Fig. 42. The Strain Cell.
be twisted. The angle of torsion is measured with the help of a graduated circle, not shown in the figure.
If a cell be made of two clean wires cut from the same piece, with water as the electrolyte there should theoretically be no P. D. between the two. But in practice a small P. D. exists between the wires, owing to slight difference in their molecular condition.
This initial difference can, however, be annulled by appropriate means, for example by subjecting them to mechanical vibration for a short time. After these precautions are taken, results are obtained which are extraordinarily consistent.
Now if one of the two wires be continuously twisted, an increasing P. D. is induced during torsion between the acted and the unacted wires. This may be measured by the deflection of the sensitive galvanometer. A curve of response could thus be obtained with electromotive force, measured by the galvanometer deflection, as ordinate, and the time during which disturbance is kept up as abscissa. Such curves were directly obtained by the recording apparatus described in my previous paper. The wire was twisted at a uniform rate. The successive dots represent the completion of 360°. To
