﻿Disruptive Voltage of Thin Liquid Films. 317 



When (18) holds this simplifies still farther to the form 

 PS-QR E 



C = 



/(K + S) * K'S' ' 



in the first factor of which pure resistances only occur. Hence 



PS-QR 

 0_ Q(R + S) ' 



which shows that the curves for the two currents are exactly 

 of the same shape. The commutator positions at which the 

 galvanometer deflexions are reduced to zero in the two cases 

 will be consequently identical. 



XXXVIII. r lhe Disruptive Voltage of Thin Liquid Films 

 between J ridio- Platinum Electrodes. Part I. Voltages 

 25-100. Bj P. E. Shaw, B.A.,I).Sc* 



I^LECTRIO-TOUCH measurements can readily be made 

 J under oil. If a telephone be included in the circuit, 

 the sounds made in contact are sharp and clear for all the 

 liquids mentioned below. The}- even excel air in this respect. 

 This property of oils to render contact distinct has been used 

 in potentiometers and other instruments. The whole instru- 

 ment, or at least the slide-wire, is immerse 1 in paraffin oil (see 

 a paper by R. A. Lehfeldt and discussion following, in Phys. 

 Soc. Proc. p. 179, 1901-1903). This previous use of oils 

 suggested the present research. 



The relation of spark-length to small P.D. of the electrodes 

 has been experimentally tested for gases by Earhart (Phil. 

 Mao-. [61 i. p. 117, 1901), Shaw (Proc. Rov. Soc. vol. lxxiii. 

 p. 337, 1903), Kinsley (Phil. Mao-. May 1905, p. 692), and 

 Hobbs (Phil. Mag. [6] x. Dec. 1905). 



Earhart discovered the knee occurring in the curve at 

 350 volts. Shaw pushed the investigation further by using 

 small- P.D. down to | volt. Kinsley threw doubt on the con- 

 stancy of potential for a given gas and given electrodes. 

 Hobbs, working more exhaustively than his predecessors, has 

 added much definite and useful information. 



The discharge in gases must bear a close relationship to the 

 discharge in liquids, but little precise work has been done on 

 the latter, though Hughes (Proc. Inst, Elec. Eng. 1892), 

 Swinton (Discussion on the above p;iper), Steinmetz (Trans. 

 Amer. Inst. Elec. Eng. 1893), before 1903, and recently, 

 C. E. Skinner (Xat. Elec. Light Assoc, Boston, Mass., 1901). 

 * Communicated by the Physical Society : read June 22, 1906. 



