600 MR. T. WRIGHTSON ON IRON AND 
Ali that was necessary was to heat by electricity in the electric welder a continuous 
bar until the position of the spot of light on the scale made it certain that the tempera- 
ture was within the welding limits in the neighbourhood of the thermo-junction. 
The pressure was then applied. 
When the bar was raised to welding heat inside the cylinder of porcelain, the 
pressure was applied. The time of its application is marked P on the photographic 
eurve in each case. The position of this point was indicated by a signal made by 
momentarily obscuring the light which feli on the galvanometer mirror, thereby 
producing a brief interruption to the continuity of the curve. 
These signals could be varied at will and proved to be of great service (see 
Plate 1, figs. 3, 4, 4a, 5, 6 and 7). 
The current which heated the bar was not entirely cut off. This was done to avoid 
any fall of temperature before the pressure was put on, and in some of the diagrams 
it will be seen that it has not always been possible to avoid a slight and uniform rise, 
which merely means that the current of the electric welder did something more than 
counterbalance the loss of heat by radiation. 
The pressure was applied when the temperature of the bar was 1347° C. (see Plate 1, 
fig. 3). A sudden rise of 27° C. showed itself, but a rise in temperature might be 
expected to take place, until all interstices between the outside of the bar and the 
envelope were filled by the plastic iron. The porcelain of course cracked, and the 
minute cracks had also to be filled before the effect of pressure in producing a fall in 
temperature could be demonstrated. 
The second time the bar was heated to 1371° C., and on pressing a rise of tempera- 
ture equal to 8° C. was shown (see Plate 1, fig. 4). 
This smal] rise appeared to indicate that the interstices were nearly, or quite filled. 
The third time the same bar was heated to 1420° C. and pressed, a distinct fall of 
27° C. was indicated by the diagram, thus for the first time realizing the anticipation 
with which I began the investigation (see Plate 1, fig. 5). 
The fourth time the bar was heated to 1400° C., when a pressure of about 1200 
pounds per square inch of the area of the bar, or 80 atmospheres, gave the remarkable 
fall of 57° C. (see Plate 1 fig. 6). 
The fifth time the bar was heated to 1300° C., a much lower temperature than the 
last, when, on being pressed, the fall in temperature was recorded as 19° C. (see 
Plate 1, fig. 7). 
If the bar experimented on be examined, a number of vein-like protuberances will 
be seen, showing that the plastic iron had to fill the crevices in the cracked porcelain, 
before the pressure could cause a fall of temperature. 
It appears from the series of experiments on this bar, that the limits of temperature 
within which the thermal expansion is negative, certainly include a range between 
1300° C. and 1420°C., and they may extend both above and below these limits, 
although, the fall being only 19° C. at the lower temperature as compared with 57° C. 
at the higher, it looks as if the lower temperature were approaching the limit, 
