One of Hitler's secret weapons, the "Luftmine" was an air dropped influence mine. The mine could be fitted with magnetic, acoustic or pressure sensitive ignition circuit. The Magnetic sensor was the first to be deployed during the early stages of the war, in November 1939 an early LMA model was dropped on a tidal mudflat of the river Thames. The British managed to make it safe and learn the mine's secrets so that countermeasures could be developed.
The magnetic sensor shown here was fitted to a later LMB type mine and is a further development of the sensor fitted to the mine defused by the British in November 1939. The operating principle is largely the same. I have linked it up with a pressure sensitive clock and counter mechanism to be able to demonstrate the arming and firing sequence of the LMB taking a series of photographs and movies. The movies are not very good, a bit shaky and out of focus, but hopefully it gives an idea of the operation of the fuse.
Let me also state that no explosives or other prohibited materials are involved in this setup. These are purely the electromechanical sensory and timing components generating low voltage electrical signals.
Here is the test setup on my work desk:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06439_zpsddc64cdb.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06439_zpsddc64cdb.jpg" border="0" alt=" photo DSC06439_zpsddc64cdb.jpg"/></a>
On the left is the pressure sensitive clock, in the center is the magnetic fuse, to the right is the target counter.
The clock was activated by the water pressure if the mine reached a depth of more than 5 meters. A little pin on the back side of the clock is pushed in by the water pressure acting against a diaprahm.
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The clock can be set to count down up to six hours or six days. A setting screw can be moved to either the T (Tagen) or S (Stunden) position for slow or fast running. The time delay was used to ensure that the mine was firmly settled and stable on the bottom of the sea. A trembling switch in the magnetic fuse mechanism will prevent it from operating, so the mechanism needs to be properly leveled and absolutely still to operate. The delay also helps to frustrate sweeping operations.
When the mine landed on the mudflats in 1939, the lack of water pressure would have prevented the clock from starting. The magnetic fuse was therefor still unarmed. There was no way of knowing this of course and the mine might still have been fitted with booby traps....
Here is a little clip of the clock counting down towards zero:
When the clock strikes zero, two electrical contacts close and a battery is connected to the fuse. This will start an arming sequence which sets the mine automatically to a preset sensitivity.
The sequence is started by blowing a fuse wire. With the fuse wire in place, the arming mechanism is locked and the magnetic balance switch is held in an open position:
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The closing of the switches in the clock causes the fuse wire to blow, releasing the setting mechanism and the magnetic balance switch:
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The fuse setting mechanism is another clockwork mechanism which is connected by a spiral spring to the magnetic balance switch. As the setting mechanism turns, it slowly adjusts the set-point of the magnetic balance switch towards the closing position.
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Watch the clip as the fuse is blown (on the left) setting off the round brass clockwork mechanism turning anti-clockwise:
A second fuse wire is fixed between two contacts on the top of the mechanism. This fuse wire is blown as the magnetic balance switch closes. This causes the direction of the setting mechanism to reverse, until it runs clockwise against a second contact on the right. The position of the second contact can be set, it effectively determines how far the magnetic balance switch is opened and therewith the sensitivity of the fuse. Once the second contact is closed, the fuse is life:
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(Note the 7.44 date on the capacitor, giving a late war manufacturing date for this particular fuse). Watch a clip of the alignment mechanism turning right until the second contact closes making the fuse life:
The signal from the magnetic balance switch is now fed to an air relay mounted on the back of the fuse:
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This air relay is like a galvanometer instrument constructed with static and rotating air coils. No iron core is used that could potentially influence the magnetic balance switch. Also note that the air relay is positioned exactly in parallel with the magnetic balance switch, again to avoid the magnetic field in the swith from unduly influencing the balance.
When a ship is passing overhead, the steel hull will produce a slight disturbance of the magnetic field, enough to move the sensitive magnetic balance switch. Note that on the above photograph, just to the left of the coils of the air relay, two balancing weigths for the magnetic balance switch can be seen. These are connected directly to the axis of the magnetic balance switch and can been seen moving when a magnetic disturbance is detected.
As soon as the magnetic balance switch closes the air relay is activated causing it to close which in turn activates a coil mounted under the magnetic balance switch ensuring that the magnetic balance switch remains firmly closed. The following clip shows the magnetic circuit firing. Best to watch the counterweights on the air relay move (Note that the air relay closes right at the start of the video, blink and you miss it.....). After about 10 seconds the power is cut and the relay resets to the rest position:
The firing pulse is now passed to the target counter; he target counter is another clockwork mechanism triggered by a small electromagnet. When triggered the clock will run for two minutes and advance the counter wheel one position. A pin can be screwed into one of twelve positions, presetting the number of "free-passes" the mine will allow before exploding. The target counter will switch off the power to the fuse so that the relay and the magnetic balance switch are released. The two minutes running time should ensure that the ship has passed and that the magnetic field is back to steady state:
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If the counter has reached it's preset number of targets (here set to five), a contact will close connecting the magnetic mechanism to the actual explosive fuse. The next magnetic disturbance will cause the mine to explode.
