HALL. CONDUCTIVITY OP MILD STEEL. 277 



in such a case the junction is pushed deep beneath the surface, the 

 observed effect is diminished and perhaps disappears. 



Accordingly it was necessary to get rid of the soldered junctions 

 upon the disk. Therefore the device was adopted of attaching the 

 copper wires to the copper surfaces by means of copper electrolyti- 

 cally deposited. The details of the operation, where one junction only 

 is to be dealt with, are as follows : Cover the surface of the disk, in 

 the neighborhood of the spot where the junction is to be, with a thin 

 layer of some non-conducting substance, paraffine for instance, and 

 then scrape off this coating from a patch about 0.5 cm. long and 

 0.1 cm. wide. Coat also, with a non-conducting substance, the copper 

 wire, and then scrape one end of it bare for a distance of about 1.0 cm. 

 Lay the bare part of the wire along the middle of the bare strip 

 on the copper disk and press it down flat in that position, the end of 

 the wire projecting a little beyond the limits of the bare strip. While 

 holding the wire firmly in place, attach it near each end of the bare 

 strip by melting at eacli spot a bit of paraffine. A few drops of sul- 

 phate of copper * are now placed upon the bare part of wire and disk, 

 above which they stand in a little mound, as in Figure 3. Another 

 copper wire, the lower end of which dips a little distance into the 

 mound of liquid, serves as the anode in the electrolysis. The exposed 

 surfaces at the bottom of the liquid are the cathode, the current 

 being cariied off, not through the thin wire, but through the body of 

 the disk, to another wire not here shown, the intensity of current 

 aimed at being 0.02 ampere per sq. cm. of the exposed surface of the 

 cathode. A run of three or four hours makes the junction sufficiently 

 strong for proper use, though perhaps it is safer to let the action go on 

 longer. After the deposit is completed the protruding end of the 

 fine copper wire is cut off and the paraffine surrounding the junction is 

 scraped away. When several junctions upon one side of the disk are 

 to be formed, it is easy to make the deposit upon all of them simul- 

 taneously. 



In the final use of the apparatus the copper wires must be kept 

 from contact with the copper surface of the disk at other points than 

 the legitimate junctions. Accordingly the wires and the disk were 

 coated with a thin layer of asphaltum varnish, baked on at a tempera- 

 ture above that to which the apparatus was to be exposed in the con- 

 duction experiments. 



* Of specific gravity 1.10, with 20 drops of strong H2SO4 added to 200 cu. 

 cm. of the solution. 



