice for several years to publish in the See also:Ann. du See also:bureau central meteorologique synchronous magnetic and earth current curves from Parc Saint Maur corresponding to the See also:chief disturbances of the year. In most cases there is a marked similarity between the curve of magnetic See also:declination and that of the north-south earth current. At times there is also a distinct resemblance between the horizontal force magnetic curve and that of the east-west earth current, but exceptions to this are not infrequent. Similar phenomena appear in synchronous Greenwich records published by Airy in 1868; these show a See also:close accordance between the horizontal force curves and those of the currents from magnetic east to west. Originally it was supposed by Airy that whilst rapid movements in the declination and north-south current curves sometimes an. 152 Feb. 2II Ma Mean Diurnal Inequalities for the year. Numerical Values of resultant current. Greenwich. Thorn-Berlin-Dresden. Thorn-Berlin-Dresden. North East North East Mean hourly values from Hour. to to Berlin Thorn to to Year. Winter. I Equinox. Summer. South West Dresto to South West den. Berlin. (Mag.) (Mag.) (See also:Ast.) (Ast.) -94 -41 -17 -13 -2o —to 81 94 51 98 2 -68 -24 -6 -13 -9 —II 84 I15 39 97 3 -44 -8 -1 —I -1 —I 84 I13 31 Io8 4 -18 +9 -20 +15 -17 +17 101 94 58 127 5 — 3o -1 — 79 +21 — 74 +32 122 58 78 23o 6 -63 -33 -139 +5 -136 +26 148 8o 139 225 7 -121 -8o -138 -36 -144 -14 166 155 206 136 8 -175 -123 -7 -98 -28 -92 203 152 185 271 9 -156 -137 +249 -156 +212 -184 305 67 272 575 10 -43 -77 +540 -184 +494 -254 557 232 628 811 II +82 +1 +722 -165 +678 -263 728 411 885 887 Noon +207 +66 +673 -107 +642 -200 675 441 848 735 1 +245 +94 +404 -20 +395 -79 400 284 510 406 2 +205 +113 +35 +55 +46 +47 98 68 Io3 125 3 +153 +97 -261 +99 -237 +132 272 136 355 324 4 +159 +108 -397 +114 -368 +167 404 218 503 492 5 -f 167 +118 -391 +108 -363 +16o 397 206 453 532 6 +125 +95 -311 +96 -287 +137 319 176 333 446 7 +43 +55 -237 +85 -216 +115 247 18o 250 312 8 — 22 +4 -191 +74 -173 +98 201 207 217 181 9 -115 -49 -168 +59 -153 +81 174 208 194 120 10 -138 - 74 -135 +4o -125 +58 138 155 149 I I I I I -136 — 70 — 84 + 18 — 79 +29 89 64 95 107 Midnight -147 -8o -43 -2 -43 +4 91 42 119 III — 59 — 56 occurred simultaneously, there was a distinct tendency for the latter to precede the former. More See also:recent See also:examinations of the Greenwich records by W. See also:Ellis (11), and of the Parc St Maur curves by Moureaux, have not confirmed this result, and it is now believed that the two phenomena are practically simultaneous. There has also been a conflict of views as to the connexion between magnetic and earth current disturbances. Airy's observations tended to suggest that the earth current was the See also:primary cause, and the magnetic disturbance in considerable part at least its effect. Others, on the contrary, have supposed earth currents to be a direct effect of changes in the earth's magnetic field. The prevailing view now is that both the magnetic and the earth current disturbances are due to electric currents in the upper See also:atmosphere, these upper currents becoming visible at times as aurora. 9. There seems some See also:evidence that earth currents can be called into existence by purely local causes, notably difference of level. Thus K. A. See also:Brander (12) has observed a current flowing constantly for a good many days from Airolo (height 116o metres) to the See also:Hospice St Gotthard (height 2094 metres). In an 8-km. line from Resina to the See also:top of See also:Vesuvius L. Palmieri (13)—observing in 1889 at three-hour intervals from 9 A.M. to 9 P.M.—always found a current See also:running uphill so long as the See also:mountain was quiet. On a long line from See also:Vienna to See also:Graz A. Baumgartner (14) found that the current generally flowed from both ends towards intervening higher ground during the day, but in the opposite directions at See also:night. During a fortnight in September and October 1885 hourly readings were taken of the current in the telegraph cable from Fort-See also: io. Artificial Currents.—The See also:great See also:extension in the applications of electricity to See also:lighting, See also:traction and See also:power transmission, characteristic of the end of the 19th See also:century, has led to the existence of large artificial earth currents, which exert a disturbing See also:influence on galvanometers and magnetic See also:instruments, and also tend to destroy See also:metal pipes. In the former See also:case, whilst the disturbance is generally loosely assigned to stray or " vagabond " earth currents, this is only partly correct. The currents used for traction are large, and even if there were a perfectly insulated return there would be a considerable resultant magnetic field at distances from the track which were not largely in excess of the distance apart of the direct and return currents(16). At