ON THE Kl'KCIFKJ RESISTANCE OF MERCURY. 365 



by finding the difference between Flat and F directly, and then when Flat was con- 

 nected to the bridge by these copper pieces. In this way we found the resistance of 

 the connexions to be '00136 B.A. unit. The temperature was about the same as 

 that at which the connexions were used. In the values of Flat given in Table VIII. 

 the resistance of these connexions has been included. 



In all cases the temperature of the room was almost exactly the same as that of 

 the wuter baths. 



One of the ebonite cups into which the ends of the tubes opened is shown in figs. I 

 and 2, Plate 19, which is drawn to scale full size. In their design two points mainly 

 were attended to. The first was, that it should be possible to reduce the mercury in 

 them very nearly to C. ; the second, that there should be no contact between 

 copper and mercury, for BENOIT has shown that the conductivity of mercury is in a 

 very short time appreciably increased by contact with copper. 



The glass tube passes through an india-rubber cork, which fits into the terminal at 

 c, G! ; the tube was usually adjusted so that its end was flush with the inner surface 

 of the terminal. 



Mercury was then poured into the cup and allowed to run slowly through the tube 

 into the second terminal until each was about two-thirds full. The top shown in 

 fig. 1 was then placed over the terminal and secured by four small screw-bolts passing 

 through the flange a, a. When these were screwed down the terminals were com- 

 pletely water-tight, and could be left covered with melting ice or water for days 

 without leakage. 



The top consists of a flat plate of ebonite, with four holes to receive the bolts. 

 Through this plate two ebonite tubes, dd, ee (figs. 1 and 3) pass. A hollow platinum 

 cup, /, about 3 '5 cm. long by rather more than 1 cm. in diameter, is secured firmly 

 into the tube d, d, and a thick piece of copper rod, g, fits the interior of the cup 

 tightly, any interstices between the two being filled with mercury ; the surface of 

 the copper rod was well amalgamated. This copper rod, g, is brazed to the copper 

 rods, g ', which form the connexion with the bridge. Pieces of india-rubber tubing, 

 hh, kk, about 10 cm. long, are fastened over the upper ends of the tubes, dd, ee ; 

 the connexion to the bridge passes through a cork, which closes the upper end of 

 the tube, hh, and the junction is made water-tight with marine glue. A thermo- 

 meter, t, graduated to fifths of a degree Centigrade, passes through ee and the 

 india-rubber tube, kk, which fits it closely, and gives the temperature of the mercury 

 in the terminal. 



In taking the observations the ice was packed closely round the terminals up to 

 the tops of the tubes, h and k, so that the copper rods were surrounded by ice for 

 12 or 14 cm. above the level of the mercury. Contact between the copper and the 

 mercury in the terminals was thus established through the platinum cup, f\ the 

 surface of this is about 12 sq. cm. This surface was amalgamated in the following 

 manner : The cups were platinised by electrolysis from a solution of platinic 



