102 Prof. W. C. Rontgen on the Change in the Double 
(4) The further question arises, how the free surface of the 
sphere would be affected if the pressure acted along an axis 
of maximum piezoelectricity. 
The quartz sphere was placed in the proper position on the 
insulated brass cylinder, the brass rod being also insulated. 
The electrical examination of the surface showed then that 
the sphere was again divided into two halves charged with 
opposite electricities, separated by a plane of no piezo- 
electricity at right angles to the direction of pressure. The 
sign of the electricity was determined by the sign of the fields 
as found above in which the points of pressure lay. If, for 
example, field 1 was above and consequently field 4 below, 
the upper half became positive, and the lower half negative. 
The maximum of electricity appeared at the points of pressure, 
and no electricity at all along the circle in which the plane ot 
no piezoelectricity at right angles to the direction of pressure 
cuts the sphere. 
If the two points of pressure were connected with the earth, 
essentially the same distribution of electricity was observed. 
(5) The sphere was subjected to pressure in a direction 
intermediate between an axis of no piezoelectricity and the 
nearest axis of maximum piezoelectricity, but still at right 
angles to the principal axis; both points of pressure were 
insulated. The sphere was again found to be divided by a 
plane passing through the principal axis into two halves 
having opposite electricities ; but the plane was no longer 
parallel or at right angles to the direction of pressure, but 
lay in the acute angle formed by the direction of pressure 
with the next but one axis of no piezoelectricity. The 
nearer the direction of pressure lay to the axis of no piezo- 
electricity the smaller was the acute angle between the 
direction of pressure and the plane of bisection. If in passing 
from one experiment to another the direction of pressure was 
changed so that, to begin with, it coincided with a direction of 
maximum piezoelectricity, and, to end with, coincided with 
the next axis of no piezoelectricity, then the position of the 
plane of bisection also revolved through an angle of 90° 
about the principal axis. 
The sign of the electricity evolved was determined from the 
sign of the fields in which the points of pressure lay. If, for 
example, the point of pressure was situated in field 1, but 
nearer to field 2 than to field 6, then the half of the sphere 
which contained a part of field 2, the whole of fields b' and 1 , 
and a part of field 5 became positively, and the other half of 
the sphere became negatively electric. 
If the pressure acted in the line bisecting the angle between 
