. Manual of physico-chemical measurements. nc of brass pins is very soluble in mercury and makes it dirty,whereas copper is only dissolved in small quantity. Steel needlesare much cleaner, but the contact is not so good ; however, if copperis deposited on them electrolytically, they may be amalgamated andthen give certain contact. Useful mercury cups can also be made by fastening pieces ofblack india-rubber tubing 1 cm. long on wood or glass by means ofsealing-wax. The side of the tube is bored through at several placesbeneath the mercury surface with an awl; surface tension preventsthe mercur
. Manual of physico-chemical measurements. nc of brass pins is very soluble in mercury and makes it dirty,whereas copper is only dissolved in small quantity. Steel needlesare much cleaner, but the contact is not so good ; however, if copperis deposited on them electrolytically, they may be amalgamated andthen give certain contact. Useful mercury cups can also be made by fastening pieces ofblack india-rubber tubing 1 cm. long on wood or glass by means ofsealing-wax. The side of the tube is bored through at several placesbeneath the mercury surface with an awl; surface tension preventsthe mercury from running out of the holes, while the ends of wirescan be conveniently stuck through them. Several mercury cups can be arranged on the same board andconnected by bridges of copper wire, many sorts of apparatus beingmade in this way. The one mostly in use is the rocker (Fig. 150),which consists of six cups which can be connected in pairs by twothree-legged bridges insulated from each other. If the rocker is ELECTRICAL MEASUREMENTS 199.
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