The circuit is as follows: From the negative D. C. bus on the power panel, to switch 3, to the “operating board” terminal, to the D. C. lead, to the D. C. post on the signal block, through the green lamp, to the D. C. jaw on the master block, through the jumper, to the A. C. jaw on the master block, through the white lamp and resistance in parallel, to the A. C. post on the signal block, to the A. C. lead, to the A. C. “operating board” terminal, to switch 8, to the A. C. bus, to switch 9, to the casemate transformer secondary, back to switch 9, to the other A. C. bus, back to switch 8, to the A. C. earth, through ground, to the D. C. earth, to switch 3, and to the positive D. C. bus on the power panel. With this circuit on, remove the 90-ohm resistance in parallel with the white lamp; the white lamp should glow more brightly, indicating continuity of circuit through the resistance as well as the white lamp. It will be observed that the above test is for only a part of the A. C. circuit. To test the firing switch and the lower contact of the automatic switch, open switches 3, 8, and 9, close 7 up, remove the jumper, put the M-AM “plug” on the A. C. jaw on the master block, close the firing switch, and trip in turn each automatic switch by raising the corresponding knob on the solenoid and observe the reading of the mil-ammeter. Close each automatic switch up before tripping the next one. The mil-ammeter reading should be from 30 to 40 mil-amperes, indicating a circuit through the firing switch and the automatic switch. 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 operating board terminal, to the M-AM lead, to the “plug,” to the A. C. jaw on the master block, through the firing switch F. S., to the A. C. bus on the operating board, to the lower point of the automatic switch which was tripped, to the middle of same, to the mine switch, through the 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” post on the power panel, to switch 7, to the positive D. C. bus on the power panel. 6. =Test of the delivery of the A. C. power to the operating board.=—_See that all the automatic switches of the operating boards are up and all the firing switches of the master blocks open._ Close switches 4 and 9 up (or down) and 8 down; close the testing switch T. S. on the master block. The white lamp should glow and the A. C. bus-bar voltage should drop appreciably.