756 REPORT — 1898. 



the polar regions. Researches have shown that the magnitude of these variations 

 of the earth currents increases with the latitude in a very high ratio. When the 

 diurnal variation in Pawlowsk has a value of O, 0008 volt, it rises in Sodankyla 

 (7° more to the north) to 0, 0600, or 75 times more. Moreover, we find from the 

 lately published third volume of the observations from the international polar 

 stations at Sodan]:yla and Kuttala that the earth currents as well as the electric 

 currents in the atmosphere depend intimately on the belt of the maximum polar 

 light. It will be very clearly seen that we also have a maximum belt of the earth 

 currents and even of the currents in the atmosphere. The paper finishes with a 

 proposition that the International Conference on Terrestrial Magnetism and 

 Atmospheric Electricity should discuss the two following questions : What signifi- 

 cation must be attributed to the earth currents and the electric current from the 

 atmosphere in the explanation of the causes of magnetic perturbations ? What is 

 to be done for the further investigation of the connection between the magnetic 

 perturbations and the electric currents ? 



The author also proposes that the International Conference should take steps for 

 establishing two international polar stations, one in the North of America, the other 

 in the North of P]urope, both situated in tlie southern border of the polar light 

 maximum belt. In connection with both these principal stations there should be a 

 northern by-statioi;, in which, us well as in the principal stations, all the magnetic 

 variations, the earth currents, and the electric currents from the atmosphere, besides 

 all metHorological observations, should be made simultaneously, and with self- 

 registering apparatus of the best construction, the details of which ought to be 

 stated by the International Conference. 



In connection with these observations it ought to be expressed as desirable that 

 all magnetic observatories should, if possible, establish similar observations with as 

 identical instruments as possible. iSince the researches of the electric currents 

 from the atmosphere require points elevated about 400 m.. it seems difficult to 

 unite these observations with those in an observatory. We, however, must 

 remember tliat the electrical resistance in the circuit, from the earth to the atmo- 

 sphere, is so great that one can put in several miles of wire without any sensible 

 augmentation of it. It is well understood that the mountain on which an apparatus 

 for out- or in-streaming of electricity is placed can without serious damage be far 

 away from the observatory. 



5. On the Interpretation of Earth-current Observations. 

 By Arthur Schuster, F.R.S. 



If two metallic plates are inserted into the ground and connected by a wire 

 an electric current is found to traverse the conductor, and this is generally called 

 the ' earth current.' It is not, however, ob^^ous at first sight what the connection 

 is between this observed current and that which traverses tlie ground when the 

 earth plates are removed. The statement sometimes made, that the observation gives 

 the difference of potential of the earth at the two points at which the plates are 

 inserted, would be true, provided the connecting wire has a sufficiently large 

 resistance, if earth currents were only due to chemical or thermo-electric forces. 

 But as there can be little doubt that earth currents are chiefly, if not entirely, 

 effects of induction, a separate investigation is necessary as to the interpre- 

 tation to be attached to the galvanometer indications. 



Let A and B be two points in a line of flow of the currents traversing the 

 earth, and consider a tube of flow passing from A to B. The specific resistance of 

 the ground being p, let p be changed to p' for the material enclosed by the tube. 

 I determine in the first place the effect of this change of resistance on the 

 distribution of currents. The change of resistance could be counterbalanced 

 by an electric force— (p — p') n acting throughout the tube, where k is the density 

 of the undisturbed currents, and it is therefore equivalent to the introduction of an 



electromotive force \{fi—p') udl in the tube of flow, dl being an element of its 



length and the integration being extended from A to B. If p and n are constant 



