Horns were made of soft metal such as lead and held a glass ampule containing battery acid, usually potassium-bichromate. The lower end of the horn contained an electric battery minus the electrolyte.Contact with the horn broke open the acid container, energizing the battery which then heated a platinum wire in a mercury fulminate detonator, thus exploding the mine.At the start of WWII, the Germans developed a new magnetic trigger for mines- one based on the mine's sensitivity to the magnetic field of a ship passing nearby. As set out here, the design of such mines fortuitously fell into British hands, allowing them to develop countermeasures for such mines:
The British experienced a stroke of luck in November 1939. A German mine was dropped from an aircraft onto the mud flats of the Thames estuary during low tide. As if this was not sufficiently good fortune, the land belonged to the army, and a base with men and workshops was at hand. Experts were dispatched from London to investigate the mine. They had some idea that the mines used magnetic sensors, so everyone removed all metal, including their buttons, and made tools out of non-magnetic brass. They disarmed the mine and rushed it to labs at Portsmouth, where scientists discovered a new type of arming mechanism.The effort to defeat these magnetic mines took several forms. One such approach was to attempt to negate the magnetic signatures of metal ships through a process known as "degaussing," as described here:
The mechanism had a sensitivity level that could be set, and the units of the scale were in milligauss. Gauss is a measurement for the strength of a magnetic field, demonstrating how it went off before coming into contact with the ship. Using the detector from the mine, they were able to study the effect of a ship passing near it. A ship or large ferrous object passing through the earth's magnetic field will concentrate the field at that point. The mine's detector was designed to go off at the mid-point of the ship passing overhead.
A steel-hulled ship is like a huge floating magnet with a large magnetic field surrounding it. As the ship moves through the water, this field also moves and adds to or subtracts from the Earth's magnetic field. Because of its distortion effects on the Earth’s magnetic field, the ship can act as a trigger device for magnetic sensitive ordnance or devices which are designed to detect these distortions. The degaussing system is installed aboard ship to reduce the ship's effect on the Earth's magnetic field. In order to accomplish this, the change in the Earth's field about the ship's hull is "canceled" by controlling the electric current flowing through degaussing coils wound in specific locations within the hull. This, in turn, reduces the possibility of detection by these magnetic sensitive ordnance or devices.Modern warships have built-in degaussing systems as set out here:
The degaussing system aboard a naval vessel consists of coils of electrical cable in different locations within the hull, a direct power source for energizing the coils and a controlling system, via switchboards, to control the amount and polarity of current through the coils.But when the "magnetic influence" mines were discovered, neither warships nor vital cargo vessels were so equipped. Degaussing of these ships was accomplished in some cases for such ships when:
The purpose of degaussing is to counteract the ship’s magnetic field and establish a condition such that the magnetic field near the ship is, as nearly as possible, just the same as if the ship were not there. This in turn reduces the possibility of detonation of these magnetic-sensitive ordnances or devices.
“The degaussing system ensures safety for the ship and the crew,” said Electricians Mate Fireman Lacy R. Tippet. “It’s a great method to keep the ship from not being detected from harmful devices.”
"Thick bands of electrical wire, aligned with the main deck, were fastened around the length of the vessel. The wire was energized with an electric current that neutralized the ship's magnetism.The famous liner turned troop transport Queen Mary was so equipped, as the caption of a nearby photo points out:
Clearly visible ... is the girdle-like degaussing ring that surrounded the ship's hull during the war. This device was designed to "degauss" the ship so that magnetic mines could find no purchase on the steel plates.Another technique was to "wipe" the hulls of ships:
It was found that 'wiping' a current-carrying cable up and down a ship' hull  temporarily cancelled the ships' magnetic signature sufficiently to nullify the threat. This started in late 1939, and by 1940 merchant vessels and the smaller British warships were largely immune for the few months at a time until they once again built up a field. Many of the boats that sailed to Dunkirk were degaussed in a marathon four day effort by degaussing stations. (the footnote at  is here)See also How to Demagnetize a Submarine which describes the process for determining the magnetic field of a ship and acting to counter it:
How could the ships natural magnetism be determined? This was done by setting up a number of magnetometers(devices for measuring the strength of a magnetic field) in a line about 400 ft. long with the magnetometers 30 ft below the water about 15 feet apart These magnetometers were connected to leads which ran to a "house" on the shore which contained a fluxmeter for each magnetometer beneath the water. Each magnetometer would send its reading to its fluxmeter in the range house which would record its reading on a moving tape. The readings from all of the fluxmeters would be combined into one reading which was called the ship's "signature". From this signature we could determine what action to take to reduce the effective magnetism to as low a condition as possible. This could be done by adding turns or decreasing the number of turns in the degaussing cable that went just inside the skin of the ship. The same effect could be accomplished by raising or lowering the current flowing through the turns. An officer from the "degaussing station" would board the ship. The ship would run the range. The recommendation would be signaled to the ship and the officer on board would make the necessary adjustments, and then the ship would run the range again to see if further adjustments needed to be made.This work also describes the effort to make "deperming" portable during WWII:
The second method for demagnetizing ships was called "deperming". This method was used always for submarines. In this method no cables were placed inside the skin of the ship. Degaussing cables are large and go through all the bulkheads of the ship. This would weaken the integrity of the submarine to withstand great pressures and depth charge attacks. In "deperming" we used the-same method that a jeweler uses to demagnetize a watch. I called my friend, Ross Maddox, a retired jeweler, and asked him about demagnetizing watches. He said that he used a coil which he plugged in to a 110 volt outlet. He would slowly move the watch into the coil and then withdraw it slowly. He had a magnetic compass that he checked the watch to see if the compass would be deflected by bringing the watch close to it. Sometimes he had to place the watch back in the coil and repeat the process before the watch would not cause the compass needle to move,
The trick for deperming submarines was to get a coil large enough and a way of putting the submarine into the coil and back out again in an easy fashion.