In this test I trigger it with a screwdriver, the fuse is sensitive enough to sense the screwdriver about 50 cm away! The operating depth of the LMB was from 5 to 35 meters, I can well believe that the fuse will sense a ship from this distance.
The vertical tube mounted on the fuse countains a trembling switch. If the magnetic fuse is not positioned horizontally (it was mounted in a gimbal, so should automatically return to the horizontal position) or if the mine is interfered with (e.g. during mine sweeping operations) this trembling switch will close. This will result in the air relay to be move to the fully open position. Apart from preventing the fuse to fire this will close another switch that energises a second coil under the magnetic balance switch to hold it fully open. Any mechanical tampering with the mine will so cause the magnetic fuse to be held in an inactive position, only after the trembling switch has returned to rest will the fuse activate again.
The recovery and dismantling of the LMA type mine in November 1939 give the allies the opportunity to develop countermeasures against the magnetic mines. Methods were developed to "degauss" ships and special mine sweeping methods were developed to safely explode the mines. They now knew that trawling or using explosives was ineffective against these mines due to the trembling mechanism. Only by generating a fake magnetic field (by passing a strong current through the seawater, using a so called J-sweep) could these mines be swept. They later also learned to perform at least 12 sweeps with more than 2 minute intervals to deal with the target counter.
Once the fuse mechanism is activated, it draws no current from it's battery, meaning that the mine could remain operational for a long time. Today however the batteries of any surviving LMB mines will have long expired, meaning that the magnetic fuse mechanism is no longer a threat. LMB's could however be fitted with mechanical booby traps, meaning that even today these mines should be treated with utmost caution. The LMB carries 750 Kg of high explosives, not to be messed with!
regards,
Funksammler
The magnetic sensor shown here was fitted to a later LMB type mine and is a further development of the sensor fitted to the mine defused by the British in November 1939. The operating principle is largely the same. I have linked it up with a pressure sensitive clock and counter mechanism to be able to demonstrate the arming and firing sequence of the LMB taking a series of photographs and movies. The movies are not very good, a bit shaky and out of focus, but hopefully it gives an idea of the operation of the fuse.
Let me also state that no explosives or other prohibited materials are involved in this setup. These are purely the electromechanical sensory and timing components generating low voltage electrical signals.
Here is the test setup on my work desk:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06439_zpsddc64cdb.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06439_zpsddc64cdb.jpg" border="0" alt=" photo DSC06439_zpsddc64cdb.jpg"/></a>
On the left is the pressure sensitive clock, in the center is the magnetic fuse, to the right is the target counter.
The clock was activated by the water pressure if the mine reached a depth of more than 5 meters. A little pin on the back side of the clock is pushed in by the water pressure acting against a diaprahm.
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06440_zpsdee7a3df.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06440_zpsdee7a3df.jpg" border="0" alt=" photo DSC06440_zpsdee7a3df.jpg"/></a>
The clock can be set to count down up to six hours or six days. A setting screw can be moved to either the T (Tagen) or S (Stunden) position for slow or fast running. The time delay was used to ensure that the mine was firmly settled and stable on the bottom of the sea. A trembling switch in the magnetic fuse mechanism will prevent it from operating, so the mechanism needs to be properly leveled and absolutely still to operate. The delay also helps to frustrate sweeping operations.
When the mine landed on the mudflats in 1939, the lack of water pressure would have prevented the clock from starting. The magnetic fuse was therefor still unarmed. There was no way of knowing this of course and the mine might still have been fitted with booby traps....
Here is a little clip of the clock counting down towards zero:
When the clock strikes zero, two electrical contacts close and a battery is connected to the fuse. This will start an arming sequence which sets the mine automatically to a preset sensitivity.
The sequence is started by blowing a fuse wire. With the fuse wire in place, the arming mechanism is locked and the magnetic balance switch is held in an open position:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06441_zps6fed18c2.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06441_zps6fed18c2.jpg" border="0" alt=" photo DSC06441_zps6fed18c2.jpg"/></a>
The closing of the switches in the clock causes the fuse wire to blow, releasing the setting mechanism and the magnetic balance switch:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06452_zpsab05aff6.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06452_zpsab05aff6.jpg" border="0" alt=" photo DSC06452_zpsab05aff6.jpg"/></a>
The fuse setting mechanism is another clockwork mechanism which is connected by a spiral spring to the magnetic balance switch. As the setting mechanism turns, it slowly adjusts the set-point of the magnetic balance switch towards the closing position.
