Physics

P. CURIE

Piezoelectricity

Crystals which have one or more axes whose ends are unlike, that is to say, hemihedral crystals with inclined faces, have a special physical property, that they exhibit two electric poles of opposite names at the ends of those axes, when they undergo a change of temperature: this is the phenomenon known as pyroelectricity.

We have found a new way to develop electric polarisation in crystals of this sort, which consists in subjecting them to different pressures along their hemihedral axes.

The effects produced are analogous to those caused by heat: during a compression, the ends of the axis along which we are acting are charged with opposite electricities; when the crystal is brought back to the neutral state and the compression is relieved, the phenomenon occurs again, but with the signs reversed; the end which was positively charged by compression becomes negative when the compression is removed and reciprocally.

To make an experiment we cut two faces parallel to each other, and perpendicular to a hemihedral axis, in the substance which we wish to study; we cover these faces with two sheets of tin which are insulated on their outer sides by two sheets of hard rubber; when the whole thing is placed between the jaws of a vise, for example, we can exert pressure on the two cut surfaces, that is to say, along the hemihedral axis itself. To perceive the electrification we used a Thomson electrometer. We may show the difference of potential between the ends by connecting each sheet of tin with two of the sectors of the instrument while the needle is charged with a known sort of electricity. We may also recognize each of the electricities separately; to do this we connect one of the tin sheets with the earth, the other with the needle, and we charge the two pairs of sectors from a battery.

Although we have not yet undertaken the study of the laws of this phenomenon, we are able to say that the characteristics which it exhibits are identical with those of pyroelectricity, as they have been described by Gaugain in his beautiful work on tourmaline.

We have made a comparative study of the two ways of developing electric polarisation in a series of non-conducting substances, hemihedral with inclined faces, which includes almost all those which are known as pyroelectric.

The action of heat has been studied by the process indicated by M. Friedel, a process which is very convenient.

Our experiments have been made on blende, sodium chlorate, boracite, tourmaline, quartz, calamine, topaz, tartaric acid (right handed), sugar, and Seignette’ salt.

In all these crystals the effects produced by compression are in the same sense as those produced by cooling; those which result from relieving the pressure are in the same sense as those which come from heating.

There is here an evident relation which allows us to refer the phenomena in both cases to the same cause and to bring them under the following statement:

Whatever may be the determining cause, whenever a hemihedral crystal with inclined faces, which is also a non-conductor, contracts, electric poles are formed in a certain sense; whenever the crystal expands the electricities are separated in the opposite sense.

If this way of looking at the matter is correct the effects arising from compression ought to be in the same sense as those resulting from heating in a substance which has a negative coefficient of dilatation along the hemihedral axis.