History of Detectors
Reprinted with permission from Modern Metal Detectors
Application of metal detectors to law enforcement and security situations has a surprisingly lengthy history. A number of instances have been recorded in which metal detection equipment was used by law enforcement personnel as a crime scene management and investigative tool. (As a reference see Karl von Mueller’s The Master Hunter Manual, 1973, Ram Publishing Company, now out of print). Metal detectors of both the walk-through and hand-held type are being used to an ever greater extent today in facilities of all types where threats of terrorism and violence are increasing at an alarming rate. Metal Detector applications are constantly being expanded and now extend into many fields. New detector advances, as well as improved operating techniques and knowledge, have increased interest in the metal detector field world-wide. Metal detectors are now an essential part of daily life in perhaps every country in the world.
Alexander Graham Bell, the inventor of the telephone, was working on an electrical induction device for locating metals in 1881 when President James A. Garfield was wounded in an assassination attempt. One bullet grazed his arm and a second lodged in his back. After attempts to locate this bullet failed and the President’s condition worsened, his doctors turned to Bell for help, asking him to bring his detector to the White House. Reports conflict concerning what happened next. One version relates that Bell was unable to perfect his instrument in time to locate the bullet. Another reports that he attempted to locate the bullet, but failed. Nevertheless, President Garfield died.
In 1890 test were made to locate sulfides through the medium of conductivity, using a telegraphic receiver connected in series with a battery and a wire brush. Electrical contacts were made in the earth, and a brush was then moved over the surface. Whenever it touched sulfides, the brush would complete the circuit, indicated by a click in the receiver. Since it could be used only on exposed mineralized surfaces, the method was of limited value. Further attempts at metal detection were made, using the Wheatstone bridge circuit for measuring resistance. Here again, conductivity was the determining factor, but the conductivity between two points on the earth’s surface had to be calculated indirectly by first measuring resistance. This method also proved impractical. Still another earth conductivity method was given considerable attention. Since electrical currents flowing through the ground cause electrical potential lines to be created, equal potential points across the ground could be measured by galvanometers and plotted. The presence of an ore body caused these lines to warp or distort. Although the method was somewhat successful, many variables were involved. In addition, water layers, areas of uneven moisture and other substances in the soil gave indications which could be misconstrued as indicating the presence of an ore body. Too, failure to indicate ore would not necessarily mean barren ground. The oxidized condition existing around sulfide ore bodies forms an almost perfect insulator that prevents accurate measurement.
Research on earth conductivity
methods is occasionally conducted. Shown on the following
page is a photo taken in 1963 of an experiment in which I
participated. This instrument involved was a crevice
detector, the brainchild of Dr. John C. Cook of Teledyne
Geotech in Garland. The closest that the early-day pioneers
came to the modern metal detectors was a method designed to
measure the distortion caused by magnetic fields generated
by an electrical conductor of very low resistance in the
earth, such as an ore body. Since this method did not
require use of any electrical contact on the ore or on the
earth, it avoided the problems caused by moisture and
Similar factors and was limited only by the short distance
in which the intensity of the magnetic field was effective.
Another promising method was that of induction balance,
which could detect the presence of gold as easily as
sulfides or other minerals. Its prime difficulty was in
obtaining the necessary depth. The idea of locating ore
bodies electromagnetically was perhaps first conceived by
Dr. Daniel G. Chilson in 1904 in Goldfield, NV. Early
experiments in conductivity of the earth, water and other
earth substances, determined that sulfides (conductive
sulfur) were the best conductors. In 1909 Chilson turned to
known radio transmission/reception techniques,
experimenting with short wave.
In 1925 an electrical gate checker was designed to help
factories cut down on rampant thefts of tools and products.
Its operation was based on the use of electromagnetic
waves. Two German physicists, Dr. Geffeken and Dr. Richter
of Leipzig, designed the original gate checker. Their work
was continued by Gebr. Wetzel of Leipzigplaqwitz. An
electromagnetic field was caused to flow across the
passageway. Metal carried by persons passing through the
door caused alteration of the electromagnetic field and a
signal was given. The apparatus, forerunner of the modern
“walk-through” detector, was adjustable to allow small
objects such as watches and keys to be taken through the
gate undetected while larger objects were detected. A small
searching coil was used to inspect those persons who
produced a signal as they passed through the doorway. This
coil could be adjusted to various sensitivities, allowing
small objects, such as coins in pockets, to pass
undetected.
About the same time, Shirl Herr was recognized, according
to reports, as the inventor of the magnetic balance, a
device used for locating underground minerals and metals.
In 1927 the spark gap metal detector was invented. A report
in Popular Science Monthly, September 1930, shows a man
using a small two-coil metal detector. (See Page 71.) The
man using the device was called an “amateur treasure
finder.” The caption said that it would find a silver
dollar buried several inches underground and that it made a
bussing noise when metal was near. The metal detector,
called a “radio prospector,” was widely sold in kits. From
the early’30s until World War II, various companies began
producing metal detector inventions based upon several of
these electrical theories. During the war there was
naturally a great interest in metal detectors, with
resultant rapid advances in their technology.
At war’s end, thousands of Army mine detectors were
available as war surplus. They were eagerly bought by
ex-military personnel whose training with the Army mine
detectors enabled them to recognize the value of such
equipment in locating buried treasure. Several companies
began producing vacuum tube and transistorized detectors
for the consumer during the ‘50s. Since the development of
transistors permitted construction of smaller and lighter
weight detectors, vacuum tube detector production ended in
the early ‘60s. But, it was not until the late ‘60s and
early ‘70s that a substantial interest in metal detectors
arose; in the ‘70s great strides in metal detector
development began taking place. Ultra-stable and very
sensitive metal detectors that featured “Good/Bad” target
identification and ground mineral rejection came into
existence during this period.
The ‘80s ushered in target analyzer designs, and each year
saw these analyzers become more accurate. The use of
computerized, microprocessor-controlled circuitry
represented a quantum leap in the analysis of data.
Garrett’s Patent #4,709,213 was the first microprocessor
metal detector technology patent granted by the United
States Patent Office. The company conducted ten years of
design and field testing before utilizing this patent in
the manufacture of a detector, an effort whose success is
told in Part III’s discussion of computerized detectors.