Magnetostriction with Small Magnetizing Fields

Piezo-electric Method of Measuring Extremely Small Magnetostriction Effects.—In this method, suggested by Pupin, the wire to be studied is surrounded by a solenoid through which an alternating current of known strength and frequency is sent, and the vibrations set up in the wire are transmitted to a piezo-electric crystal and the resulting electric charge is amplified and measured. (1) For elongations down to 2 × ${10}^{-9\mathrm{}}$ the wire was hung directly from a quartz crystal. To measure the charge produced by a given magnetizing current, an electromotive force of the same frequency and adjusted by means of a Pupin wave balance to the proper phase, was fed to the amplifier through a vacuum tube in parallel with the one connected to the crystal, and the potential was varied until the note produced by the crystal was balanced out. The force corresponding to a given charge was determined with the aid of a condenser, one plate of which was suspended from the crystal. (2) By using the torsion of a Rochelle salt crystal, the sensitiveness was extended to 2 × ${10}^{-11\mathrm{}}$, and (3) by tuning both the crystal and the wire to resonance with the frequency used, a sensitiveness of 3 × ${10}^{-12\mathrm{}}$ was reached. The last two arrangements were calibrated in absolute units by comparison of the results with those obtained with quartz. The smallest change previously measured was about ${10}^{-8\mathrm{}}$.