PART I. ON MAGNETIC STORMS. CHAP. II. igi 



travelled nearly the same way, and in a short space of time the relative positions of the sun and the 

 earth, which should be decisive for the form of the system, would undergo only slight alteration. 



With reference to this cyclo-median perturbation, I have made a number of experiments with my 

 magnetic terrella, and will here give some of the results of these. 



With a suitable proportion between the stiffness of the cathode rays and the intensity of the mag- 

 netisation, the rays strike the terrella in lower latitudes, and form a well-defined luminous area. 



Fig. 66 shows an area such as this. In making the experiments, an influence-machine was used 

 as the source of electricity, and a discharge-tube similar to that shown in fig. 37. The four positions 

 of the terrella, shown in the four photographs in fig. 66, were such that in No. i, the magnetic south 

 pole (answering to the terrestrial-magnetic north pole) was in such a position that, considering the cathode 

 as representing the sun, there was noon there. In the positions 2, 3, and 4, the terrella is so turned 

 that at the same south pole it is respectively 6 p. m., midnight, and 6 a. m. 



Fig. 66. 



The tension employed between the anode and the cathode was about 10,000 volts. The terrella 

 was magnetised with a current of 3.2 amperes, and the gas-pressure in the tube was 0.0011 mm. 



The photographs were taken from the same position in all four cases, i. e. so that the line from 

 the centre of the terrella to the camera made an angle of 45 with the line from the centre to the cen- 

 tral point of the cathode. The characteristic changes undergone by the luminous area during the turning 

 of the terrella, are distinctly seen. It is especially noticeable that the strength of the light is greatest 

 in the polar regions, and that the luminous point towards the east near the equator moves from southern 

 to northern latitudes during the turning of the terrella. 



By studying this phenomenon more closely, I have found out that under certain circumstances, several 

 such characteristic luminous areas may be obtained on the terrella. 



By employing an inductorium as the source of electricity, and a very strong current for the mag- 

 netisation of the terrella, I have found three distinct, and possibly more, such areas, arranged one after 

 the other round the terrella from west to east. In order to make sure that these different luminous 

 areas were not due to the almost simultaneous appearance of cathode rays of various degrees of stiff- 

 ness, during each discharge from the inductorium I 1 ), I have repeated all the experiments, employing as 

 the source of the current a high-tension direct-current machine, system Thury, Geneva. This machine, 

 when in regular work, can supply J /3 ampere at 15,000 volts, but with lower current strength can go 

 up to 20,000 volts. 



It now turned out that I obtained exactly the same kind of light-figures on the terrella as I did when 

 employing the inductorium as the source of the current. 



(!) See Birkeland, "Sur un spectre des rayons catodiques". Comptes Rendus. 28 Sept., 1896. 



