Dictograph "Carbon Ball" Microphone
|This is an early
microphone made by the Dictograph Corporation in the 1920's. Dictograph was primarily a manufacturer of office equipment,
and early their product line included
wax-cylinder dictation machines and intercom systems. They
also made some very interesting radio speakers in the 1920's, and were
one of the first manufacturers of electrical hearing aids in the late
1890's. They sold
eavesdropping equipment to law enforcement agencies beginning about
Much more information on the history and products of the Dictograph Corporation can be seen at this excellent Australian site operated by telephone collector, Bob Estreich.
The microphone shown above is covered by US Patent number 1,551,723 which was issued to Fredrick Wohlers on Sept. 1, 1925, however, the application was filed more than four years earlier on April 14, 1921. This patent was a improvement on a 1907 design (patent 844,635) which was described as a "carbon ball transmitter", was issued to Kelley Turner and Norton Johnston of the General Acoustic Company (which later became Dictograph).
|The Dictograph microphones have a unique internal configuration which
consisted of two semi-circular graphite blocks, each with four spherical
recessions which held a number of tiny carbon spheres. A
conductive graphite diaphragm bridged the two blocks and allowed an
electrical path to be established between the two blocks. It's
position was carefully adjusted with metal and paper shims to prevent
the carbon balls from escaping. Movement of the diaphragm caused
the amount of pressure on the balls to vary, which varied the electrical
resistance of the unit.
The entire system was very simple, and in its most basic form, it utilized only a microphone, a speaker (or earphone), and a battery. There was no electronic amplification whatsoever used in this system. It is a low-impedance system, relying on a considerable amount of current for its operation. The efforts of the designers to create a microphone which would handle that level of current can be seen in the multiple carbon chambers, and the carbon balls themselves. The carbon granules found in telephones and conventional carbon microphones are much finer, resembling very fine sand (but black).
These were brute force systems, and audio quality was not a priority.
|The slots around the periphery of the microphone are unique to the Dictograph design. Most had six of these slots, some had four or eight. Some were plain, with no decoration, some had elaborate designs. This particular microphone has a push-to-talk button just below the grill.|
|This microphone, unfortunately, is missing it's balls, so it is inoperative. The object at the upper right is the retainer for the diaphragm. It is threaded on its outer rim, and it screws into the housing. The decorative nickel-plated grill has a threaded stud on it's reverse side with screws into the diaphragm retainer.|
|This picture shows the interior of another Dictograph microphone from a mid-1920's office intercom unit as seen below. This microphone still has it's carbon balls in place. A pencil inscription inside is dated 11/16/31. The word "balls" is readable at the right. Possibly, it was serviced on that date, and its balls were replaced.|
|Most of the
Dictograph units that are seen are enclosed in wood boxes suitable for
the office desk. This unit, known as the Mastercall is very unusual, having not only the
standard Dictograph carbon ball microphone, but it also has a horn
loudspeaker. This would have been appropriate for a large work
area such as a store, warehouse, or factory. A buzzer can be seen just
above the microphone, and below the microphone is a push button to
operate a buzzer at a remote location.
The design of the horn speaker is similar to one of their radio speakers, but it has a swivel mount to allow it to be aimed. The DC resistance of the horn driver is only about 3 ohms, in keeping with the high current that this system would have been expected to carry. The equivalent driver for a radio application would be about 1000 ohms.