Changes occurring in Iron Wire at a low red Heat. 473 



Royal Society, Mr. Gore kindly furnished me with his own ap- 

 paratus. By attaching to the movable cross piece a light 

 mirror, from which a brilliant ray of light was reflected to a 

 scale on a distant wall, the effect sought was not only vastly 

 magnified, but one or two new facts also revealed themselves, 

 (i) During the heating of the wire a slight and momentary re- 

 trogression of the beam was noticed at the temperature corre- 

 sponding to the powerful jerk that occurred on cooling : some 

 smaller tremblings of the beam were noticed at higher and lower 

 temperatures ; but these seemed due to irregular heating and 

 cooling, (ii) It was evident that the anomalous deportment of 

 the iron occurred approximately at the critical temperature 

 when iron undergoes its principal magnetic change. 



Mr. Gore having stated in a letter to me, written in May 

 1872, that he had no intention at present of making any 

 more experiments in the direction of his discovery, and adding 

 the subject was quite open to me, I felt at liberty to pursue 

 the inquiry thus suggested. It was not, however, till this 

 autumn that I could find the necessary leisure ; and the follow- 

 ing results were then obtained. 



My best thanks are here due to Professor Guthrie for his 

 hearty welcome to use his laboratory at South Kensington, 

 where the experiments have been conducted. 



I. 



Employing twenty Grove cells, I have had no difficulty in 

 obtaining this anomalous behaviour with moderately thick iron 

 wires. These have the advantage of allowing the effect to be 

 studied more leisurely, the phenomenon sought for occurring 

 several seconds after the interruption of the current. The tem- 

 perature at which the momentary jerk occurs seems to be lower 

 in thick wires than in thin ones, the critical point being a mode- 

 rately bright or cherry-red heat in thin iron wire, say No. 23, 

 and a very dull red heat in thick wire, say No. 20 ; the latter 

 wire is, in fact, in the stage just preceding obscurity when the 

 effect occurs. The internal temperature of the thicker wires is 

 no doubt masked by the cooling of their surface, whereas in thin 

 wires the cooling throughout is extremely rapid, and moreover 

 the transitions of temperature cannot be so well noted. 



II. 



With No. 21 hard iron wire I have had no difficulty in ob- 

 taining the jerk during heating. In this case the movement is 

 in the reverse direction of that which occurs during cooling; 

 that is to say, it indicates a momentary retraction, occurring, as 

 closely as can be judged, at the same temperature at which the 

 elongation takes place in cooling. 



