Electrical Effect on the Double Refraction of Quartz. 97 
compressed between them. This arrangement ensured that 
the points of pressure formed a narrow line upon the plate, 
parallel to the axis of the disk, and that the pressure was 
exerted upon the disk exactly in the direction of a diameter. 
The disk was attached by means of a straw about 4 centim. 
long to a divided circle, so that its axis coincided with the 
axis of rotation of the divided circle. Consequently by 
rotating the circle the direction in which the pressure was 
exerted upon the disk could be changed and the angle of 
rotation read off on the divided circle. 
The frame of the compression-apparatus and the mounting 
in which the divided circle turned were rigidly attached by 
means of clamps to a stand. To ensure that the pressure 
was always applied in the direction of a diameter the following 
plan was adopted. The quartz plate was covered with a thin 
layer of wax, in which, by means of a fine needle, a considerable 
number of diameters were traced. With the aid of these lines 
it was easy to ensure that the direction of pressure coincided 
with a diameter. One of the two silver wires was connected 
with the delicate electroscope previously mentioned; the other 
was put to earth. The experiment, then, consisted in deter- 
mining the direction of no piezoelectricity. The results of a 
single series of experiments will serve as example : — 
Reading of Divided 
Circle. 
o 

20 
35 
27 
Piezoelectricity due to 
increased pressure. 
Strongly negative. 
Less strongly negative. 
Feebly positive. 
Feebly negative. 
Having thus found that one of the directions sought lies 
between the last two positions of the disk, the following ex- 
periments were made : — 
Reading 
of circle. 
o 
29 
Piezoelectricity due to in- 
creased pressure. 
Reading 
of circle. 
Piezoelectricity due to in- 
creased pressure. 
Trace of negative electricity. 
209 
Trace of positive electricity. 
30 
Nothing. 
210 
Nothing. 
31 
Trace of positive electricity. 
211 
Trace of negative electricity . 
89 
Trace of positive electricity. 
269 
Trace, of negative electricity. 
90 
Nothing. 
270 
Nothing. 
91 
Trace of negative electricity. 
271 
Trace of positive electricity. 
149 
Trace of negative electricity. 
329 
Trace of positive electricity. 
150 
Trace of positive electricity. 
330 
Nothing. 
151 
Trace of positive electricity. 
331 
Trace of negative electricity. 
