Earth Currents
EARTH CURRENTS. 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 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 10 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 1000 ohms and the three galvanometers as shunted. If i' be the current recorded, e the E.M.F. of the cell, then e = i' (1 + g/s) {1000 + 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 = 0.240 (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 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 10 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 ρ and ρ' 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'ρ / ρ', where i' is the observed current in the wire. As ρ' 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 1., 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 0° 29' N. lat. 5° 12' E. long.
Dresden 1° 28' S. lat. 0° 21' E. long.
Thus the Berlin-Dresden line was directed about 8° 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 = 0.147i' + 0.435i, and J' = 0.989i' − 0.100i,
where i and i' 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.
Table I.
Mean Diurnal Inequalities for the year. Numerical Values of resultant
current.
Greenwich. Thorn-Berlin-Dresden. Thorn-Berlin-Dresden.
Hour. North
to
South
(Mag.) East
to
West
(Mag.) Berlin
to
Dresden. Thorn
to
Berlin. North
to
South
(Ast.)
East
to
West
(Ast.) Mean hourly values from
Year. Winter. Equinox. Summer.
1 −94 −41 −17 −13 −20 −10 81 94 51 98
2 −68 −24 −6 −13 −9 −11 84 115 39 97
3
−44 −8 −1 −1 −1 −1 84 113 31 108
4 −18 +9 −20 +15 −17 +17 101 94 58 127
5 −30 −1
−79 +21 −74 +32 122 58 78 230
6 −63 −33 −139 +5 −136 +26 148 80 139 225
7 −121 −80 −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
11 +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 68 103 125
3 +153 +97 −261 +99 −237 +132 272 136 355 324
4 +159 +108 −397 +114 −368 +167 404 218
503 492
5 +167 +118 −391 +108 −363 +160 397 206 453 532
6 +125 +95 −311 +96 −287 +137 319 176 333 446
7 +43 +55 −237
+85 −216 +115 247 180 250 312
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 +40 −125 +58 138 155 149 111
11 −136 −70 −84 +18 −79 +29 89 64 95
107
Midnight −147 −80 −43 −2 −43 +4 91 42 119 111
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 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 10 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.
Table II. - Mean Numerical Value of Resultant Current.
Jan. Feb. March April May June July Aug. Sep. Oct. Nov. Dec.
152 211 293 328 313 314 337 300 258 235 165 132
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 Magnetism, Terrestrial). 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 11-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 800 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 0.5 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 Ann. du bureau central météorologique synchronous magnetic and earth current
curves from Parc Saint Maur corresponding to the chief disturbances of the year. In most cases there is a marked similarity between the curve of magnetic
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 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 occurred simultaneously, there was a distinct tendency for the latter to precede the
former. More recent examinations of the Greenwich records by W. 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 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 atmosphere, these upper currents becoming visible at times as aurora.
9. There seems some evidence that earth currents can be called into existence by purely local causes, notably difference of level. Thus K.A. Brander (
[12]) has observed a current flowing constantly for a good many days from Airolo (height 1160 metres) to the Hospice St Gotthard (height 2094 metres). In an
8-km. line from Resina to the top of Vesuvius L. Palmieri (
[13]) - observing in 1889 at three-hour intervals from 9 A.M. to 9 P.M. - always found a current
running uphill so long as the mountain was quiet. On a long line from Vienna to 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 night. During a fortnight in September and October 1885 hourly
readings were taken of the current in the telegraph cable from Fort-William to Ben Nevis Observatory, and the results were discussed by H.N. Dickson (
[15]), who found a marked preponderance of currents up the line to the summit. The recorded mean data, otherwise regarded, represent a "constant" current, equal
to 29 in the arbitrary units employed by Dickson, flowing up the line, together with the following diurnal inequality, + denoting current towards Fort-William
(i.e. down the hill, and nearly east to west).
