The first metal detector Dr. Bell's used on President Garfield was made using a large primary coil with a smaller secondary coil fixed to it. The coils were mounted on a handle for scanning; the unit is to the left of the picture.
There is a similar set of coils, to the right of the picture, mounted on a platform. The small coil on this unit was adjusted to balance the electrical circuit for a null in the buzzer signal heard in the earphone. The balancing adjustment was made without the presence of metal near the portable coil set. The portable unit when brought in the presence of metal unbalanced the circuit and produced a note in the earphone. The range was only an on inch or two. The figure below is the circuit used with this four-coil detector.
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to see the magnetic field distortions and listen To Dr.Bells metal detector probing a bullet.
Bell and his assistant worked many days with different coil designs. They experimented with a number of circuit configurations and components. They tried different numbers and types of batteries and condensers. In frustration they returned to a coil and circuit that Dr. Bell had created while working on the telephone the previous year in England. They discovered that it had a consider table improvement in range, more than double that of anything they had worked thus far. This was at a time when the president was doing poorly. They hurried through the night to produce this new detector. A re-creation of it is shown above. It consisted of two large coils, a primary coil connected to the rheotome (buzzer), a condenser, and a battery, and the secondary coil connected to an earphone.
In the evening of 7/26/1881, Bell and Tainter brought apparatus very much as pictured above to White House and tested it on President Garfield. They scanned the President, back and fourth many times adjusting their equipment. They had a lot of trouble with this experimental run. They had wired the condenser incorrectly to the rheotome and didn't realize it until several days later. The rheotome was acting up mechanically first indicating some signal then none. The loud noise from the rheotome, located in the same room, interfered with the observer trying to listen for the low buzzing signal in the earphone. The rheotome, the batteries, and condenser were moved to an adjacent room with the door closed. All in all they believed they detected a slight evidence of the bullet, but it was not reliable enough.
The two coils are mounted in a pair of sliding blocks of wood with a handle. They are connected together with ebonite thumbnuts. The blocks are adjusted back and forth to null the buzzing sound with no metal present. Dr.Bell and his assistant spent considerable time sliding the blocks back and forth to get a perfect null from the detector. At one time Bell uses a hammer for a fine adjustment to sock the coil to a null condition. On 8/1/1881 At the White House most of the equipment was placed in another room to eliminate the noise coming directly from the buzzer. This equipment was placed on a small table with an assistant attending them while the observer used the portable coil to scan for the bullet in the president. Dr. Bell was still unable to find a definite location for the bullet. Sadly the president died several days later of infections cause by poor medical practice and not directly from the bullet.
The Circuit for the four coil metal detector used for 7/26/1881 scans of President Garfield.
A cross sectional view of the two-coil unit.
Bell's simpler circuit for the double coil metal detector.
