554 



P.lRKF.I.AXn. Till: XOKWKGIAX AI'ROKA POLARIS KXPKIMTIOX, I QO2 - 



In order to obtain clear phenomena in the experiments, it is important that the discharge-tube 

 shall have been exhausted for several days, and that during this time the terrella shall have been 

 frequently magnetised, thus becoming' heated and giving off gases. The discharges, moreover, must 

 have taken place abundantly, so that superfluous gas is removed from the electrodes and the inner 

 surfaces of the discharge-tube. 



One drawback in the photographing <>l tin- various light-phenomena was the rather bright, dis- 

 turbing reflections trom the plaU -glass sheets. They have been to some small extent removed in the 

 retouching of the prints. 



Fig. 200 is a photograph taken during an experiment with a terrella No. 5, which was 5.5 cm. in 

 diameter, and suspended in such a manner that the magnetic axis coincided with the axis of rotation, 

 The first series of experiments in the following pages, until stated otherwise, have been made with this ter- 

 rella, of which the magnetic moment fur different magnetising currents is given graphically in fig. 70 (Part I). 



In the experiment shown in tig. 200, the terrella is provided with two fixed screens, one hori- 

 zontal round the equator, anil one vertical. On the horizontal screen moreover, there are fixed 5 short 

 thick pieces of metal wire, coated, as are also the screens and the terrella, with tungstate of lime. The 

 picture is interesting in that it shows how the rays from the cathode are thrown upon the walls of the 

 discharge-tube; and it also shows how the rays are drawn in towards the terrella in the form, previ- 

 ously often mentioned, of two luminous horns near the poles. The dark space between these luminous 

 horns widens greatly if the magnetisation of the terrella is increased. The rays are thrown down in 

 abundance upon the under surface of the discharge-tube, and similarly up towards the top surface. The 

 rays are moreover thrown forcibly against the left side surface, looking from the cathode towards the 

 terrella; and the terrella is magnetised with the south pole uppermost. On the right side surface there 

 is no appearance of any corresponding great precipitation of rays. It will be seen that here no perceptible 

 rays reach the back surface of the discharge-tube. 



A great number of experiments have been made with terrella No. 5, and photographs have been 

 taken trom various points simultaneously, during each separate experiment. Such photographs have been 

 taken, tor instance, of 12 different positions, with the vertical screen tui'iu d right round, 30 each time. 



