When the variations in the pressure upon the transmitter diaphragm in either hand set varies the resistance of the corresponding branch circuit a slight variation in the current from the battery is produced. The internal resistance of the battery is sufficient to produce a slight variation in its terminal voltage. The resulting variations in the line voltage, and hence in the drop across the receivers, produce the usual vibrations in the receiver diaphragms. These variations also produce slight variations in the current through the primary winding of the induction coil, resulting in greater variations across the terminals of the secondary winding. Since the secondary winding is in series with the battery, the practical effect is to amplify the variations in the line voltage, and hence in the talking currents. Successful working of the relays is obtained only by a careful adjustment of the screws which regulate the throw of the armatures. The relay “A” is located in front of the “+” battery post, the relay “C” in front of the “G” post. In addition to the above matériel there are necessary for the mine system certain electrical instruments, as well as tools, appliances, and supplies requiring no special description, which are enumerated in the supply list. (Appendix 8.) Figures 17a and 17b, at the end of the book, show the construction of an improvised mine target. CHAPTER III. LOADING ROOM DUTIES. =Making a telegraph joint.=—The insulation is removed from the ends for 1½ inches and the wires brightened. The ends to be joined are placed across each other about one-third distance from the insulation, making an angle of about 45° with each other. The wires are grasped firmly at the junction and each free end wound tightly around the other wire for four turns; the winding should be in opposite directions. The ends of the wires are trimmed down so they will be smooth and present no sharp points. When wires are joined with brass jointers three-fourths inch of each wire is bared and the wires are inserted in the jointer; each end is crimped with pliers in the direction of the longer axis; the rest of the jointer is crimped and the ends or sharp points rounded off. When brass jointers are used care should be exercised not to crimp them too hard, as the wires may be partly cut through and finally broken. Special care must be used with the fuse leads, as the secondary circuit of the mine transformer can not be tested after the compound plug is assembled.