Vol. 6, 1920 PHYSICS: E. H. HALL 145 
1 
For both of these groups 1.5 is a critical value for q. Examination of 
equation (19) shows that, when q = 1.5, C becomes infinite unless X'o 
at the same time becomes 0; and if X'o becomes 0 while 1.5, C becomes 
c 
Fig. 3 
indeterminate. In both figure 2 and figure 3, therefore, g = 1.5 would 
imply a line coincident with the C axis. 
If q in equation (19) has a value between 1.5 and 4, C will have the 
same sign as K\, which is positive for the first group and negative for the 
second group. Accordingly, since negative values of C are meaningless, 
1.5<g<4 is possible for the first group but not for the second group. 
Cases in which Ki is not 0. There are in Bridgman's list five metals 
for which K2, or B, is not zero. These are aluminium, gold, iron, molyb- 
denum, and thallium. Finding the value of C, for a given combination 
of q and X'o, in these metals, is a somewhat roundabout, though not diffi- 
cult, process. The value of C2 is found by use of equation (15), then the 
value of C\ by use of (14), then the value of C by use of (17). The q- 
constant lines on the {C-\'o) plane are no longer straight, as they are in 
figures 2 and 3. Figure 4 shows their general shape for g = 0, g = 0.5 and 
q = 1, in the range from C = 0 to C = 0.20. It is to be noted that, though 
Ki is positive in iron and thalHum while negative in aluminium, gold. 
Fig. 4 
and molybdenum, the lines in question are of the same general shape 
and arrangement for all five metals. The value of is positive for all. 
Examination of equation (17) shows that, when g = 1.5, C is 00 or 
is indeterminate. In the latter case, we may have either X'o = 0 or Ci = 
546C2. 
