MR, W. CROOKES ON MOLECULAR PHYSICS IN HIGH VACUA. 
6 57 
another tube was taken having the upper pole an aluminium wire, and the lower one 
a ring’, fig 19. It was only exhausted to such a point that the induction spark should 
pass freely from one pole to the other in the form of a luminous band of light, this 
being the form of discharge usually considered most sensitive to magnetic influence. 
This tube was also mounted over an electro-magnet, and the two sets of apparatus 
being actuated successively with the same coil and battery the following observations 
were made. 
620. The tubes will be distinguished by the terms “high vacuum” (fig. 18) and 
“low vacuum” (fig. 19). The rotation produced in each tube will be recorded in the 
direction in which it would be seen by an observer above, looking vertically down on 
the tube, his eye being in a line with the terminals and with the axis of the magnet. 
When the rotation thus viewed is in the direction of the hands of a watch, it is called 
direct ; the opposite movement being called reverse. To facilitate a clear appreciation 
of the actions, an outline sketch accompanies each experiment. The shape of the tube 
shows whether it is the high or low vacuum tube, and the letter D or If shows the 
direction of rotation. 
Fig. 20. 
Case a. 
Case b. 
Case c. 
Case cl. 
621. a. Upper pole of electro-magnets north. 
Induction current passing through tubes so as to make the top electrode 
positive. 
Rotation in the high vacuum direct. 
Rotation in the low vacuum direct, 
h. Upper pole of magnets north. 
Top electrode of tubes negative. 
Rotation in high vacuum direct . 
Rotation in low vacuum reverse . 
c. Upper pole of magnets south. 
Top electrode of tubes positive. 
Rotation in high vacuum reverse. 
Rotation in low vacuum reverse. 
d. Upper pole of magnet south. 
Top electrode of tubes negative. 
Rotation in high vacuum reverse. 
Rotation in low vacuum direct. 
4 P 2 
