Page:EB1911 - Volume 08.djvu/842

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