352 Iron and Steel at Welding Temperatures. [Feb. 21, 



presence of carbon, does not possess the property of welding. 

 Further, it was not practicable to experiment with wrought' iron in 

 the same .way as with cast iron on account of the difficulty of dealing 

 with that substance in its liquid form. 



Professor Roberts-Austen has, however, given metallurgical re- 

 search a recording pyrometer, and this has enabled the author to 

 resume the investigation at the Mint, where he had the advantage ot : 

 Professor Roberts- Austen's assistance and advice. The method 

 adopted was the heating of bars in an electric welder, and as soon as 

 the junction of the bars was at a welding temperature, end pressure 

 was applied by mechanical power, and the weld effected. 



The temperature at the point of welding was observed by placing a 

 thermo-junction at this point consisting of a platinum wire twisted 

 into a second wire of platinum alloyed with 10 per cent, of rhodium. 

 The electric current produced at the thermo-junction deflected a 

 galvanometer, which by means of a mirror threw a spot of light 

 upon a sensitised plate which moved by clockwork uniformly in a 

 direction transverse to the spot of light. This produced a curve, the 

 ordinates of which represented time and temperature. 



These curves appear to show that a molecular lowering of tempe- 

 rature took place immediately the pressure was applied to the bar 

 when in the welding condition. 



Photographic curves are exhibited which show that this fall in 

 temperature varied in these particular experiments from 57 C. to 

 19 C., according to the circumstances of temperature and pressure. 



This appears to prove that wrought iron at a welding temperature 

 possesses the same property of cooling under pressure which was 

 proved by Lord Kelvin to exist in freezing water, and on which 

 demonstration the generally received theory of regelation depends. 



The author distinguishes the process of melting together of metals 

 from that of welding. 



Either process forms a junction, but the latter takes place at a 

 temperature considerably below the melting point. 



The well-known and useful property of welding in iron appears, 

 therefore, to depend, as in the case of regelation in ice, upon this 

 critical condition, which exists over a limited range of temperature 

 between the molten and the plastic state. 