Hour 1 2 3 4 5 6 7 8 9 10 11 12
A.M. −21 −41 +13 +23 +55 −3 +25 −32 −59 −62 −46 +6
P.M. +24 +18 +115 +18 +75 −5 +50 −9 −56
−37 −28 −34
There is thus a diurnal inequality, which is by no means very irregular considering the limited number of days, and it bears at least a general resemblance to that shown by Weinstein's figures for an east-west line in Germany. This will serve to illustrate the uncertainties affecting these and analogous observations. A constant current in one direction may arise in whole or part from plate E.M.F.'s; a current showing a diurnal inequality will naturally arise between any two places some distance apart whether they be at different levels or not. Finally, when records are taken only for a short time, doubts must arise as to the generality of the results. During the Ben Nevis observations, for instance, we are told that the summit was almost constantly enveloped in fog or mist. By having three earth plates in the same vertical plane, one at the top of a mountain, the others at opposite sides of it, and then observing the currents between the summit and each of the base stations, as well as directly between the base stations - during an adequate number of days representative of different seasons of the year and different climatic conditions - many uncertainties would soon be removed.
10. Artificial Currents. - The great extension in the applications of electricity to lighting, traction and power transmission, characteristic of the
end of the 19th century, has led to the existence of large artificial earth currents, which exert a disturbing influence on galvanometers and magnetic
instruments, and also tend to destroy metal pipes. In the former 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 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 if 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 felt varies with the 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 absence or presence of "boosters," and finally on the
sensitiveness of the magnetic instruments. At the U.S. Coast and Geodetic Survey's observatory at Cheltenham the effect of the 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 Toronto, Washington (Naval Observatory), Kew, Paris (Parc St Maur), Perpignan, Nice, Lisbon, Vienna, Rome, Bombay
(Colaba) and 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 gas and water pipes, numerous observations have been made, especially in Germany and the 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 conditions, if the joints between contiguous pipes forming a main present appreciable resistance, whilst the surrounding earth through moisture or any other cause is a 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 considerable as to be the alleged cause of the ultimate failure of water pipes to stand the pressure they are exposed to.
Bibliography. - See Svante August Arrhenius, Lehrbuch der kosmischen Physik (Leipzig, 1903), pp. 984-990. For lists of references see J.E.
Burbank, Terrestrial Magnetism, vol. 10 (1905), p. 23, and P. Bachmetjew [8]. For papers descriptive of corrosion of
pipes, etc., by artificial currents see Science Abstracts (in recent years in the volumes devoted to engineering) under the heading "Traction, Electric;
Electrolysis." The following are the references in the 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 Erdströme im deutschen Reichstelegraphengebiet (Braunschweig, 1900);
[5] Phil. Trans. R.S. vol. 158 (1868), p. 465, and vol. 160 (1870), p. 215; [6]
Mém. de l'Académie St-Pétersbourg, t. 31, No. 12 (1883); [7] T. Moureaux, Ann. du Bureau Central
Mét. (Année 1893), 1 Mem. p. B 23; [8] P. Bachmetjew, Mém. de l'Académie
St-Pétersbourg, vol. 12, No. 3 (1901); [9] Terrestrial Magnetism, vol. 3 (1898), p. 130; (
[10])
Journal Tel. Engineers (1881); (
[11]) Proc. R.S. vol. 52 (1892), p. 191; (
[12]) Akad. Abhandlung (Helsingfors, 1888); (
[13])
Acad. Napoli Rend. (1890), and Atti (1894, 1895); (
[14]) Pogg. Ann. vol. 76, p. 135; (
[15]) Proc. R.S.E. vol. 13, p. 530;
(
[16]) A. Rücker, Phil. Mag. 1 (1901), p. 423, and R.T. Glazebrook, ibid. p. 432; (
[17]) J. Edler, Elektrotech. Zeit. vol. 20
(1899); (
[18]) L.A. Bauer, Terrestrial Magnetism, vol. 11 (1906), p. 53.
(C. Ch.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
