1912-13.] Torsional Oscillations of Metallic Wires. 
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XXI. — On the Deviation of the Torsional Oscillations of Metallic 
Wires from Isochronism. By Professor W. Peddie. 
(Read December 18, 1912. MS. received January 21, 1913.) 
§ 1. In former papers {Trans. P.S.E., 1898; Phil. Mag., 1894) it was 
shown that the period of the inward oscillation of an iron wire was distinctly 
greater than the period of its outward oscillation over the same range, and 
that the complete period was slightly greater when the range was relatively 
great than when it was small. In these respects the material deviated 
from the conditions of isochronism which are characteristic of the simple 
harmonic vibrations of a material which practically follows Hooke’s Law 
during distortion. 
As the method of observation which was formerly employed did not 
give much more than qualitative results, and was quite inadequate for 
the determination of the actual law of oscillation at all stages, a different 
method was employed in the work described below. A description of 
that method, and of the apparatus, is first given ; and then, by means of 
curves, the experimental results obtained in the cases of three different 
materials are shown, and the nature of their deviation from those char- 
acteristic of simple harmonic motion is exhibited. Finally, a theoretical 
discussion is given of a molecular condition under which the observed 
laws of torsional oscillation might be simulated. 
Experimental Discussion. 
§ 2. In fig. 1 the disposition of the apparatus is exhibited. Above, on 
the right, a horizontal wooden arm carrying the torsion head can be 
seen. The head is coned strongly, and works in a similarly coned bearing 
fitted in the wooden arm. A pin-vice, carefully centred, clamps the 
upper end of the wire, and the oscillator is clamped to its lower end by 
a similar vice. The oscillator consists of a circular brass ring of 
rectangular section, and equal brass pins, arranged equidistantly and 
concentrically round the ring at angular intervals of 10°, project down- 
wards from it. Beneath it is placed an ebonite disc which rests by 
screw feet on a vertically adjustable stand. Narrow radial slits are 
cut in the upper surface of the disc, their middle points lying on a 
circle of the same radius as that of the circular row of pins in the 
oscillator, but the angular distance of successive slits is 12° instead of 10°. 
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