the Mercury Unit with the British Association Unit. 25 



Tube V., against W.M. 10 b.a.u. and E 10 b.a.u. in parallel. 



Tube VI., against E 1 b.a.u. 



When coils were connected in parallel, their terminals 

 rested solidly on copper disks, about j inch in diameter, well 

 amalgamated and covered with mercury, in boxwood cups. 

 The rods connecting these cups to the bridge were short, stout 

 copper rods, whose calculated resistance was '000156 at 16° 0. 

 This is, of course, always taken into account. 



After cleaning the tube as described above, each end was 

 thrust through a perforated cork, which was then fitted into 

 an end-piece similar to those used by Lord Rayleigh, and 

 the outer surfaces of the corks were covered with melted 

 paraffin. The mercury used was new and was distilled in a 

 vacuum, the temperature of distillation being low. Before 

 filling the tube, the mercury was gently warmed and was 

 then poured into one of the end-pieces, the other end being 

 raised, in order to allow the column to flow in slowly. If 

 any specks or small bubbles were noticed on the sides of the 

 bore of the tube, the filling was discontinued and the tube 

 again cleaned. The terminal cups were about two thirds full 

 of mercury. These were corked, the tube laid in the notched 



Scale |-. 



A. Copper connecting Eod. 



B. Ice cup. 



C. Hard rubber terminal. 



D. Hard rubber. 

 T. Tube. 



partitions in the trough and well covered and surrounded 

 with crushed ice. About four hours afterwards, the cups 

 on the connecting rods (see figure) having been previously 

 filled with ice_, in order to have them cooled down before 

 placing them in position, the resistance measurements were 

 commenced and usually occupied about twenty minutes. 



The galvanometer used was an Elliott, having a resistance 

 of 1^ ohms. A difference on the bridge-wire, amounting to 

 one part in 100,000 of the resistance, being measured gave 

 readable deflexions on either side of the " balance" position. 



Three complete sets of observations were always taken 



