1891.] Mr Parker, On Contact- and Thermo-Electricity. 281 



experiments of Brown and Pellat on the effects of reducing the 

 pressure of the air, we conclude that within attainable limits of 

 pressure, AjZ, the electromotive force of contact of the metal 

 and air is nearly independent of the pressure of the air. It will 

 now be shewn directly from theory not to be unreasonable to 

 suppose that this may be the case. 



If, for shortness, we take AjZ to be a volt, or 10 8 times the 

 electromagnetic C. G. S. unit, and suppose the distance, d, of the 



two electric layers to be „ .,,-, cm., the attraction between the 

 J 3 x 10 



metal and the air per square centimetre will be 



1 (A/Z\* 



or about 40,000 x 10 6 dynes. Hence since the pressure of one 

 atmo is only about 10 6 dynes per square centimetre, we can partly 

 understand why the reduction of the pressure to 1 cm. or 2 cms. of 

 mercury produces so little effect. 



VOL. VII. pt. v. 22 



