560 Intelligence and Miscellaneous Articles. 



combining, in various ways, the four electrodes of our two lines, I 

 succeeded in separating the two classes of currents, and in drawing 

 from iny observations the following conclusions : — 



(1) The difference in the electrical potential of the earth for 

 two points at a distance of 1 kilometre, and in the most favour- 

 able direction, is in any case less than 0*001 volt for a time of 

 magnetic calm— that is to say, free from magnetic disturbances ; 

 while the difference of the electromotive force of the electrodes 

 may reach 0*05 volt. During magnetic storms the earth currents 

 become stronger ; but even then their potential rarely exceeds this 

 latter limit, 0*05, in lines a kilometre in length. 



(2) The electromotive difference of the electrodes placed at a 

 depth of 2 metres below the surface, and connected by well-insu- 

 lated wires, is for some time so constant that we may look upon 

 any appreciable change in the ordinary current due to the elec- 

 trodes as the manifestation of an earth current properly so 

 called. 



(3) The ratio between the force of the branch earth current 

 and of the current due to the electrodes is independent of the re- 

 sistance of the lines and of the dimensions of the electrodes, and 

 only increases with the distance of these latter. In order to be 

 certain about an observation of an earth current properly so called 

 during a time of magnetic calm, if indeed an earth current does 

 exist at such periods, the lines should be 50 kilometres in length. 

 Only by using earth plates more uniform than they are usually, is 

 it possible to observe this current in shorter lines, that is 1 kilo- 

 metre in length. 



(4) The resistance of the earth for electrodes a metre square 

 and at a distance of 1 kilometre, is equal to from 30 to 60 ohms. 

 Assuming that the total resistance of the circuit, including the 

 galvanometer, does not exceed 100 ohms, we could observe earth 

 currents in such lines by means of a mirror-galvanometer, in 

 which the value of a division of the scale is equal to 0*000002 

 ampere. The deflections by earth currents will rarely exceed 

 + 250 parts of the scale even during magnetic storms. 



Since the publication of the paper mentioned, the calculation of the 

 observations made regularly during the period from September 1882 

 to September 1883 has been so far advanced as to enable me to 

 communicate to the Academy some interesting results which I have 

 deduced from them : — 



1. The earth-current at Paulowsk in our lines of one kilometre 

 in length is usually manifested not as a current which flows for 

 some time in a certain direction, and the force of which varies 

 slowly, but as more or less strong alternating currents, which change 

 rather rapidly their direction in space. 



2. The component of the earth current which appears in the 

 E.-W. line is generally stronger than that observed in the N.-S. 

 line ; the direction approaches therefore the parallels rather than 

 the meridians. 



3. The observations of the term-days, taken separately each day, 



