TRANSACTIONS OF SECTION A. SO-J 



TUESDAY, SEPTEMBER 13. 



A discussion was held in conjunction with the International Magnetic Con- 

 ference and Section G on the Magnetic and Electrolytic Actions of Electvic 

 Kailways. 



Communications were made by Dr. 0. Schott, I'rofessor A. AY. lllicker, Sec. U.S., 

 Dr. Eschenhagen, Dr. von Bezold, Mr. W. II. Preece, F.R.S., Signor Luigil'alazzo, 

 and Professor A. J. Fleming, F.K.S. See p. 758. 



The following Report and Papers were read r — 



1. Report on Electric Standards. 

 See Reports, p. 145. 



2. On Standard High Resistances. By F. B. Fawcett. 



The author finds that the electrical resistance of metallic films deposited on 

 glass in high vacua may be rendered constant by prolonged heating in oil under 

 reduced pressure. Resistances of a megohm and upwards liave thus been con- 

 structed which have not varied more than 0-001 per cent, in the four months since 

 they were finished. The most suitable alloy seems to be one of gold and platinum, 

 and for this the temperature coefficient is about 001 per cent, per degree Centi- 

 grade. The coefficient is, however, somewhat dependent on the thickness of the 

 film, being less for thin films than for thicli ones. 



3. On the Electric Conductivity and Magnetic Permeability of a Series 

 of New Alloys of Iron, By Professor W. F. Barrett, W. Brown, 

 and R. A. Hadfield. 



WEDNESDAY, SEPTEMBER 14. 



The following Papers were read : — 



I. The Drop of Potential at the Carbons of the Electric Arc. 

 By Mrs. Ayrton. 



The carbons employed were of the same kind as those used for all my previoits 

 experiments on the arc, namelj-, solid 'Apostle' carbons, the positive 11 mm. and 

 the negative 9 mm. in diameter. In order to eliminate errors caused by the differ- 

 ence in hardness or construction of the carbons, only those were used with which 

 it was found that the P.D. between the carbons was within half a volt of that 

 given by the equation which I published in 189o connecting that P.D. with the 

 length of the arc and the current flowing. 



To find the fall of potential between each of the carbons and the arc a third 

 carbon was used, varying in diameter between 0-5 mm. and 2 mm. This was 

 brought up to the crater of the positive or the white-hot spot on the negative car- 

 bon, and the P.D. between it and the main carbon was observed just before it 

 touched the main carbon. The P.D. was mea.sured by means of a high-resistance 

 d'Arsonval galvanometer having a resistance of a million and a half ohms in 

 circuit. 



Experiments were made with arcs of 1, 2, 3, 4, 5, 0, and 7 mm., and with cur- 

 rents of 4, 5, 6, 7, 8, 9, 10, 12, and 14 amperes. 



The drop of potential at the positive carbon, I find, is affected both by the length 

 of the arc and the value of the current, and the connection between this P.D. in 



