232 
ME. J. E. PETAVEL ON THE HEAT DISSIPATED BY A 
in all the experiments, but owing to two slight accidents the calibration is not 
quite the same throughout. The wire was calibrated four times during the course of 
the work. 
It is, of course, necessary to eliminate the cooling effect of the terminals, and to 
ensure this, the heat dissipated by the middle third of the wire was alone measured. 
In the experiments made in 1898 the radiating wire was placed vertically. At the 
pressure of one or two atmospheres that was then used, consistent results could be 
obtained by this disposition, and it had the advantage that the wire was an axis 
of symmetry with regard to the convection currents. At high pressures, however, 
owing to increased convection, it is impossible to keep a sufficient length of radiating 
surface at a uniform temperature unless the axis of the radiator is horizontal. For 
the reasons given in Part I. (see page 506 and Plate 20), the absolute value pf 
the emissivity is rather greater vdien the radiator is horizontal than when it is 
vertical. 
The pressures recorded were obtained from the readings of an air manometer, 
which was calibrated in the ordinary way by weighing mercury. The glass mano¬ 
meter tube ended in an air bulb, contained in a strong steel vessel resembling a small 
Cailletet bottle. The relative volumes of the air bulbs were regulated so that one 
instrument began reading at 7 atmospheres and the other at 100. The tubes were 
so arranged that the level of the mercury in the reservoir could be estimated, and 
the correction for the weight of the mercury column calculated. An ordinary 
Bourdon gauge was attached to each manometer for convenience of reference during 
the experiment. 
All joints and connections were made metal to metal without intermediate 
packing. Where high pressures are to be used, the importance of designing the 
connections in this manner cannot be exaggerated. A metallic joint once made will 
remain tight for any length of time. And on the other hand, if at any time it is 
necessary to change the connections, the joint can be broken and re-made in a few 
minutes. 
ExperimentaI Worh. 
The same routine was observed throughout in taking the observations. The 
enclosure having been filled with gas at about the required pressure, the water 
circulations were started. As soon as the temperature had become constant, the 
pressure of the gas, and the temperatures of the standard resistance, of the pressure 
gauge, and of the enclosure, were read, and any other observations that might be 
of importance, sucli as the barometric pressure, the temperature of the room, of the 
circulating water, &c., recorded. 
In all cases two readings of the current and electromotive force were taken, so as 
to cancel any thermo-electric effects by reversing the current. The differences in 
the readings shown in Table I. were due in great part to inequalities in the resistance 
