Mr. Thomas Andrews. [June 16,. 



The appearance of the spark-discharge changed with the pressure 

 in the usual manner. At ordinary pressures the discharge was a 

 bright white spark, gradually passing, as the pressure was lowered, 

 into a purplish glow. In the case of the flat and convex discs of 

 Part I, the purple discharge, at first central and narrow, gradually 

 spread out and at low pressures filled the space between the plates. 

 The appearance of the spark could not, however, be carefully 

 examined, as the circuit was always broken as soon as discharge 

 occurred in order to avoid damage to the surfaces of the plates. 

 When these were examined by breathing on them after a number 

 of sparks had passed at low pressures, it was always found that 

 the part of the surfaces most affected by the discharge was an annulus 

 at some little distance from the centre. With the flat plates used in 

 Part TI, the discharge was generally diffused over the space between 

 the plates. 



Observations of spark discharge between plates in air at different 

 pressures have been made by Warren de la Rue and H. Miiller* and 

 by Macfarlane.f Their results do not present the features to which- 

 attention has been called in the present paper, nor could they do so r 

 as the curves for potential difference and pressure for the spark- 

 lengths they examined would show no evidence of a minimum poten- 

 tial difference at the lowest pressures they considered. In both cases 

 the spark-length was about 0'13 inch, and the lowest pressure 20mm. 

 of mercury. 



In conclusion, I have to express my thanks to Professor J. J. 

 Thomson, to whom I have been indebted for advice and suggestions 

 at every point of this investigation. 



VII. " Electro-chemical Effects on Magnetising Iron. Part I V." 

 By THOMAS ANDREWS, F.R.S., M.Inst.C.E. Received May 

 16, 1892. 



Influence of Magnetisation on Corrosion of Steel. 



In connexion with, and during the progress of, the electrical portion 

 of my research on " Electro-chemical Effects on Magnetising Iron," 

 Parts I, II, and III (' Roy. Soc. Proc.,' vols. 42, 44, and 46), numerous 

 gravimetric experiments were conducted, with the object of investi- 

 gating the influence of magnetisation on the corrosion of iron and 

 steel. I selected as the corrosive fluid a solution of cupric chloride, 

 being partly guided in this choice by the results obtained with 

 solutions of this salt in the electrical portion of the investigation. 



# ' Phil. Trans.,' vol. 171, pp. 7582, 1880. 



t ' Trans. Boy. Soc. Edinb.,' vol. 28, p. 642, 1878. 



