In such cases, the ship's gyrocompass sends a disabling signal that closes a solenoid valve to block mercury flow between the tanks.
2.
In some cases, records show, railroad workers have accidentally disconnected the warning system, or disabled signals during maintenance without providing alternate ways to warn drivers, like flagging them at the crossing.
3.
For many, the power interruption was only a momentary flickering, but in the subways the impact rippled through the system, as disabled signal lights brought whole lines to a halt at the height of the evening rush.
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