required to produce Sparks in Air and other Gases. 107 



determined by means of one of his recently constructed Abso- 

 lute Electrometers. It was found that the electrostatic force 

 per unit length diminished at first rapidly, with increasing 

 difference of potential, then slowly, and approached a limit the 

 value of which was supposed by Thomson to be about 130 

 C.G.S. electrostatic units per centimetre. 



In 1882 Bailie* published an account of some experiments 

 in which sparks nearly 1 centim. long were passed between 

 two plates. His results are expressed in the formula 



V 2 = 10500 (7 + 0-08) ZC.G.S. electrostatic units, 



provided I is not greater than 1 centim. 



Similar expressions have been given by Macfarlanef ; 



namely, for sparks less than 1 centim. long he deduces from 

 his observations, 



V = 66-940\A 2 + 0*20503 1 C.G.S. electrostatic units. 



The agreement between these expressions is not satisfactory ; 

 they give, for instance, for a spark 0'1 centim. long, values 

 which differ by about 10 per cent. 



In gases other than air very much less has been done ; 

 indeed no direct observations appear to have been made. 

 Macfarlanef compared the difference of potential necessary to 

 give a 0*5 centim. spark in air with the difference required to 

 give a spark of same length in several other gases. Calling 

 this difference unity for air, he finds for 



Carbon dioxide. Nitrogen. Hydrogen. Coal-gas. 



•951 -930 -634 -935 



From which it appears that hydrogen offers a much smaller 

 resistance to the disruptive discharge than any other one of 

 the gases experimented upon. 



Instruments. 

 The electrometer used in the present work was a very fine 

 instrument, made from designs of Prof. Rowland by T. Edel- 

 mann, of Miinchen, on the principle of Sir Wm. Thomson's 

 Absolute Electrometer. But it was much larger than, and 

 differed in other and important respects from, those generally 

 in use. One feature was the attachment of the movable disk 

 to the end of the arm of a balance, from the other end of 

 which depended, on knife-edges, a regular balance-pan. This 

 whole system was inclosed in a metallic case, through a hinged 



* Ann. de Chim. et de Phys. xxv. p. 486. 



t Trans. Roy. Soc. Edinb. xxviii. p. 636 (1877). 



J Phil. Mag. [5] x. p. 389 (1880). 



