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Bible Encyclopedias
Earth Currents
1911 Encyclopedia Britannica
or After the invention of telegraphy it was soon found that telegraph lines in which the 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 English telegraphists, e.g. W. H. Barlow (1) and C. V. Walker (2), who were in charge respectively of the Midland and South-Eastern telegraph systems. Barlow noticed the existence of a more or less regular diurnal variation, and the result - confirmed by all subsequent investigators - that earth currents proper occur in a line only when both ends are earthed. Walker, as the result of general instructions issued to telegraph clerks, collected numerous statistics as to the phenomena during times of large earth currents. His results and those given by Barlow both indicate that the lines to suffer most from earth currents in England have the general direction N.E. to S.W. As Walker points out, it is the direction of the terminal plates relative to one another that is the essential thing. At the same 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 differences in the geological structure of the intervening ground might be of importance. This is a point which seems still somewhat obscure.
Our present knowledge of the subject owes much to practical men, but even in the early days of telegraphy the fact that telegraph systems are commercial undertakings, and cannot allow ' T. Albrecht, Resultate des internat. Breitendienstes, i. and (Berlin, 1903 and 1906); F. Klein and A. Sommerfeld, fi ber 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. Newcomb, " Statement of the Theoretical Laws of the Polar Motion " ( Astronomical Journal, 1898, xix. 158); F. R. Helmert, " Zur Erklarung der beobachteten Breitenanderungen " ( Astr. Nachr. No. 3014); J. Weeder, " The 14-monthly period of the motion of the Pole from determinations of the azimuth of the meridian marks of the Leiden observatory " ( Kon. Ak. van Wetenschappen to Amsterdam, 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 S Cassiopejae mit dem grossen Zenitteleskop " ( Mitteilungen der Nikolai-Hauptsternwarte zu Pulkowo, 1907); J. Larmor and E. H. Hills, " The irregular movement of the Earth's axis of rotation: a contribution towards the analysis of its causes " ( Monthly Notices R.A.S., 1906, lxvii. 22); A. S. Cristie, " The latitude variation Tide " ( Phil. Soc. of.Wash., 1.895, Bull. xiii, 103); H. G. van de Sande Bakhuysen, "Uber die Anderung der Polhohe " ( Astr. Nachr. No. 3261); A. V. Backlund, " Zur Frage nach der Bewegung des Erdpoles " ( Astr. Nachr. No. 3787); R. Schumann, " Uber die Polhehenschwankung " ( Astr. Nachr. No. 3873); " Numerische Untersuchung " ( Ergdnzungshefte zu den Astr. Nachr. No. i I); Weitere Untersuchungen (No. 4142); Bull. astr., 1900, June, report of different theoretical memoirs.
the public to wait the convenience of science, was a serious obstacle to their employment for 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 ordinary duties." Some valuable observations have, however, been made on long telegraph lines where special facilities have been given.
Amongst these may be mentioned the observations on French lines in 1883 described by E. E. Blavier (3), and those on two German lines Berlin-Thorn and Berlin-Dresden during 1884 to 1888 discussed by B. Weinstein (4).
2. Of the experimental lines specially constructed perhaps the best known are the Greenwich lines instituted by Sir G. B.
Airy (5), the lines at Pawlowsk due to H. Wild (6), and those at Parc Saint Maur, near Paris (7).
Experimental Lines
At Greenwich observations were commenced in 1865, but there have been serious disturbances due to artificial currents from electric railways for many years. There are two lines, one to Dartford distant about io m., in a direction somewhat south of east, the other to Croydon distant about 8 m., in a direction west of south.
