6. =The range-finding system=, the same as or similar to that used for the guns, enabling accurate plotting of the positions of the individual mines, and consequently permitting vessel tracking and observation firing. 7. =The searchlights=, for illuminating the mine fields at night. 8. =The rapid-fire guns=, for the protection of the mine fields. CHAPTER II. MATÉRIEL OF THE SYSTEM. =The generating set.=—This consists of a D. C., shunt-wound generator driven by a kerosene oil engine, or of a direct-connected gasoline set. (For method of operation of a Hornsby-Akroyd oil engine, see Appendix 2.) =The storage battery.=—This is a 40-cell chloride accumulator, with a normal charge and discharge rate of 5 amperes. The voltage may be taken at 2 volts per cell; the internal resistance is negligible. Directions for setting up, care, and usage of the storage battery are given in Appendix 3. The 5-ampere battery is the standard equipment at the present time, but the new installations will have batteries with a normal charge and discharge rate of 15 amperes. =The motor-generator, D. C.-A. C.=—This is a D. C.-A. C. (60-cycle, single phase) machine, running on D. C. voltage (80-110) and designed to give one-half kilowatt at 80 volts. To insure against breakdown two of these motor-generators are supplied to each casemate. =Starting switch.=—This is a 4-point lever switch and is used to start the motor-generator and to accelerate it to full speed. To insure against breakdown two of these motor-generators circuit to the fourth point. Resistances are connected between the points, as shown in figure 1. The contact made at point 1 is not broken as the lever is moved to its successive positions. It is seen that the total resistance is 8 ohms; it is all in the armature circuit when the switch blade is in the first point; 4 ohms when in the second point; 2 ohms when in the third point; none when in the fourth point. The operation of closing the lever short circuits in turn the resistances 4, 2, and 2. =The casemate transformer.=—This is a step-up transformer, of the oil-insulated core type, and is rated at 60 cycles, 500 watts, 80 volts primary and 500 volts secondary, when carrying full load. [Illustration: FIG. 1.—Starting switch.] =The power panel.=—This panel is shown in figure 2, its wiring diagram in figure 18 at the end of the book. It consists of an enameled slate panel upon which the apparatus is mounted. It is 32 inches wide, 69 inches high, and is set up with its face 34 inches from the wall in rear. [Illustration: FIG. 2.—POWER PANEL.] Across the top are two lamps, a double circuit breaker, a D. P. D. T. switch, and a single circuit breaker. Below these there are an ammeter, an A. C. voltmeter, and a D. C. voltmeter. Below the ammeter is a battery rheostat and below the D. C. voltmeter a field rheostat. On a bracket at the side there is a mil-ammeter, with a 16 c. p., 110-volt lamp in series with it.