About this time the higher ups decided that it would be wise to train a group of four officers and thirty enlisted men into a unit that could "deperm" submarines using portable equipment that could be deployed in any harbor and not confined to any one specific place such as Pier 33 in San Francisco Bay. As submarines required depermming about every six months, such a unit could be located in the South Pacific and move northward as the war proceeded against Japan. This could make the submarine fleet much more effective by not having to come to San Francisco or Pearl Harbor, or down to Australia to be depermed.The Navy also developed a class of ships for degaussing purposes, see here for discussion USS Magnet (ADG-9):
Later assigned to SevRon 6 to service, in close proximity, the striking forces as they moved closer to Japan, she established and operated degaussing ranges and provided facilities for inspection, calibration, and adjustment of shipboard degaussing equipment in forward areas, primarily for minesweepers. After the War ended, she operated with mine groups at Okinawa and, after 29 January 1946 at Sasebo, Japan as those groups cleared the waters to allow safe passage to both military and merchant shipping.Another example here. Judging by the few records located concerning the activities of such ships, it appears that the crews may not have had the sense of how important their work was...
Top photo is USS Ampere (ADG-11).
Other photos are of USS Surfbird (ADG-383), including what I assume are her degaussing cables.
Of course, no discussion of degaussing would be complete without a brief mention of the "Philadelphia Experiment", in which is has been alleged that a Navy destroyer was made to disappear in Philadelphia and teleport to Norfolk. Some claims are remarkable:
The genesis of the Philadelphia Experiment myth dates back to 1955 with the publication of The Case for UFO’s by the late Morris K. Jessup.Needless to say, the Navy disputes the accuracy of the claims of those who promote the teleportation story. See here.
Some time after the publication of the book, Jessup received correspondence from a Carlos Miquel Allende, who gave his address as R.D. #1, Box 223, New Kensington, Pa. In his correspondence, Allende commented on Jessup’s book and gave details of an alleged secret naval experiment conducted by the Navy in Philadelphia in 1943. During the experiment, according to Allende, a ship was rendered invisible and teleported to and from Norfolk in a few minutes, with some terrible after-effects for crew members. Supposedly, this incredible feat was accomplished by applying Einstein’s “unified field” theory. Allende claimed that he had witnessed the experiment from another ship and that the incident was reported in a Philadelphia newspaper. The identity of the newspaper has never been established. Similarly, the identity of Allende is unknown, and no information exists on his present address.
Some researchers have erroneously concluded that degaussing has a connection with making an object invisible. Degaussing is a process in which a system of electrical cables are installed around the circumference of ship's hull, running from bow to stern on both sides. A measured electrical current is passed through these cables to cancel out the ship's magnetic field. Degaussing equipment was installed in the hull of Navy ships and could be turned on whenever the ship was in waters that might contain magnetic mines, usually shallow waters in combat areas. It could be said that degaussing, correctly done, makes a ship "invisible" to the sensors of magnetic mines, but the ship remains visible to the human eye, radar, and underwater listening devices.Or, in a slightly more interesting variation:
Personnel at the Fourth Naval District believe that the questions surrounding the so-called “Philadelphia Experiment” arise from quite routine research which occurred during World War II at the Philadelphia Naval Shipyard. Until recently, it was believed that the foundation for the apocryphal stories arose from degaussing experiments which have the effect of making a ship undetectable or “invisible” to magnetic mines. Another likely genesis of the bizarre stories about levitation, teleportation and effects on human crew members might be attributed to experiments with the generating plant of a destroyer, the U.S.S. Timmerman. In the 1950’s this ship was part of an experiment to test the effects of a small, high-frequency generator providing l,000 Hz instead of the standard 400 Hz. The higher frequency generator produced corona discharges, and other well known phenomena associated with high frequency generators. None of the crew suffered effects from the experiment.And the need for degaussing today? See here, where it is noted that the sensitivity of magnetic sensors has increased, thus limiting the effectiveness of degaussing.
Still, degaussing saved thousands of lives and a salute should be given to those men who toiled in anonymity to make sure our ships were safe.