184 
Mr Almy, On a Diminution of the Potential 
Table I. Magnet 'perpendicular to tube. 
Spark gap 
(without 
magnet) . . . 
11-5 
11*4 
11*5 
11 
11-2 
11*3 
11-5 
11-5 
11-5 
Spark gap 
(with 
magnet) . . . 
11-5 
9 
8 
7 
4 
3 
2 
2 
1-9 
Potential- 
diff. 
104 
84 
76 
68 
42 
33 
25 
25 
24-5 
Diminution 
in P.-D. ... 
0 
16 
24 
34 
62 
69 
80 
80 
80-5 
Magnetic 
field in- 
tensity ... 
0 
40 
48 
54 ! 
62 
69 
75 
120 
420 
Table II. Magnet parallel to tube 
Spark gap (with- 
out magnet)... 
Spark gap (with 
magnet) 
12*5 
10 
7 
12-5 
5 
3 
2 
1-7 
Potential-diff. . . . 
110 
90 
68 
51 
33 
25 
21 
Diminution of 
P.-D 
0 
20 
42 
59 
77 
85 
88 
Intensity of 
magnetic field. .. 
0 
180 
216 
290 
300 
312 
420 
The magnetic intensities given are those at the centre of 
the cathode ; spark gaps in millimetres, potential differences in 
static (c.g.s.) units, according to Heydweiller (cf. Wied. Ann. Vol. 
48, p. 214). 
That the same sort of effect is produced with continuous 
discharge was shown by using a large Holtz machine to give the 
discharge. Potential differences were measured with a Kelvin 
absolute electrometer; gas pressure being about O il mm. 
That the relatively large change in potential difference is 
only produced at fairly small gas pressures became ver}^ soon 
evident. A series of observations of the diminution produced 
by a certain magnetic field, normal to the tube, of intensity large 
enough to be safely above the “ critical ” intensity, with various 
