98 Prof. W. C. Rontgen on the Change in the Double 
On releasing the compressing-screw electricity of the opposite 
kind was obtained in every case. 
These experiments leave no doubt that the angle between 
the three axes of no piezoelectricity is 120° ; the single devia- 
tion observed, which did not amount to 1°, may very well be 
set down to experimental error. 
Pressures exerted upon the plate in directions lying 
between the axes of no piezoelectricity produced a powerful 
evolution of electricity. Since the electroscope employed was 
not adapted for quantitative experiments, I was not able 
to determine how the quantity of piezoelectricity varied 
with the direction ; it is, however, certain that the largest 
quantity was produced in these intermediate directions. For 
an accurate quantitative determination it would be absolutely 
necessary to take account of the inductive action which the 
electricity excited on the surfaces near the points of pressure 
would exert upon the electrode connected with the electro- 
scope. This action will be considered later on. 
The experiments with the sphere of quartz required a 
somewhat more complicated apparatus. Here also it was 
necessary that pressure should be exerted upon the sphere as 
nearly as possible along a diameter, and that the resulting 
piezoelectricity should be observed. After numerous experi- 
ments with different forms of apparatus, the following arrange- 
ment was adopted as the most convenient. The sphere was 
placed upon the stage of an old and large microscope, and 
pressure exerted upon it by lowering the heavy body of the 
microscope down upon it. 
The stage is provided with the necessary screws for exact 
adjustment ; in the circular opening of the stage can be 
placed either a round thick disk of brass or a disk of ebonite 
which fits it exactly. The disk of brass is provided at its 
centre with a depression 0*25 centim. wide, which gives better 
support to the sphere laid upon it ; the sphere therefore does 
not touch the disk in one point only, but in a small circle. A 
small thick brass cylinder 1 centim. long, 0*4 centim. thick, is 
cemented on to the ebonite disk, which is also provided at the 
top with a small depression which lies exactly in the axis of the 
disk of rubber and upon which the sphere rests. The brass 
disk was used when the lower side of the sphere did not require 
to be insulated, and the rubber disk when insulation was 
necessary. 
The microscope-tube, which moved accurately in a solid 
stand by means of rack-work, was fitted at its lower end with 
a piece of brass rod rounded at the lower end and insulated 
by means of ebonite, by means of which the pressure upon 
