^g^ H1KKI LAM>. II I K XnKWIX'.lAX AURORA I'OI.AKIS KXI'KIHTIOX, IQO2 I 903. 



auroral /one on the < arth, forming almost a circle round the point of intersection of the elementary 

 magnetic axis with the earth's surface. The terrclla employed, however, could not he magnetised suffi- 

 ciently strongly, hut, a^ \ve shall see, \ve can easily form an idea as to how much the terrella must be 

 magnetised in order that the ring shall have its correct si/.e. It is also no doubt possible that by se- 

 lecting a somewhat stronger iron core lor the terrella than the one here employed, and employing a 

 stronger magnetising current, a precipitation-ring with a diameter of 45 , might be obtained, which would 

 remain long enough to allow of its being photographed before the terrella became too hot. Indeed I 

 have already, as will be seen below, realised the conditions necessary for this purpose. 



There is another result which may also be directly deduced from our photographs, a result which 

 we have moreover demonstrated many times under the most varied conditions. 



It appears that the more /lie trnrlla is magnetised, the narrowr or thinner docs the hand of light in 

 tin- ri/ii; I'l-cninc, and the smaller the number of rays that arc drawn in towards the terrella in the pre- 

 cipitation-ring. This last circumstance may be at any rate partly accounted for 1>3' the fact that the 

 discharge-tube was not large enough for the highest magnetising of the terrella, as the rays describe large 

 arcs before they go in towards the precipitation-ring. 



With reference to photograph No. 2 in fig. 218, \ would point out, as being of interest in this 

 connection, that aurora that occurs in low latitudes on the earth, must, according to our theory, be due 

 to stiller rays than aurora that only occurs in the ordinary auroral /.one; and the farther the northern aurora 

 extends towards southern latitudes, the greater will he its width and we should expect that it will be seen simul- 

 tanenelv in the xenith over a greater area of the earth. Theory, in this case, is in harmony with experience. 



In order, as we have said, to obtain an estimate of the extent to which the terrella must be mag- 

 netised to give the precipitation-ring a spherical diameter of 45 , the magnetic intensity was measured 

 at the poles of the terrella by means of a LKXARD spiral. An intensity of 1600 C. G. S. answered 

 to a magnetising current of m amperes, 2400 C. G. S. to 20 amperes, and 2800 C. (1. S. to 30 amperes. 

 The relative proportions of the intensities were controlled by induction experiments with a small, flat 

 coil, which was also placed at the pole, exactly where the LKXARD spiral had been used. 



The sixc of the precipitation-rings was then measured from the photographs, and their spherical 

 diameter calculated in the various experiments, measuring along the middle of the band of light, the 

 middle photographs, Nos. 2, 5 and 8, in figs. 217 and 218, being taken for this purpose. In this way 

 the following values were obtained for the spherical diameters: 



Answering to 1800 volts, 73, 68, and 63"; 

 2400 , 88, 72, 66, 



for magnetising currents to the terrella of respectively 10, 20, and 30 amperes. 



From these values we may conclude by extrapolation that with cathode rays answering to 2000 

 volts and a field-intensity of 4500 C. G. S. at the poles of the terrella, we should certainly obtain a 

 small precipitation-ring with a spherical diameter of about 45. The error in this determination is prob- 

 ably no greater than that in the assumption that the auroral zone upon the earth has a spherical dia- 

 meter of 45. We shall later on have an opportunity of controlling experimentally the result of this 

 extrapolation. 



We will now assume that with the above-mentioned magnetisation of the terrella, corresponding to 

 4500 lines of force at the poles, and with rays of 2000 volts, we obtain a comparatively correct idea of 

 what lakes place when the earth is irradiated by corpuscular rays from the sun; and upon this basis we 

 will see what degree of stiffness these rays from the sun may then be assumed to possess. We pre- 

 suppose then, that the magnetic field of force round the earth is similar in form to the field of force 

 round our terrella, and that thus the magnetic field at great distances from the earth is not in any very 

 essential degree aftected by possible current-systems outside the earth. 



