STEEL AT WELDING TEMPERATURES. 599 
spot of light to move horizontally across the plate in the photographic slide, which 
slide moves vertically upwards at a uniform speed controlled by clockwork. The 
spot of light thus describes a curve, the ordinates of which represent respectively 
temperature and time. By this means a photographic record is produced. A base 
line (also produced by photography) represents the zero of temperature. When the 
indications of the thermo-junction are calibrated the diagram affords a complete record 
of the changes of temperature in the material in contact with the thermo-junction. 
Plate 1, figs. 1 and 2, were the first taken, and some transient surprise was caused 
by finding that on the pressure being applied, by a hand lever acting on a pinion and 
rack, there was a rise instead of a fall of temperature. On reflection it appeared that 
this would certainly be due to the distortion and crushing up of the soft heated iron 
when the bars were pressed together at the welding temperature. Work was thus 
done, and the heat evolved masked any true fall of temperature. 
To get rid of this heat of distortion it was necessary to confine the bar when pressed 
in some rigid envelope, which, from the conditions of the experiment, must, because 
of the electric current used for the heating, be a non-conductor of electricity. 
After many attempts and failures the plan was adopted of placing the bar inside a 
close-fitting cylinder of porcelain, outside of which was closely fitted a strong steel die. 
Both porcelain cylinder and steel die were cut through the centre in a plane trans- 
verse to the axis. This plan enabled the wires of the thermo-junction to be led to the 
centre of the bar, where the maximum heat was evolved. The bar was perforated by 
a minute hole, into which the wires forming the thermo-junction passed from each 
side. 
Fig. 3. 



PORCELAIN 




































Pipe-clay insulators encircled the wires and rested in grooves between the faces of 
the transverse cut, through the die, so as to prevent contact between the die and the 
wire. 
After the temperature of welding had been noted on a series of photographic plates, 
there was no object in severing the bar so as to make a welded junction for each 
experiment, 
