October 



1922] 



SCIENCE 



467 



that the earth-current toward the south was 

 strongest between eleven o'clock and noon, 

 while the magnetic force toward the west was 

 at its maximum between one and two P.M. 



If a curve be drawn showing the rate of 

 change of magnetic force throughout the 

 twenty-four hours, we have the dotted curve, 

 which resemibles Curve A so closely as to give 

 proibability to our assumption that the mag- 

 netic deviation is caused by the earth-current, 

 and that the magnetic force is always changing 

 at a rate which is proportional to the intensity 

 of the earth-current. 



This relation between the rate of change of 

 the magnetic declination and the intensity of 

 the north-south earth-current has also been ob- 

 sei'ved by Bauer^ in data taken from the 

 records of Observatorio Del Ehro, at Tortosa, 

 Spain, but his conclusion drawn from this rela- 

 tion differs from that given above. Bauer says : 



The general conclusion is that the north-south 

 earth-current might he the result of electro-mag- 

 netic induction, caused hy the fluctuation during 

 the day of the west-east component of the earth's 

 magnetism. 



In reaching this conclusion Bauer has over- 

 looked a very elementary and very fundamental 

 law of electromagnetic induction, viz., that a 

 current induced by a change in a magnetic 

 field is in the opposite direction to the current 

 which would be required to produce the given 

 change in the magnetic field. If the north- 

 south earth-current and the westward deflection 

 of the magnetic needle are causally related, the 

 magnetic deviation is induced by the current. 

 The contrary can not be true. 



In the case of the west-east component of 

 the earth-current conditions are somewhat dif- 

 ferent. This current weakens the magnetic 

 component toward the north by a very small 

 fraction of its total force, the diurnal range 

 of this weakening being less than one two- 

 thousandth part of the whole, and probably 

 not all of this is attributaible directly to the 

 earth current. The resultant effect of the total 

 earth current at a given place is to make the 

 day side of the earth electropositive to the 

 night side, and this condition would, of itself, 

 weaken the north magnetic force on the day 



iTerr. Mag., XXVII, 14 (March- June, 1922). 



side of the earth. That is, a positive charge 

 carried around by the earth's rotation from 

 west to east would set up a north-south mag- 

 netic field in the opposite direction to the prin- 

 cipal magnetic field of the earth. Such a 

 charge would not, however, affect the east-west 

 magnetic field of the earth. 



That the day side of the earth is, on the 

 whole, electropositive to the night side is plain- 

 ly shown 'by a series of observations which the 

 present writer has carried on for about three 

 years, and for which be has continuous photo- 

 graphic records for more than two years. That 

 this difference" of electric potential on the oppo- 

 site sides of the earth is due to the electrostatic 

 induction of the negative charge of the sun 

 seems beyond question. A smaller, but still a 

 very definite effect of the moon's induction is 

 also shown by the records. 



In Figure 2, Curve A shows the diurnal 

 variation of the earth's potential at Palo Alto 

 for one year, from August, 1920, to July, 

 1921. Curve B shows the corresponding mean 

 variation of the west-east earth current at 

 Berlin for the year 1884. It is seen that while 

 the earth-current reaches its maximum inten- 

 sity at between ten and eleven A.M., the elec- 

 tropositive potential of the earth is at its maxi- 

 mum at 'between one and two P.M. In both 

 cases the time of maximum has a seasonal 

 variation. The same may be said of the varia- 

 tion in the north-south magnetic component of 

 the earth's magnetic field, which for the year 

 1884 attained its maximum between eleven 

 o'clock and noon at Vienna and Wilheknshaven, 

 as shown by the dotted curve C. In this case, 

 as in the case of the variation in magnetic 



