Wires led out from the top and bottom of each tube, and were brought down to metal buttons arranged in a row along the front edge of the base-plate, as shown in the illustration of a mercurial rheostat (Fig. 1). Each tube represented a certain resistance--one or more ohms, as required. The outlet wire was attached to the button at one end of the row, and the inlet could be moved along from button to button, until the required amount of current was obtained. The mercurial rheostat was an expensive, cumbersome, and treacherous thing to handle; it was liable to break, and its weight often prohibited its use in places where the more convenient and easily handled German-silver rheostats are now in universal employment. Overheating the mercury in the columns caused it to expand, and sometimes, before the switch could be thrown open, an end would be forced out and the mercury would climb over the edge of the glass columns. All metals have a certain amount of resistance for electric currents, and some have more than others. German-silver, for instance--a metal made of a mixture of other metals with about eighteen per cent. of nickel (see Appendix)--is considered to be the best commercial resistance medium, while pure copper is regarded as the best commercial conductor. Unalloyed copper is universally employed for electric conductors of high voltage; but for telegraph and telephone work, galvanized iron wire is still used extensively. The finer the wire, the higher is its resistance, and the more resistant the metal, the greater are the number of ohms to a given length. To nine feet and nine inches of No. 30 copper wire there is one ohm resistance, while to No. 24--which is six sizes coarser--there is one ohm to thirty-nine feet and one inch. In many cases it is necessary to use the coarser wire and greater length, as the current would superheat or burn the fine wire, while the coarser would conduct it safely. For very high voltage and amperage--such as used in traction cars, in power stations, and in manufacturing plants--castings of German-silver are employed and linked in series. They are more easily handled than the coils of wire, and a greater number of them can be accommodated in a small space. [Illustration: _=Fig. 2=_]