Radio uses of piezo-electric crystals

Piezo-electricity is a phenomenon which has been known for many years but which is having some remarkable new applications. Certain crystals undergo a slight expansion or contraction when an electrical voltage is applied to them and, vice versa, produce a slight voltage when compressed or pulled. A piece of quartz crystal 1 or 2 inches long has a high natural frequency of the same order as the frequencies of currents used in radio communication. It has been found that the frequency of vibration of the piece of quartz is extraordinarily constant and that it is very useful as a radio standard. In association with a small electron tube it acts as an oscillator or generator of a current the frequency of which is that of mechanical vibration of the piece of crystal. As the frequency thus produced is accompanied by numerous harmonics, the crystal is a standard giving several radio-frequencies. It is thus a remarkable supplement to the wave meters which have hitherto been used as standards of radio-frequency.