a distance of See also:half a mile or more from an electric tram line the disturbance is usually largest in magnetographs recording the vertical component of the earth's field. The magnets are slightly displaced from the position they would occupy it undisturbed, and are kept in continuous oscillation whilst the trams are running (17). The extent of the oscillation depends on the damping of the magnets. The distance from an electric tram line where the disturbance ceases to be See also:felt varies with the See also:system adopted. It also depends on the length of the line and its subdivision into sections, on the strength of the currents supplied, the amount of leakage, the See also:absence or presence of " boosters," and finally on the sensitive- ness of the magnetic instruments. At the U.S. See also:Coast and See also:Geodetic Survey's observatory at See also:Cheltenham the effect of the See also:Washington electric trams has been detected by highly sensitive magnetographs, though the nearest point of the line is 12 m. away (18). Amongst the magnetic observatories which have suffered severely from this cause are those at See also:Toronto, Washing- ton (See also:Naval Observatory), See also:Kew, Paris (Parc St Maur), See also:Perpignan, See also:Nice, See also:Lisbon, Vienna, See also:Rome, Bombay (Colaba) and See also:Batavia. In some cases magnetic observations have been wholly suspended, in others new observatories have been built on more remote sites. As regards damage to underground pipes, mainly See also:gas and water pipes, numerous observations have been made, especially in Germany and the See also:United States. When electric tramways have uninsulated returns, and the potential of the rails is allowed to differ considerably from that of the earth, very considerable currents are found in neighbouring pipes. Under these condi- tions, if the See also:joints between contiguous pipes forming a See also:main present appreciable resistance, whilst the surrounding earth through moisture or any other cause is a See also:fair conductor, current passes locally from the pipes to the earth causing electrolytic corrosion of the pipes. Owing to the diversity of interests concerned, the extent of the damage thus caused has been very variously estimated. In some instances it has been so consider- able as to be the alleged cause of the ultimate failure of water pipes to stand the pressure they are I A.M. P.M. Hour -2I +24 8 -32 -9 7 +25 +5o 9 o 12 1- 62 116 +6 exposed to. - 37 -28 I -34 BIBLIOGRAPHY.—See Svante August See also:Arrhenius, Lehrbuch der kosmischen Physik (See also:Leipzig, 1903), PP. 984-990. For lists of references see J. E. Burbank, Terrestrial Magnetism, vol. to (1905), p. 23, and P. Bachmetjew (8). For papers descriptive of corrosion of pipes, &c., by artificial currents see Science Abstracts (in recent years in the volumes devoted to See also:engineering) under the heading " Traction, Electric; See also:Electrolysis." The following are the references in the See also:text:—(1) Phil. Trans. R.S. for 1849, pt. i. p. 61; (2) Phil. Trans. R.S. vol. 151 (1861), p. 89, and vol. 152 (1862), p. 203; (3) Etude des courants telluriques (Paris, 1884) ; (4) Die Erdstrome See also:im deutschen Reichstelegraphengebiet (Braunschweig, 1900) ; (5) Phil. Trans. R.S. vol. 158 (1868), p. 465, and vol. 16o (187o), p. 215; (6) Mein. de l'Academie St-Petersbourg, t. 31, No. 12 (1883); (7) T. Moureaux, Ann. du Bureau Central Met. (Annee 1893), I Mem. p. B 23; (8) P. Bachmetjew, See also:Mina de l'Academie St-Petersbourg, vol. 12, No. 3 (1901) ; (9) Terrestrial Magnetism, vol. 3 (1898), p. 13o; (10) Journal Tel. See also:Engineers (1881); (11) Proc. R.S. vol. 52 (1892), p. 191; (12) Akad. Abhandlung (See also:Helsingfors, 1888) ; (13) Acad. Napoli Rend. (189o), and Atti (1894, 1895); (14) Fogg. Ann. vol. 76, p. 135; (15) Proc. R.S.E. vol. 13, p. 530; (16) A. Rucker, Phil. Mag. I (1901), p. 423, and R. T. Glazebrook, ibid. p. 432; (17) J. Edler, Elektrotech. Zeit. vol. 20 (1899) ; (18) L. A. See also:Bauer, Terrestrial Magnetism, vol. I I (1906), p. 53. (C. CH.)
EARTH-See also:NUT, the English name for a plant known botanically as Conopodium denudatum (or Bunium flexuosum), a member of the natural order See also:Umbelliferae, which has a See also: See also:Boswell See also:Syme, in English See also:Botany, iv. 114, says: " The See also:common names of this plant in England are various. It is known as earth-nut, See also:pig-nut, ar-nut, See also:kipper-nut, See also:hawk-nut, See also:jar-nut, earth-chestnut and ground-nut. Though really excellent in See also:taste and unobjectionable as See also:food, it is disregarded in England by all but pigs and See also:children, both of whom appreciate it and seek eagerly for it." Dr Withering de-See also:scribes the roots as little inferior to chestnuts. In See also: Additional information and CommentsThere are no comments yet for this article.
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