Refraction of Quartz produced hy Electrical Force. 101 
The further result was obtained that the most energetic 
evolution of electricity took place when pressure was exerted 
in directions at right angles to the principal axis, and bisecting 
the angles between the planes of no piezoelectricity; these are 
the directions already designated as axes of maximum piezo- 
electricity. 
I scarcely need remark that diminution of pressure produces 
electricity opposite in kind to that produced by increase of pres- 
sure ; the same holds good for all the following experiments. 
(3) A pressure exerted in the direction of an axis of no 
piezoelectricity produces no electricity at the points of 
pressure. The questiou, however, arises whether under these 
circumstances there is no piezoelectricity at other points of 
the sphere*. To answer this question, the sphere was so 
placed upon the metal disk that the principal axis was at right 
angles to the direction of pressure, and so that the pressure 
was exerted in the direction of an axis of no piezoelectricity. 
The electroscope was connected with the insulated electrode 
previously described, which was successively placed in contact 
with different points of the sphere, the direction of pressure 
remaining unaltered. 
The following results were obtained: — The plane of no piezo- 
electricity in which the pressure is exerted divides the sphere 
into two halves, which both become electric all over, the one 
half being positive, the other half negative. The sign of the 
electricity produced upon each half is determined by the sign 
of the electricity excited on that one of the two fields, divided 
by the direction of pressure, which is situated on that half. 
Thus, for example, if the pressure takes place in the plane 
dividing the field 1 from 6, and 3 from 4, then the half upon 
which the fields 1 and 3 lie becomes positive, and the other 
half negative. 
Maximum electricity is evolved at the ends of an axis of 
maximum piezoelectricity lying at right angles to the direc- 
tion of pressure — in the above example therefore in the 
middle of fields 2 and 5. There is no electricity produced 
upon the circle in which the plane of no piezoelectricity 
containing the direction cuts the sphere. 
The same result was obtained when the sphere was placed 
upon the insulating brass cylinder, or if the support and the 
brass rod exerting the pressure were connected with the 
earth. 
* Messrs. J. and P. Curie have found that a pressm-e exerted in the 
direction of an axis of no piezoelectricity upon a parallelepiped of quartz 
produces electricity at the ends of the axis of maximum piezoelectricity 
at right angles to the direction of pressure. 
