Along the front of the base-board small metal contact plates, or binding-posts, are arranged (see Binding-posts, chapter iii.) and the wires led to them from the tubes, as shown in the drawing. The top or drop wires in the tubes are provided with metal buttons at the ends; or the end of the wire may be rolled up so as to form a little knob. The manner of connecting the wires was freely explained in the resistance-coil descriptions and may be studied out by examining the drawing closely. In this resistance-apparatus there are two ways of cutting out a medium--first, by lowering the wire in the tube so that both contact-points meet; and second, by cutting out the first tube altogether by connecting the incoming wire with the second binding-post. Then again the resistance may be regulated quite accurately by raising or lowering the wires in the liquid. For example, there is too much resistance if the current has to travel through all the tubes. If it is too strong when one tube is cut out, the wire in tube No. 1 is lowered so that the contacts are an inch apart. Then the more accurate adjustment is made by dropping the wire in the second tube, as shown in Fig. 18. The wires leading out at the top of the tubes are pinched over the edge to hold them in place. They should be cotton insulated and the part that is in the liquid should be coated with hot paraffine. The water may be made a slightly better conductor if a small portion of sulphate of zinc, or sulphate of copper, is added to each tubeful. [Illustration: _=Fig. 18=_] Hittorf’s resistance-tube is one of the oldest on these lines, and two or more of them are coupled in series, as described for this water-tube resistance; glass tubes are employed that have one end sealed with a permanent composition, as described for Fig. 18. A metallic cadmium electrode is placed at the bottom of the tube, and the tube is then filled with a solution of cadmium iodide one part and amylic alcohol nine parts, and then corked. A wire passing down through or at the side of the cork is attached to another small piece of metallic cadmium, which touches the top of or is suspended a short distance in the liquid. As the alcohol is volatile the cork cannot be left out of the tube, and the wire must be drawn through the cork with a needle so that no opening is left for evaporation. A number of these tubes may be made and coupled in series and the wires led down to the contact-points of a switch. Chapter VIII THE TELEPHONE