RESEARCHES IN CONTACT ELECIRICITY, 277 
EXPERIMENT III. (March 5). 
(Curve symbol x), 
Temperature, ; : 5 : 64:6 473 31 22°2 
Deflection, . 5 ; ; : oe 26 15'2 12 
EXPERIMENT IV. (March 6). 
(Curve symbol \ ). 
Temperature, ‘ : 4 ; 66°5 36°7 34:2 25 
Deflection, . J : i : 47°5 di 16 3 
EXPERIMENT V. (March 13). 
(Curve symbol A). 
Temperature, . : , 60°4 5d'd 38 30 23:°2 
Deflection, ; ; } 37 36 20 9°3 6 
The reduced means for curve VI. (symbol ©) are as follows :— 
Temperature, . : ; 63:1 54°6 45-7 33°4 23°2 
Deflection, ; ; : 38°5 28°9 22°4 149 72 
The temperature of the room, and therefore of the upper surface, was 
12°C., at which point then the curve should. meet the line of temperatures. 
The mean curve is obviously best represented by a straight line, whose tangent 
of inclination to the temperature line is—°76, expressed in diagram units. 
In seeking for an explanation of the results of these experiments, we 
must not neglect the possible effects due to surface oxidation, or to the 
change in density of the gas condensed upon the metallic surface. If the 
negative character of heated iron to cold iron disappeared on the cooling of 
the former, then the effect must be the result of some temporary change 
accompanying the heating—such for example as the mere change of temper- 
ature, or the driving off of the condensed gases at the higher temperature, 
or of both causes combined. Experiment, however, clearly proved that 
the originally heated surface, when cooled to the temperature of the colder 
surface, retained its strong negative characteristics with no appreciable diminu- 
tion; from which it would appear that the observed phenomena are to be 
attributed mainly to a permanent change of surface condition depending upon 
the temperature to which that surface has for a brief period been subjected— 
probably to oxidation, It was also found by trial that no appreciable increase 
in the deflection corresponding to a given temperature resulted when a 
considerable interval of time was suffered to elapse between the polishing 
of the heated surface and the making of contact between it and the upper 
and colder surface. Whether the instantaneous contact was made fifteen 
minutes (the usual interval) forty minutes, or sixty minutes after polishing, 
