The circuit is as follows: From the negative D. C. bus on the power panel, to switch 7 closed up, through the mil-ammeter and its protective lamp, to the terminal bar, to the M-AM lead, to the plug, to the upper point of the power switch P, through the solenoid, to the middle of the testing switch T, to the upper point of same, to the upper point of the automatic switch, to the middle of same, to the mine switch, through same, to the terminal bar, through the 19-conductor and the single-conductor cables, to the mine transformer primary, to the mine case, to ground, to the D. C. “earth” terminal on the power panel, to switch 7, and to the positive D. C. bus on the power panel. With from 80 to 110 volts these readings should normally be between 30 and 40 mil-amperes. A mine may be fired if the reading with 80 volts is between 14 and 120 mil-amperes. These limits increase with the testing voltage. If the mine tests within the firing limits, the solenoid should be adjusted if the current is above its normal setting (0.075 amperes). If the test indicates that the mine can not be fired, the mine switch should be opened. 4. =Test of the automatic switch, red lamp, and bell.=—Throw the D. C. power on the busses of the operating board by closing switch 3 up. Open the bell switch. Next close the testing switch down on the mine block under test. The red lamp should glow and the corresponding automatic switch trip. (For circuit see fig. 18.) Now close the bell switch, throwing the bell in parallel with the red lamp; the bell should ring. Next open the bell switch and repeat the test for each mine block in turn. 5. =Test of the alternating circuit.=—This circuit is tested with D. C., as follows: Connect the A. C. and D. C. jaws on the master block with a jumper, open the power switches, close switches 3, 8, and 9 up on the power panel. The green and white lamps of the operating board under test should glow. A break or an excessive resistance in the casemate grounds, or elsewhere in the circuit, will be indicated by the lamps not glowing, or glowing dimly.