METEOROLOGY. ool 



Airy on his long line undoubtedly show the presence of terrestrial cur- 

 rents, but there is no means of separating the one from the other. 



3. In magnetic calms the difference of electric potential between 

 ground plates 1 kilometer apart, so far as it is due to terrestrial cur- 

 rents, is less than 0.001 volt, while so far as due to the plates themselves 

 it can easily amount to 0.05 volt, or even more ; at times of magnetic 

 perturbations it is only rarely that the first dilierenco increases to an 

 equality with the second. 



4. In such short lines it is therefore imperative that the observation 

 be so arranged that we maj' distinguish between the terrestrial and the 

 ground- i)late current, and this is easily done by making different com- 

 binations of the four plates by means of the four connecting cables. 



5. For i)lates 1 meter square and L kilometer apart the resistance of 

 the earth may be estimated at from 30 to GO ohms ; if, therefore, the 

 resistance of the cable and that of the galvanometer coil be each about 

 40 ohms, so that the total resistance of plate and wire be about 100 

 ohms, then a galvanometer must have a sensitiveness of 0.000002 

 ampere for one division of the scale in order that the deviations in 

 times of great magnetic perturbations shall not frequently exceed the 

 limits of the scale, supposing the latter to be 250 divisions long, cor- 

 responding to 0.05 volt. 



6. The ratio of the strength of the earth current in any conductor to 

 the strength of tbe ground-plate current is independent of the resist- 

 ance of the conductor and of the size of the earth ])late, but increases 

 with the distance between the latter; therefore an increase in the size 

 of the ground plates, or the introduction of resistance in the conductor, 

 do not tend to weaken the ground- plate current in comparison with the 

 terrestrial current. 



7. If we desire to observe the terrestrial current during magnetic 

 calms, the distance between the plates must be at least 50 kilometers; 

 for shorter lines, say from 1 to 5 kilometers, the terrestrial current can- 

 not be satisfactorily observed unless the ground plates are much more 

 nearly alike than ordinarily attained. {Z. 0. G. M., xix, p. 53.) 



352. II. Wild, from a further study of observations made up to Sep- 

 tember, 1883, draws the following conclusions: 



1. The earth current appears not like a current of nearly uniform in- 

 tensity, but as alternating stronger and weaker currents, which rapidly 

 change their direction in space. 



2. The east-to-west components are stronger than the north-and-south 

 component, or the direction of the current is nearer the parallels than 

 the meridians. 



3. Only by taking the mean of the twenty-four term days during the 

 year do we find traces of slight diurnal change whose amplitude corre- 

 sponds nearly to 0.0008 volt. The diurnal change of the earth current 

 is therefore not the cause of the diurnal change in the magnetic ele- 

 ments. 



