d8 line 12 through relay r2 line 14 diode d10 and line 16 back to battery ba2 upon energization of relay r2 its contacts r2a and r2b are closed and tape player 18 starts in the manner previously explained and its head will automatically change tracks through operation of solenoid sol and stepping relay sr1 which being in parallel with relay r2 is energized at the same time b relay r3 will also become energized through diode d24 as explained above and open its contacts r3b and close contacts r3a to connect the right channel of the tape player to the speaker system terminals tb9 and tb10 c since position 4 of the player track switch tsw is the only position receiving plus 24 volts via sw8 and diode d17 stepping relay sr1 will continue to step and move the player head by automatically energizing and deenergizing the solenoid coil sol until track 4 is reached when this occurs stepping relay sr1 will remain energized through the track switch contact rc and diode d16 and the player head will stay on track four of the tape and broadcast the message recorded on its second or right channel operation of the device to warn of a major hazard is as follows assume that sensors l and 1a are temperature sensors and one is so located in the mine as to be activated by a fire in the area switch sw1a is closed by the sensor leading 24 volt power from line 10 to line 23 to line 24 to relays br and r1 and through line 26 to the tone modulator tm to energize these parts the return current flow path being through wires 14.