598 MR. T. WRIGHTSON ON IRON AND 
and only affects a portion of the mass, so that methods which are available for 
measuring changes of volume in cast iron are inapplicable. 
The only method of proof left open was to show that pressing the welding masses 
is attended by a fall in their temperature. 
Unfortunately, fourteen years ago, when I made my earlier experiments, the 
investigation could not be continued as there was no suitable pyrometer with which 
such a delicate experiment could be carried out. 
The recent successful application to metallurgical research, by Professor RoBERTs- 
AusTEN, of a recording pyrometer,* led me to resume the investigation of the 
question. 
This pyrometer, which depends upon the use of a thermo-junction, consisting of a 
platinum wire twisted with another wire of platinum alloyed with 10 per cent. 
of rhodium, had been of service in the investigation of critical temperatures in various 
metals and alloys, and as Professor Roperts-AUsTEN offered to place his laboratory 
and appliances at my disposal, and to aid me by advice, the opportunity for 
conducting the new experiments was gladly accepted. 
The first experiments were made by placing the thermo-couple between the two 
welding faces of bars heated in an ordinary smith’s fire. The wires of the thermo- 
couple were carried from the smith’s shop to the pyrometer in the laboratory, but 
signalling was necessary to ensure the taking of the photographic record of the 
temperature at the exact moment the smith applied pressure, and it soon became 
evident that the arrangement was awkward. The results were far from being 
uniform, although on several occasions a distinct fall of temperature was apparent 
when welding was effected, which encouraged the expectation that with a more 
perfect system of work the true facts of the case would be revealed. 
After full consideration it was decided that the only satisfactory way to proceed 
would be to use the electric welding apparatus of THompson Houston with alternat- 
ing currents. The Electric Welding Company readily put one of their admirable 
apphances and a suitable dynamo at my disposal; the manager, Mr. ARMSTRONG, and 
the electrician, Mr. REF, assisted in installing it at the Mint, and helped in every 
way to make the arrangement effective. By the kindness of Mr. R. A. Hix, Super- 
intendent of the Operative Department, the composite alternating dynamo was driven 
from the Mint engines, and the conductors from the dynamo were carried over the 
intervening buildings to Professor Roperts-AusTEN’s laboratory, where they were 
connected to the electric welder. 
The wires of the thermo-junction, which was placed in contact with the surfaces to 
be welded, were carried from the welder, round the walls of the laboratory to the 
galvanometer placed inside the camera of the recording pyrometer. 
It may be well to point out that the deflection of the galvanometer mirror causes a 
* ¢Proc. Roy. Soce.,’ vol. 49, 1891, p. 347, and ‘ Proe. Inst. Civil Engineers,’ vol. 10, 1891-2, Part 4, 
where a full description of the pyromeier is given. 