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06442_zps2f42087c.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06442_zps2f42087c.jpg" border="0" alt=" photo DSC06442_zps2f42087c.jpg"/></a>
Watch the clip as the fuse is blown (on the left) setting off the round brass clockwork mechanism turning anti-clockwise:
A second fuse wire is fixed between two contacts on the top of the mechanism. This fuse wire is blown as the magnetic balance switch closes. This causes the direction of the setting mechanism to reverse, until it runs clockwise against a second contact on the right. The position of the second contact can be set, it effectively determines how far the magnetic balance switch is opened and therewith the sensitivity of the fuse. Once the second contact is closed, the fuse is life:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06454_zpse1e2a847.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06454_zpse1e2a847.jpg" border="0" alt=" photo DSC06454_zpse1e2a847.jpg"/></a>
(Note the 7.44 date on the capacitor, giving a late war manufacturing date for this particular fuse). Watch a clip of the alignment mechanism turning right until the second contact closes making the fuse life:
The signal from the magnetic balance switch is now fed to an air relay mounted on the back of the fuse:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06464_zps9732c6ad.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06464_zps9732c6ad.jpg" border="0" alt=" photo DSC06464_zps9732c6ad.jpg"/></a>
This air relay is like a galvanometer instrument constructed with static and rotating air coils. No iron core is used that could potentially influence the magnetic balance switch. Also note that the air relay is positioned exactly in parallel with the magnetic balance switch, again to avoid the magnetic field in the swith from unduly influencing the balance.
When a ship is passing overhead, the steel hull will produce a slight disturbance of the magnetic field, enough to move the sensitive magnetic balance switch. Note that on the above photograph, just to the left of the coils of the air relay, two balancing weigths for the magnetic balance switch can be seen. These are connected directly to the axis of the magnetic balance switch and can been seen moving when a magnetic disturbance is detected.
As soon as the magnetic balance switch closes the air relay is activated causing it to close which in turn activates a coil mounted under the magnetic balance switch ensuring that the magnetic balance switch remains firmly closed. The following clip shows the magnetic circuit firing. Best to watch the counterweights on the air relay move (Note that the air relay closes right at the start of the video, blink and you miss it.....). After about 10 seconds the power is cut and the relay resets to the rest position:
The firing pulse is now passed to the target counter; he target counter is another clockwork mechanism triggered by a small electromagnet. When triggered the clock will run for two minutes and advance the counter wheel one position. A pin can be screwed into one of twelve positions, presetting the number of "free-passes" the mine will allow before exploding. The target counter will switch off the power to the fuse so that the relay and the magnetic balance switch are released. The two minutes running time should ensure that the ship has passed and that the magnetic field is back to steady state:
<a href="http://s672.photobucket.com/user/Funksammler/media/DSC06443_zpsbf650d5c.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC06443_zpsbf650d5c.jpg" border="0" alt=" photo DSC06443_zpsbf650d5c.jpg"/></a>
If the counter has reached it's preset number of targets (here set to five), a contact will close connecting the magnetic mechanism to the actual explosive fuse. The next magnetic disturbance will cause the mine to explode.
In this test I trigger it with a screwdriver, the fuse is sensitive enough to sense the screwdriver about 50 cm away! The operating depth of the LMB was from 5 to 35 meters, I can well believe that the fuse will sense a ship from this distance.
The vertical tube mounted on the fuse countains a trembling switch. If the magnetic fuse is not positioned horizontally (it was mounted in a gimbal, so should automatically return to the horizontal position) or if the mine is interfered with (e.g. during mine sweeping operations) this trembling switch will close. This will result in the air relay to be move to the fully open position. Apart from preventing the fuse to fire this will close another switch that energises a second coil under the magnetic balance switch to hold it fully open. Any mechanical tampering with the mine will so cause the magnetic fuse to be held in an inactive position, only after the trembling switch has returned to rest will the fuse activate again.
The recovery and dismantling of the LMA type mine in November 1939 give the allies the opportunity to develop countermeasures against the magnetic mines. Methods were developed to "degauss" ships and special mine sweeping methods were developed to safely explode the mines. They now knew that trawling or using explosives was ineffective against these mines due to the trembling mechanism. Only by generating a fake magnetic field (by passing a strong current through the seawater, using a so called J-sweep) could these mines be swept. They later also learned to perform at least 12 sweeps with more than 2 minute intervals to deal with the target counter.
Once the fuse mechanism is activated, it draws no current from it's battery, meaning that the mine could remain operational for a long time. Today however the batteries of any surviving LMB mines will have long expired, meaning that the magnetic fuse mechanism is no longer a threat. LMB's could however be fitted with mechanical booby traps, meaning that even today these mines should be treated with utmost caution. The LMB carries 750 Kg of high explosives, not to be messed with!
regards,
Funksammler
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