472 



Mr. J. E. Petavel. An Experimental 



is formed by a point or line of very high intrinsic brilliancy rotating 

 at a speed of from 50 to 450 revolutions per second. 



The existence of the above mentioned phenomenon forms a serious 

 objection to the use of the electric arc as a primary standard, but it 

 does not, per se, render its use impossible.* Finally, the variation of the 

 intrinsic brilliancy of the crater is a question which in itself offers con- 

 siderable interest. 



The points on which the present work bears are threefold — 



1. What is the average intrinsic brilliancy of a normal (silent) 



arc ? 



2. When the conditions are carefully specified, are the variations still 



too great to allow of the use of this source of light as a 



standard ? 



3. What variations can be obtained by the use of excessive currents 



and current densities, and by surrounding the arc with an en- 

 closure maintained at a very high temperature 1 



To obtain consistent results the observations must be made on a very 

 small area selected from the central portion of the crater. 



The diaphragm used for this purpose is shown in fig. 3. The 

 opening, d, is 1 "47 sq. mm. in cross-section, f The diaphragm is 

 shaped much like the tuyere of a blast furnace, and being kept cool 

 by a water circulation it can be placed within a very short distance of 

 the crater of the arc. 



The next question involved was the determination of the best relative 

 position of the carbons. In the usual arrangement of the arc the centre 

 of the crater is hidden by the negative carbon. A modification of this 

 arrangement, used by Blondel, which consists in slanting the carbons 

 and placing the positive slightly behind the negative, was found not 

 to be entirely satisfactory. An attempt was made to take the observa- 

 tions through a hole drilled out of the negative carbon (see fig. 1), but 

 when the arc was started this hole became filled with mist, and the 

 plan had to be given up. 



Fig. 2 shows diagrammatically the next arrangement which was 

 tried. The carbons %i 3 ?i 2 , %, are negative, and form the edges of an 

 equilateral pyramid, the axis of which is in the prolongation of the 

 positive carbon P. The summit of the pyramid is at the point P. 

 The crater formed on the positive carbon by these three arcs is in 



toI. 58, p. 174, 1895 ; vol. 60, p. 377, 1897 ; see also the account of the discussion 

 on this subject on the 19th February, 1897, at the Societe Francaise de Physique. 



* On this subject see Captain Abney, ' Journ. of the Inst. Elec. Eng.,' vol. 28> 

 p. 443, 1899. 



f It has been shown by Professor S. P. Thompson (see ' Phil. Mag.,' vol. 37, 

 p. 120, 1893) that when diaphragms of very small diameter are used the thickness 

 of the plate in which the aperture is pierced introduces a serious error in the 

 measurements. This difficulty was avoided by counter-sinking the opening. 