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 amplitude at the time of the resultant current. The lines laid down at Pawlowsk in 1883 lay nearly in and perpendicular to the geographical - meridian, a distinct desideratum, but were only about 1 km. long. The installation at Parc Saint Maur, discussed by T. Moureaux, calls for 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'Arsonval galvanometer. Light reflected from the galvanometer mirrors falls on photographic paper wound round a drum turned by clockwork, and a continuous 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 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 Daniell cell is put in a circuit including 'coo ohms and the three galvanometers as shunted. If i' be the current recorded, e the E.M.F. of the cell, then e too°+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
240(i/i') for the N.-S. line, and E = o
3 71 (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 wire which do not satisfy this limitation. During thunderstorms surgings may arise, at least in overhead wires, without these being actually struck. Again, if the circuit includes a variable magnetic field, electric currents will be produced independently of any direct source of potential difference. In the third circuit at Parc Saint Maur, where no earth plates exist, the current must be mainly due to changes in the earth's vertical magnetic field, with superposed disturbances due to atmospheric electricity or aerial waves. Even in the other circuits, magnetic and atmospheric influences play some part, and when their contribution is important, the galvanometer deflection has an uncertain value. What a galvanometer records when traversed by a suddenly varying current depends on other things than its mere resistance.
Even when the current is fairly steady, its exact significance is not easily stated. In the first place there is usually an appreciable E.M.F. between a plate and the earth in contact with it, and this E.M.F. may vary with the temperature and the dryness of the soil. Naturally one employs similar plates buried to the same depth at the two ends, but absolute identity and invariability of conditions can hardly be secured. In some cases, in short lines (8), there is reason to fear that plate E.M.F.'s have been responsible for a good 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 io m. apart may mean very different currents travelling through the earth, according to the chemical constitution and condition of the surface strata.
According to 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 cable formed of actual soil, equal in 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, water channels and so on, it is clear that. even the most careful observations convey but a general idea as to the absolute intensity of the currents in the earth itself. In Schuster's 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 Nancy, the one an 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 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 month of the 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 units used in Table I. the mean values of these two " constant " currents were respectively 777 and 559
6. Diurnal Variation. - Probably the most 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. lat. 5° 12' E. long.
Dresden.. I° 28' S. lat. o° 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 =0.1 47 i '+ 0 435 i, and J'=0.9891' - o
10oi, where i and s' are the observed currents in the Thorn-Berlin and Dresden-Berlin lines respectively, both being counted positive when flowing towards Berlin.
It is tacitly assumed that the average earth conductivity is the same between Berlin and Thorn as between Berlin and Dresden. It should also be noticed that local time at Berlin and Thorn differs by fully 20 minutes,while the crests of the diurnal variations in short lines at the two places would probably occur about the same local time. The result is probably a less sharp occurrence of maxima and minima, and a relatively smaller range, than in a short line having the same 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 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 hour of the day, for the year as a whole, and for winter (November to February), equinox (March, April, September, October) and summer (May to TABLE I.
August). There is a marked double period in both the N.-S. and E.-W. currents. In both cases the numerically largest currents occur from To A.M. to noon, the directions then being from north to south and from west to east. The currents tend to die out and change sign about 2 P.M., the numerical magnitude then rising again rapidly to 4 or 5 P.M. The current in the meridian is notably the larger. The numerical values assigned to the resultant current are arithmetic means from the several months composing the season in question.
7. The mean of the 24 hourly numerical values of the resultant current for each month of the year a deducible from Weinstein's data - the unit being the same as before - are given in Table II.
There is thus a conspicuous minimum at 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 Terrestrial magnetism). There is also considerable resemblance between the curve whose ordinates represent the diurnal inequality in the current passing from north to south, and the curve showing the hourly change in the westerly component of the horizontal magnetic force in similar European latitudes.
8. Relations with Sun-spots, Auroras and Magnetic Storms. - Weinstein gives curves representing the mean diurnal inequality for 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 II-year sun-spot period. When we pass to the large and irregular earth currents, which are of practical 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 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 size and rapidity, the accompanying aurora was extraordinarily 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 order of o.5 volts per mile have been recorded by various authorities, e.g. Sir W. H. Preece (10).
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