The Coherer is a form of detector used in early continuous wave radio receivers. It is a glass tube filled with sharply cut silver and nickel shavings. Silver electrodes make contact with the shavings on both ends. One electrode is connected to the antenna and the other to ground. A series combination of a battery and a relay coil is also attached to the two electrodes.

   When the oscillating signal from a spark transmitter is received, the shavings tend to cling to each other, reducing the resistance of the coherer. The battery supplies more current, and the relay is actuated, giving an audible click. The clapper of an electric bell mechanism then strikes the coherer, shaking up the filings and raising the resistance of the coherer to the original value. 

     In the apparatus above in the Kenyon College collection, the tapping mechanism is at the bottom, the coherer is the glass tube on the right-hand side about half way up, and the relay is at the top. This system was sold by the C. H. Stoelting Co. of Chicago for $12.00 in 1912. Soon afterward, the development of the crystal rectifier and tuned circuits made it possible to detect and hear amplitude-modulated voice and music signals.
                             Middlebury College                                                                 University of Toronto

   This Coherer was given to the Greenslade Collection by Antoine DuBourg.

   The bell mechanism to vibrate the filings in the coherer tube is concealed under the metal box on the right-hand side. On its top, stamped in tiny letters, is "L.E. Knott" of Boston. The workmanship, while good, is not up to production standards, suggesting that this might have been built to a special order by the firm in the early years of the twentieth century.

The coherer was invented by David Edward Hughes (1831-1900), who also invented one form of microphone.

Return to Electricity Home Page | Return to Home Page