March 2, 1899] 



NATURE 



429 



(9) Platinoid wire obtained from Messrs. Glover. 



(10) German silver wire obtained from Messrs. Glover. 



(11) Reostene ' wire obtained from Messrs. Glover. 



(12) Manganin wire obtained from Messrs. Glover. 



5. The size of the wire used, except for (5) (7) (8) above, was 

 about No. 18 standard gauge. A piece of the wire was taken 

 and drawn through a draw plate till it was reduced to about 

 No. 24 standard gauge. This process of wire drawing subjects 

 the wire to longitudinal extension and to lateral compression. 

 Lord Kelvin in his experiments (" Mathematical and I'hysical 

 Papers," vol. ii. , and section 3 above) showed that thermo- 

 electric differences were in the same direction for longitudinal 

 extension and transverse compression. For drawn and undrawn 

 wires the direction of the current through the hot junction is 

 from itndrawn to drawn for copper, reostene, and lead, and 

 from drawn to undrawn for platinoid, German silver, and man- 

 ganin. The magnitude of the thermo-electric difference per 

 degree difference of temperature is given in the following table : 



6. The effect of hardening by twisting has been partially 

 tried. Thus two pieces of laboratory copper wire were taken, 

 and one was in successive experiments twisted i turn, 3 turns, 

 5 turns, 7 turns, 83 turns per cm. The wire with Si turns per 

 cm. got quite brittle, and broke when an attenipt was made to 

 put more twists into it. The twisted wire was then heated 

 red-hot by an electric current, and allowed to cool. Tliis 

 partially annealed it. 



The results are given in the following table : — 



Number of turns Theimo-electric difference between 

 in twisted wire untwisted and twisted copper wire 

 per centimetre. in mikrovolt per degree. 

 I 0-0054 



3 00223 



5 ■■• 0-0262 



7 0-0419 



8"S 0-0594 



8-5 and partially annealed ... 0-0345 



7. The effects of twist on the drawn copper wire were also 

 tried, and it was found that i, 2, 3 turns per cm. in the drawn 

 wire slightly diminished the thermo-electric diffeience obtained 

 between the undrawn wire and the drawn wire ; but that 4 and 

 5 turns per cm. in the drawn wire gave the same thermo- 

 electric difference as was found between the undrawn wire and 

 the untwisted drawn wire. 



8. The drawn and twisted copper wire was annealed by 

 putting a gradually increasing current through till it got red- 

 hot, and then, without breaking the circuit, the current was 

 gradually reduced till the wire was at the temperature of the 

 laboratory. Trying it in this condition along with the undrawn 

 and untwisted copper wire, the current through the hot junction 

 was found to be reversed, being from the drawn twisted and 

 annealed wire to the undrawn wire. The thermo-electric 

 difference was 0-0081 mikrovolt per degree. 



9. Similar experiments on platinoid wires as those described 

 in Section 7 on copper wires gave similar results. Thus 

 I, 2, 3 turns per cm. in the drawn platinoid wire diminished the 



istene belongs to the nickel steel gioup, with certain other metals as 



alloy. 



NO. 



1 53 1, VOL. 59] 



thermo-electric difference obtained between the drawn wire and 

 the undrawn wire ; but 4 and 5 turns per cm. in the drawn wire 

 gave the same thermoelectric difference (1-477 mikrovolts per 

 degree) as was found between the untwisted drawn wire and 

 the undrawn wire. 



10. The drawn and twisted platinoid wire was partially 

 annealed, and the thermo-electric difference between it and the 

 undrawn platinoid wire was thereby reduced from I -477 

 mikrovolts per degree to 0-567 mikrovolt per degree. 



11. A beginning has been made of determining the thermo- 

 electric differences between free wires and wires previously 

 permanently elongated i, 2, 3, iStc, per cent, bya simple longi- 

 tudinal stress ; also wires while (a) under stress, stretching 

 them within their limits of elasticity ; and {/>) under stress, 

 stretching them beyond their limits of ela.sticity. 



February 9. — "On the Recovery of Iron from Overstrain." 

 By James Muir, B.Sc, Trinity College, Cambridge (1851 

 Exhibition Science Research Scholar, Glasgow University). 

 Communicated by Prof. Ewing, P'.R.S. 



It is known that iron which has been overstrained in tension 

 — that is to .say, strained beyond the yield-point so that it suffers 

 a permanent stretch — possesses different elastic properties from 

 the same iron in its primitive condition. Ultimately, the material 

 is "hardened" by stretching, its elastic limit being raised and 

 its ductility diminished -, but first of all very imperfect elasticity 

 is exhibited, and the elastic limit may be found to be reduced to 

 zero. The material, in fact, assumes a semi-plastic state ; so 

 that a stress-strain curve obtained from a recently overstrained 

 bar of iron or steel, shows a marked falling away, even for small 

 loads, from the straight line which would indicate obedience to 

 Hooke's law. 



It is the recovery from this semi-plastic state induced by over- 

 strain to a condition of perfect or nearly perfect elasticity with 

 raised elastic limit, that is referred to in the title of this 

 paper. Such recovery is known to be effected by mere lapse of 

 time. 



This slow recovery of elasticity with lapse of time is first 

 illustrated in the paper by means of stress-strain curves obtained 

 at succeeding intervals of time. Recovery under continued stress 

 is next considered, and the marked hysteresis in the relation of 

 extension to load, exhibited by overstrained iron, is illustrated 

 by means of a closed cycle. It is then shown that by exposing 

 an overstrained specimen of iron or steel for three or four 

 minutes to a temperature of 100° C , a very perfect restoration 

 01 elasticity is effected ; in the case of semi-mild steel, a more 

 perfect restoration than was brought about by a fortnight's rest 

 at the normal atmospheric temperature. So moderate a tem- 

 perature as 50° C. is also shown to have a large influence in 

 hastening recovery from overstrain. 



It is next shown that by striking a recently overstrained 

 specimen with a hammer so as to make it ring, the material 

 of the specimen becomes less elastic. That is, the effect of 

 mechanical vibration is opposite to that of increase of 

 temperature. 



The influence of magnetic agitation was also tried, but with 

 a coil giving a field strength of 140 C.G.S. units at its centre, 

 no effect was found to be produced on the elastic condition of 

 recently overstrained material ; the process of recovery seemed 

 to be neither accelerated nor retarded. 



Compression experiments are also described in the paper ; an 

 instrument specially designed by Prof. Ewing having been em- 

 ployed to measure the small compressional strains. The semi- 

 plasticity of recently overstrained iron is thus shown, and the 

 effect of moderate temperature in restoring elasticity demonstrated, 

 by means of compression curvei. The lowering of the com- 

 pression yield-point which accompanies the raising of the 

 tension one (due to tensile overstrain) is also indicated. 



Physical Society, February 24.— Prof. Perry, F.R.S., 

 Vice-President, in the chair. —A paper by Mr. E. F. J. Love, 

 on the Joule-Thomson effect and its connection with the 

 characteristic equation, and some of its thermo-dynamical con- 

 sequences, was read by Mr. Watson. The author points out 

 that the results of the original Joule-Thomson investigation of 

 the thermal effects of fluids in motion has been utilised hitherto 

 almost exclusively for the one purpose of determining the re- 

 lation between various gas-thermometer scales and the absolute 

 scale of temperature. He proceeds to deduce further conse- 

 quences from those results, indicating the relation between the 

 formula assigned to the Joule-Thomson effect, regarded as a 



