332 THE ELECTRIC TELEGRAPH. 



result as the foregoing, which regards the tube as a single 

 truncated cone, whose top and bottom are to each other as 

 the least and greatest sections, wherever these may occur in 

 the tube : but the difference, however slight, is in the right 

 direction towards the truth. Before the tubes are used, they 

 are furnished at the ends with glass angular pieces, made to 

 fit tightly by india-rubber packing or cement, and are 

 filled with mercury for receiving the thick terminals of 

 the measuring apparatus. These terminals are either of 

 copper amalgamated or of copper electro -nickeled and the 

 nickel amalgamated.* 



A correction for resistance which the current meets with 

 in passing between the ends of the tube and the mercury 

 cup, is made as follows : 



It is accepted that this resistance is equal to the resistance 

 of a hemispherical shell, whose inner radius is r the radius 

 of the bore of the tube and whose outer radius is very 

 great, and, therefore, to be regarded as infinite in comparison 

 with r. The resistance, d y, of a shell of the thickness d x, 

 and radius x, is expressed by 



dx 



* In using mercury resistances it is very necessary to amalgamate the ends 

 of the leading wires. If this precaution is disregarded, a condensed layer of 

 gas remains adhering to the ends, immersed in the mercury, although they 

 may be perfectly clean, and occasions a considerable resistance. The high 

 conducting power of copper recommends it for connections of this kind, but 

 it is doubtful whether the copper will not after a time dissolve in the mercury 

 and the amalgam entering the tube lessen its resistance. The best way to 

 avoid this is to cover the surface of the copper first, in the galvanic way, with 

 a thick coating of metallic nickel, and then to amalgamate the nickel. Nickel, 

 although, like platinum and iron, it is not easily amalgamated by the methods 

 which Gmelin and others give, may be readily amalgamated on its surface by 

 galvanic deposit. For this purpose a small quantity of a solution of nitrate of 

 mercury in a porous pot, is placed in a glass of acidulated water in which 

 a zinc plate is immersed. A wire from the zinc is attached to one end of the 

 terminal to be amalgamated, the other end of it being dipped into the mercury 

 solution. In the course of a few minutes a thick coating of mercury is 

 obtained on the end, which should be stirred in a vessel containing metallic 

 mercury, and then washed. 



