The coherer.

From the book 'Harmsworth's Wireless Encyclopedia". Published in 1923.

 

The coherer is
an electric wave detecting device, so named by Lodge. Most coherers depend upon the fact that certain conductors when in a state of fine division are practically non-conductors for small electro-motive forces when loosely packed, but are good conductors for large electro-motive forces. A mixture of particles of metal and poor conductors exhibit the same phenomena. C. and S. A. Varley, in their British patent taken out in 1866 for a lightning arrester, had two copper points, nearly touching, fixed in a small box filled with powdered carbon. In their patent specification they pointed out the fundamental principle of the coherer.
  Hughes, in 1878, discovered that a tube of glass loosely filled with zinc and silver filings was sensitive to electric sparks at a distance.
But it was not until 1890 that the first real step was taken to make use of the phenomena of powdered conductors. In that year Branly published the account of his researches, in which he confirmed Hughes' observations and added that the electric spark had the power at a distance to change the conductivity of the powdered conductors. Branly carried out his experiments with a number of metals, and in 1889 Sir Oliver Lodge discovered a cohering effect when a very minute spark or scintilla passed between knobs, in connexion with his lightning guard experiments, as described in 1890 to the Institute of Electrical Engineers. In 1894 he exhibited his adjustable point device, the end of a fine spiral spring lightly touching an aluminium plate, which he called a coherer.
Marconi about the same period carried out a series of experiments, using different metal filings, and finally he brought out his first detector for wireless telegraphy. Marconi's coherer consisted of a sealed glass tube in which were placed two silver plugs attached to platinum wires. The inner ends of the plugs were cut at an angle, and brought within two millimetres of each other. The space between was filled with nickel and silver filings. Branly had found that after the filings in his coherer had been made conducting by an electric current, it required a slight mechanical shock to restore the filings to their original non-conducting state. Lodge provided this shock by means of an electric bell or a clockwork tapper, and an electro-magnetic tapper was employed by Marconi.
A circuit employing a coherer
Fig. 1       CIRCUIT EMPLOYING A COHERER.
In this diagram is shown how a coherer may be used in an aerial circuit with a relay to a recording instrument, M.

  The application of the coherer to wireless telegraphy is as follows. The coherer is connected in series with a single cell, or with a shunted cell, and galvanometer or other recording electrical device. In a second method the coherer is connected in series with a single cell and a telegraphic relay, operating with a current of less than a milliampere. In a third method a Bell telephone is in series with the coherer and a single cell and high resistance.
  The resistance of the loosely packed filings is so great that the circuit of the cell to the relay, for example, is not completed, and the relay does not act. When ether waves act on the aerial of the receiving station, the oscillating electrical potentials set up in it act across the coherer, breaking down the resistance and completing the cell and relay circuit. In effect, the coherer is a closed tap which is opened by the action of the ether waves cohering or sticking together the metal filings. The filings remain in a cohered condition until they are decohered, and to detect the train of signals, therefore, the mechanical decoherer must be made part of the circuit.
The coherer has the disadvantages of being uncertain in its sensitiveness, and responding too readily to local and atmospheric disturbances.
  Various forms of construction and various materials have been tried to overcome these defects, but they have not been very successful. The Castelli coherer, with iron, mercury and carbon electrodes, has the advantage of being self-restoring. The Italian navy coherer, a modification of Castelli's, consists of a small glass tube with two iron or steel plugs between which is a small drop of mercury, and automatically decoheres.
Another form of self-decoherer is the Lodge-Muirhead. This consists of a steel disk slowly rotated by clockwork and just touching a globule of mercury which is covered with a thin film of paraffin. Normally the oil insulates the wheel from the mercury, but when oscillating potentials are set up in the receiving circuit, the insulation is broken down and the coherer circuit is completed. The steady rotation of the steel disk continually restores the coherer to a sensitive condition.
  Walter's adjustable tantalum coherer also makes use of mercury. A tantalum wire, sealed in glass, just dips below the surface of the mercury, and the arrangement of the coherer is such that it will withstand shocks or tapping without affecting the sound.
  The theory of the coherer is not fully understood. Lodge suggested that the surfaces of the metal were welded together, a suggestion that has been supported by a number of others. Sundorph, Tommasina and others assert that a chain of conducting particles are formed. But with both Lodge's theory and that of Sundorph, large differences of potential are necessary to observe the phenomena, much greater than those due to ether waves. It is clear that the action is an electrical one and depends upon light contact, and probably there is a passage of negative ions from one surface to another.

