Efficient protection for war industries against sabotage is given by a tamper-proof wire fence that sends a signal to patrol stations or towers as soon as anyone approaches near enough to cause the slightest variations. It is virtually a fence with ears, thwarting all efforts to climb or jump over, dig under, or saw or cut through it. Even a dog or rabbit coming in contact with the fence sets off the alarm. Specially developed magnetic pickups using sensitive phonograph needles, are attached to the fence at intervals of 1,000 feet. These detect any vibrations which pass into the fence from the ground, and relay them to amplifiers located at remote points. A loud speaker enables the guard to hear and usually identify the nature of the disturbance; a relay closes, ringing a bell in the guard tower, and a pilot light also indicates that the system has been disturbed. If desired, a signal may be transmitted to a central station over the telephone system. Developed by the Du Pont company, and manufactured by Automatic Alarms company, the acoustic alarm fence can be installed over ground that is flat or rolling, rough or smooth. It remains in constant operation regardless of weather conditions, and cannot be outwitted even by experts acquainted with its construction. Fences around ordnance plants may be up to 20 miles long.