From A. C. bus on power panel to switch 8 closed up, to “operating board” terminal, to A. C. lead, to A. C. post on signal block, to white lamp and resistance in parallel, to A. C. jaw, through firing switch, F. S., to A. C. bus on operating board, to lower point of automatic switch when it is closed down, to middle point of automatic switch, through mine switch to terminal bar, through 19-conductor and single-conductor cables, through mine transformer primary, to mine case, to ground, to A. C. “earth” terminal on power panel, to switch 8, and to other A. C. bus on power panel. The white lamp glows after the mine has been fired. =The mine block= (see figs. 4 and 18).—This consists of an enameled slate block, 6 inches wide and 9½ inches high, on which are mounted four switches. 1. The upper switch is the “mine switch.” When it is open the corresponding mine is cut out and can not be fired. It is placed horizontally on the blocks of the old model and vertically on those of the new model. 2. The right-hand switch, a S. P. S. T. knife switch, is the “power switch.” When it is closed the D. C. power is on the block and the automatic switch will function when the corresponding mine is struck. When it is open the mine can be fired by raising the automatic switch release, thus tripping the automatic switch. 3. The central switch is the “automatic switch,” a single-pole double-throw switch, operated by the plunger of a solenoid. Through its lower arm there passes an insulated pin which, when the switch is down, makes connection between two contacts to the right and left of this arm. If for any cause the current through the solenoid rises above that for which it is set (normally 0.075 ampere), its plunger is drawn up and the switch is tripped. Such rise in current is produced when a mine is struck, the resistance through the circuit-closer circuit being far less than that through the primary coil of the transformer. Such would also be the case when a mine cable is grounded. When the automatic switch is tripped, the D. C. circuit to the mine is broken at its upper contact (see fig. 18) and D. C. circuit through red lamp and bell is made through the insulated through pin in the lower arm, thus giving warning. If at the same time A. C. power be on the busses and the firing switch on the master block be closed, A. C. will be thrown on the mine through the lower contact of the automatic switch, and the mine will be fired. Just above the plunger of the solenoid there is a red knob attached to the tripping bar of the automatic switch release. This enables the automatic switch to be released by hand in observation firing and in testing.