396 



Mr. L. Schwendler on a 



0*0264 grm., having a calculated resistance =0*109 S. U. and 

 a measured resistance =0*143 S. U. at 66° F. ; gives the unit 

 for light-intensity *. 



Photometric Measurements. — Having now a constant light, 

 it became possible to measure the variations of light which the 

 combustion standards invariably show. 



For instance, one of Sugg's candles was compared with the 

 P. L. S. (A) with the result shown in the following table : — 



Distance in millimetres. 



Remarks. 



P. L. S. (A) 

 with 6' 15 webers. 



Sugg's candle. 



These readings were taken i- 1 g 

 in about five minutes. © W 



millim. 

 117 

 120 

 112 

 110 

 120 

 120 

 120 

 120 

 126 

 128 

 117 

 120 

 123 

 127 



The P. L. S. (A) was kept at the 

 same position =100 millims. 



Sugg's candle was moved in order to 

 get the light equal. 



The variations observed were actually 

 in the candle and not in the pla- 

 tinum standard, as the eye could 

 easily discern. 



This gives as an average : — 



1 Sugg's candle = 1-44 P. L. S. (A) with 6*15 webers. 



* In order to show that a platinum light-standard can easily be repro- 

 duced, I will give here some actual measurements : — 



The platinum sheet out of which the P. L. S. (A) was cut weighed 

 0-0364 gram per square centimetre. From this the weight of the part 

 which becomes hot, calculated, gives 0-0264 gram. The resistance of 

 the standard, measured at 66° F., gaye 0-143 S. XL, including contact 

 resistances. 



Now another piece of platinum sheet 26 x 28 millims. was found to 

 weigh 0*265 gram. • The piece cut off which actually becomes hot = 0*026 

 gram, which agrees within 0-0004 gram with the weight found by calcu- 

 lation for the P. L. S. (A) actually used. 



Taking the specific resistance of mercury =96190 I , nor . 



„ „ of platinum (annealed) = 9158 f. atu u *> 



S. TT. 



the calculated resistance of the platinum which becomes hot= 0-109 ) at 



measured resistance, including contact resistance =0'143 J 66°F.- 



or contact resistance probably =0*034 S.U. 



It is therefore much more accurate to define the P. L. S. by weight 

 than by resistance. 



