c ————e 
Sl os < st lal eee 
ON MOLECULAR PHENOMENA IN MAGNETISED IRON. 149 
sooo Of its length. Earlier experiments were made with the wire hori- 
zontal, the tension being applied by means of a pulley; the same results 
were obtained, but the vertical method is obviously the best. A length 
of 50 centims. of the same wire gave similar results, the amount of the 
anomalous retraction and expansion being proportional to the length of 
the wire. When the load was increased to 590 grams with the same wire, 
the retraction on heating vanished, and the sudden elongation on cooling 
had increased to no less than 5}, of the original Jength of the wire, or 
0°32 per cent.: the permanent elongation being 3:5 mm. at each heating, 
or upwards of +4, of the length of the wire. The diagram, Fig. 1, 
shows the actual change in the dimensions of the wire. It will be 
observed that a load of 90 grams will cause the jerk on heating to vanish 
with this wire, and that the jerk on cooling grows less as the experiment 
is repeated ; at each experiment the microscope was adjusted to zero so 
that the total permanent elongation is the sum of that in each experi- 
ment, 
II. The permanent stretching of the wire under these loads occurs 
only when the critical temperature is reached. At this temperature an 
abrupt and remarkable softening or plasticity of the iron or steel occurs 
which renders it extraordinarily ductile. A comparative experiment was 
made with copper wire heated to the same temperature. The copper 
wire used was 50 centims. long and rather thinner, 0°76 mm. diameter. 
When heated it would only bear a load of 510 grams without rupture ; 
on heating to redness no jerk or stoppage of the expansion, and on cooling 
no stop in the contraction was noticed, the permanent elongation being 
08 mm. or ;}; of the length of the wire. With the iron wire, which 
was considerably stouter (0°9 mm. diameter), of the same length and 
under the same load of 510 grams, the permanent stretching was 1‘8 mm., 
or about 54, of its length, so that the iron appears to be far more ductile 
and plastic than copper when both are at a red heat. 
Mr. H. Tomlinson! has, in fact, already published some interesting 
‘experiments on the enormous loss of rigidity which occurs in iron at 
the critical temperature. A torsionally vibrating iron wire has a 
logarithmic decrement at about 1,000° C., ten times greater than that of 
a tin wire at the temperature of the air, though tin has the highest 
internal friction of any metal yet examined at ordinary temperatures. 
Mr. Tomlinson finds two temperatures, one about 550° C. and the other 
about 1,000° C., when there is a sudden rise in the internal friction of 
iron. In a series of interesting papers communicated to the Physical 
Society, Mr. Tomlinson bas added much to our knowledge of the physical 
changes which occur in iron at the critical temperature. Mr. Tomlinson 
_ places near 1,000° C. the remarkable alterations he has observed in an 
_ iron wire under stress or strain. At this temperature ‘when stretched 
by a slight weight it suddenly unstretches, when under a slight bending 
stress it suddenly unbends, when under a slight twisting stress it suddenly 
“untwists, whilst on the contrary, if it has been previously bent or twisted 
permanently and then released from stress, it suddenly bends more or 
twists more as the case may be.’? Opposite changes occur in cooling. 
One of us* has suggested that the probable explanation of the effects 
observed by Mr. Tomlinson is due to the difference in the rate of heating 
and cooling between the interior and exterior of the wire. Such is 
» Phil. Mag., Feb. 1888. 2 Phil. Mag., Sept. 1887. 
* Newall: Phil, Mag., Nov. 1887. 
