154 PHENOMENA, ATOMS, AND MOLECULES 



undergoes slight changes in resistance, and it was with this object, as well 

 as to test the purity of the gases, that the lamps were subjected to the 

 aging process. 



The lamps were then set up on the photometer one by one, and measure- 

 ments of current, voltage, candle power and color were made. About 

 30 to 50 sets of readings were taken with each lamp, raising the voltage 

 usually in steps of 2.5 to 5 volts at a time. The temperature was thus 

 gradually raised from 800° K., up to about 2900°. Then a series of points 

 was taken at descending temperatures and finally the temperatures were 

 raised to temperatures of 3000° and more, frequently repeating some of 

 the measurements at lower temperatures to see if the filament had under- 

 gone any change. The readings up to 3000° K. could be taken without 

 haste, as the filament undergoes only relatively slow changes below this 

 temperature. Above this temperature the readings were taken as rapidly 

 as possible, and at wider intervals (usually 10 volts). These precautions 

 were especially necessary with the lamps containing less than 100 mm. of 

 gas. In those containing nearly atmospheric pressure of either hydrogen 

 or nitrogen, there was never any perceptible darkening of the bulb, and 

 the loss of material from the filament was extremely small, even when the 

 temperature was raised several times very close to the melting point of the 

 filament. 



From the data thus obtained, the power consumption (in watts per 

 centimeter of length), the temperature, and the resistance (per centimeter) 

 were determined. The watts per cm. were plotted on semi-logarithmic 

 paper against temperature and smooth curves drawn through the points. 



Most of the points fitted very closely (within i or 2%) with the 

 smoothed curves, and it was only rarely, at very low and at very high 

 temperatures, that deviations as great as 5% were observed. 



The results of these experiments are summarized in Table I. The 

 energy radiated from the filament per second (in watts per centimeter of 

 length) is given in the second column under the heading W/,.. This was 

 obtained from the lamps with well exhausted bulbs. The resistance (in 

 ohms per centimeter) of the filament is given in the third column headed R. 

 These values represent the averages of all the lamps. There was no ap- 

 preciable difiference in R for the gas filled and the vacuum lamps. The next 

 six columns contain the data obtained from the bulbs containing hydrogen. 

 The figures give directly the energy carried from the filament by the gas. 

 the values tabulated being obtained by subtracting W^ from the observed 

 total power consumption (in watts per cm.). 



The last five columns contain similar data obtained from the nitrogen 

 filled lamps. 



Sini]'le ol)servation of these figures shows the very striking difiference 



