CHAPTER IV. 



EXPERIMENTS MADE WITH THE TERRELLA ESPECIALLY FOR THE PURPOSE 

 OF FINDING AN EXPLANATION OF THE ORIGIN OF THE POSITIVE AND 



NEGATIVE POLAR STORMS. 



108. In the following pages, we shall describe a series of experiments that were made for the 

 purpose of gaining a clear idea of the course of the rays about our magnetic terrella. 



It is, of course, of great importance to calculate, as STORMER has done, the separate possible paths 

 that electric corpuscles from a distant cathode may describe about an elementary magnet, under the 

 influence of the magnetic forces originating from the magnet; and in so doing he has thrown much 

 light uppn my earlier experiments, and, on some essential points, has supported the theory which it 

 is my intention to work out. But as long as the mathematical problem is not entirely solved, so that 

 the distribution of nil paths in space is found, the utility of such calculations as an endeavour to 

 explain, lor instance, the positive and negative polar storms, is very limited. The experimental investi- 

 gations with a magnetic terrella in a large discharge-tube are another matter. There it is possible, by 

 various means, to see how the rays group together round the terrella, and even to photograph the 

 phenomena. 



It is apparent that in this way a full, clear idea of the phenomena may be obtained, so that the 

 results, as we shall now see, may be successfully transferred to the relations between the sun and the 

 earth, as regards the various terrestrial-magnetic and auroral phenomena that have been observed. 



We can, as will be seen, guard against the liability of our discharge-tube, owing to its compa- 

 ratively narrow proportions, having any injurious influence upon the range of the conclusions that can 

 lie drawn from any of the results, and those who will closely follow the entire series of elaborate ex- 

 periments which have been made, will end by seeing how great difficulties resolve themselves into 

 nearly perfect lucidity. Some of the experiments last described, were made some time after the first 

 series; I have not, however, on that account, omitted any of the previous results, as I considered it 

 best that the method adopted could be plainly traced. 



The experiments now to be described have nearly all been made with the machine shown in 

 lig. 67 (Section I), generating a direct current with a tension of up to 20 ooo volts. The arrangemen 

 of the sets of apparatus was also similar to that shown in the figure, but the discharge-tube now was not 

 cylindrical but prismatic, composed of flat plates of glass, so that the photographs taken of the terrella 

 should not be contorted by the passage of the light through the curved glass of unequal thickness. The 

 prismatic discharge-tube, which is shown in fig. 200, was formed of plates of glass, 20 mm. in thickness, 

 cemented together with cementium, and finished outside with picei'n. There was no great difficulty 

 in keeping the tube air-tight, even if it were exhausted down to a pressure answering to 0.0005 mnl - 

 of mercury ; but with a low pressure such as this, there was vapour in the discharge-tube, of which 

 the pressure may well have been several times as great as that mentioned above; but in the experi- 

 ments here described it had no disturbing effect upon the results, as we generally worked with greater 

 pressure. 



