132 I>R E. H. ORIFFITHS AND MR. EZER GRIFFITHS ON THE 



For preliminary experiments a 10-ohm coil of constantan wire was used, but was 

 replaced in the final form of apparatus by a 20-ohm coil of bare manganin wire, as it 

 was essential to eliminate, as far as possible, sources of thermo-electric forces in the 

 potential circuit. 



The diameter of the circle in which the wire was formed was approximately 7 mm. ; 

 the number of turns being 59. The upper end of the wire coil was situated about 

 20 mm. below the surface of the block ; two straight manganin leads (l mm. diameter) 

 projecting from the coil terminated at their upper extremities at the junctions with 

 the current and potential leads. 



Both potential and current leads were of manganin, the latter being 1 mm. 

 diameter, and to further diminish the heating effect of the current, two leads were 

 connected in parallel. Thus six leads extended up the central quartz tube to a 

 distance of 30 cm. These leads were insulated by perforated mica discs. A solid wad 

 of such discs was fixed between the top of the rack and the junctions to the current 

 and potential leads, in order to diminish the passage for convection currents. 



As the resistance of the coil had to be observed in situ at each temperature and at 

 frequent intervals, four brass cups, amalgamated inside and containing mercury, were 

 soldered on the current and potential leads outside the apparatus ; plugs enabled us 

 to isolate these circuits, when a resistance had to be taken, from the various 

 connections to battery, &c. Heavy leads from a dial resistance box terminated in a 

 pair of brass cups alongside those above referred to. 

 If 



R be resistance of coil, 

 r, and r a current leads, 



a and r 4 potential leads from cups to coil, 

 then, if 



we have 



N, = r l + r 2 , 

 N< = r 3 +r 4 , 



As the absolute value of R was required, the resistance of a reference heating coil 



f the same construction and about the same value as the one used) was determined 



by means of the dial box, and then forwarded to the National Physical 



tory, where its value was determined in international ohms ; on its return we 



d our previous determination by the dial box. 



This enabled us to reduce our determinations of the resistances of the heating coils 

 used in the work to international ohms. 



