PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 579 



A great difference is discernible between the precipitation in Nos. 7 and 8, although the position of the 

 screen is very little changed. 



Nos. i, 2, 3 and 4 of fig. 211 were taken in order to determine more exactly the position of 

 the ray-precipitation now under discussion. In i and 2, the conditions are the same as in 8 and 9 

 respectively of fig. 210, except that the hour-angle of the screen is 75; and in Nos. 3 and 4, they are 

 also the same, except that the hour-angle of the screen is 60. The white wedge of light on the hori- 

 zontal screen (Nos. i & 3) is a patch belonging to precipitation B, formed by rays which have passed 

 through the slit. 



It will be seen from all these photographs that under these conditions the precipitation is well 

 defined on the eastern side, and its strength is greatest on the terrella at a place answering to between 

 5 and 6 p. m. 



Nos. 5, 6, 7 and 8 were taken from a series of experiments, made for the purpose of finding out 

 whether rays that come in towards the polar regions of the terrella from the left side, seen from the 

 cathode, could also form precipitation of the same kind as the rays that came through the slit on its 

 right side, looking from the cathode. 



It appeared that with the highest magnetising that the terrella could stand, a quantity of rays were 

 drawn in towards the terrella on the left side too, descending fairly perpendicularly, so as to give the 

 distinct impression that even the large discharge-box of sheets of plate-glass, which was employed 

 in all the experiments described here, was not large enough, i. e. high enough above the poles, to 

 allow of the position of the precipitation upon the terrella being accurately determined, as it might 

 have been if the rays could have moved towards the terrella, unhindered by the sides of the discharge- 

 box. A great many experiments were made, however, so the results described below may be considered 

 sufficiently certain. 



Photographs 5 and 6 are of experiments in which the pressure was o.ooi mm., the discharge-current 

 20 milliamperes, the tension 3000 volts, and the magnetising current 25 amperes. They were taken from 

 places with hour-angles of 250 and 315. The hour-angle of the screen was 115. With this high 

 magnetisation of 25 amperes, and still more with 35 amperes, which was used subsequently, the small 

 luminous patch, described in Section I of this work at the bottom of page 298, came out. In the present 

 case, this little ring became a rather compressed oval, a great part of it being visible upon both sides 

 of the screen. In No. 5 we distinctly see the one part, but in No. 6 the continuation of the precipitation 

 is no more than just visible. With 33 amperes and rather softer rays, this half of the oval was just as 

 bright as the other part on the other side of the screen (see No. 5). 



In this photograph there is also distinctly seen in the precipitation, the shadow of the conducting- 

 vvire for the current to the terrella. The shadow shows how the rays descend almost perpendicularly 

 towards the terrella; but a twisting of the rays can also be proved resembling that of a helix. 



Photographs 7 and 8 show results of experiments made with a pressure of 0.009 mm., a discharge- 

 current of 22 milliamperes, and a magnetising current of 30 amperes. The screen has an hour-angle of 

 70 (it is still the wing with the hole in it from which the angle is measured), and the photographs were 

 taken from places with hour-angles of 180 and 320. 



In No. 7 we see the continuation of the precipitation which produced the oval in No. 5. The pre- 

 cipitation now entirely disappears from the vertical screen where there had previously been precipitation 

 from the returning rays that passed through the slit. 



The shadow of a conducting wire is now seen in the precipitation, showing that the rays have 

 curved round from the left side of the screen, looking from the cathode, to far back on the right side. 



