TRANSACTIONS OF SECTION A. 8U7 



The method employed for finding the above falls of potential at each of the 

 carbons was not original. It was first used by Lecher ten years ago, and has 

 since been employed by Uppenborn, by I'rofessor Silvanus Thompson, and by many 

 others. These, however, all used a third carbon placed in a stationary position in 

 the arc, while I considered that more accurate results could be obtained by bringing 

 the point of the third carbon up to touch the hottest part of the main carbon, and 

 observing the last P.D. registered by the voltmeter before it fell to zero. Very 

 accurate and definite results were obtained in this way compared with any I could 

 get by the ordinary method, but in interpreting these results it is necessary to take 

 into account the disadvantages under which all experiments made with a bare 

 carbon dipping into the arc labour. 



1. The third carbon may not take up the exact potential of the part of the arc 

 in which it is placed. 



2. If it does, it will bring all the parts of the arc that it touches to practically 

 one potential, which will be greater than the least, and less than the greatest of 

 the potentials that existed in that portion of the arc before it was occupied by the 

 e.xploring carbon. 



3. It repels the arc, and, therefore, makes its real length greater than its 

 apparent length. Hence the insertion of the exploring carbon usually increases 

 the P.D. between the main carbons by from a half to two volts. 



As regards the first disadvantage, it is, of course, possible that there is a 

 contact P.D. between the carbon vapour and the solid exploring carbon, but, even 

 it it exists, this P.D. is probably small compared with the P.D.'s with which we 

 are dealing, and I have, therefore, neglected it in the calculation of the equations. 



The second point appears to be much more important, for it is quite possible 

 that the variable part of the fall of potential at each of the carbons, given by 

 equations (1) and (2), may be mainly produced by the exploring carbon being 

 in comuiunication with the arc, not only at its tip, but also along a part of its 

 length. For whether the fall of potential at the carbon itself be a constant or 

 not, it is quite certain that the potentials of the other points at which the arc 

 touched the exploring carbon varied both with the current that was flowing and 

 with the length of the arc before tliat carbon was inserted. Hence a portion of 

 the variation in the values given bv equations (1) and (2) must have been created 

 by the use of a bare carbon in the arc, and it is at least possible that the whole of 

 those variations were created in the same way. 



Many attempts have been made to explore the arc with insulated conductors, 

 notably by Uppenborn, who tried wires embedded in clay, steatite, and glass tubes, 

 and had to abandon them all. I myself have tried asbestos coverings for the 

 oarbons, but the asbestos melted and fell in drops like metal. Now that the 

 importance of insulating the exploring carbon is so very apparent, however, it will 

 be worth while making a very great effort to hnd some insulating material that 

 will stand the heat of the arc, and will yet not have to be so thick as to disturb it 

 unduly. This I hope very shortly to do, and thus to completely solve the question 

 of the constancy of the drop of potential at the carbons of the arc 



:2. Some Experiments on the Effect of Pressure on the Tliermal Con' 

 ductivities of Hacks. By Dr. C. H. Lees. 



The experiments were made by the ' lamellar ' method, the rocks tested being 

 cut in the form of flat circular discs and placed between discs of steel, the tem- 

 peratures of which were indicated by thermometers placed in holes in them. Two 

 discs of rock between three discs of steel were used in each experiment, the central 

 disc of steel having embedded in it a coil of insulated wire through which an 

 electric current of known amount could be sent. Through the upper and lower 

 steel discs, which were hollow, a flow of water was maintained. Thermal con- 

 tact between steel and rock discs was improved by a thin layer of glycerine. The 

 results show an increase of conductivity as the pressure is raised from zero to 800 

 or 900 lbs. per square inch, which is very small in the case of granite and marble. 