The decoherer is a  device to tap or vibrate a coherer; It is well known that various metallic filings and borings, when enclosed in a glass tube or otherwise kept in contact, cohere or stick together when brought under the action of electromagnetic waves.
  Certain metals, however, show the reverse action, and Professor Bose has demonstrated that if potassium or arsenic powder is confined between two metal electrodes and its electrical resistance measured, this resistance will increase when the powders are subjected to a wireless wave. This is therefore a case of metals which are decoherers.
Early form of decoherer
Fig. 2   EARLY FORM OF DECOHERER.
Sir Oliver Lodge carried out experiments in the early days of decoherers with a device made on the lines indicated above, which is operated by a clockwork.

  The various forms of mechanical appliances used to vibrate coherers are better known, however, as decoherers.
A single blow or tap with a pencil on a coherer tube will effectively shake apart the filings, but for anything other than experimental working it is necessary that this action should be automatic, and that immediately the particles have cohered they should be shaken apart. This cohesion and decohesion should go on automatically and rapidly as long as the coherer is being subjected to a wireless wave, and the coherer should stay in its high-resistance state with its particles well shaken apart as soon as the wave ceases.
  The earliest decoherers to be used, were either electric vibrators, of the bell type, or mechanical ones depending on a clockwork-driven, cogwheel rubbing on a spring attached to the coherer or its stand.
Lodge employed the latter—Fig. 2, in which C is a clockwork motor driving a four-bladed spider wheel, S. The tips of the spokes of this wheel were arranged so that they just touched the tip of a long arm of whalebone, wood, or ebonite, W, which was rigidly attached to one of the V blocks in which the coherer tube, A, rested.
Lodge preferred the mechanical vibrator to the electrical because of the absence of the small sparks produced at the bell contacts, which he found affected his coherer.
Electrical method of decohering
Fig. 3   ELECTRICAL METHOD OF DECOHERING.
Mounted on a wedge-shaped block is a tapper which, being adjusted, strikes the coherer, C. This method was employed in the experiments of Marconi and Popoff.

  Popoff and Marconi both employed the electrical method, having a relay connected in series with the coherer, so that when the coherer closed the circuit of the relay the tapper started.
In the Marconi apparatus a very sensitive relay of about 1,000 ohms was connected in series with the coherer and additional external resistances, whilst a single dry cell provided the direct current which passed through the coherer and relay.
  The tapper (Fig. 2) was mounted on a wedge-shaped block so that it could readily be adjusted to strike the underside of the coherer, C.
In the Lodge-Muirhead mercury coherer (Fig. 4) a small steel disk, D, having carefully rounded edges, was mounted over an ebonite cup, E. In the bottom of the cup, E, was a small hole which held a pellet of mercury in light contact with the disk. The disk was slowly revolved by clockwork. A few drops of paraffin oil were then placed in the cup, forming a film over the surface of the mercury and also on the steel disk. When a wireless wave passed, the film of oil broke down and the disk and mercury cohered. As soon as the wave passed the revolving disk carried the paraffin over the surface of the mercury, thus effecting decohesion. S. G. Brown employed alternating current magnetic fields to produce decohesion in iron filings, both by passing alternating currents through field coils

 

Lodge-Muirhead mercury coherer.
Fig. 4.   LODGE-MUIRHEAD MERCURY COHERER.
Beneath the disk, D, is a mercury pellet, M. These two are separated by a film of paraffin oil, P. The disk is rotated by Morse clockwork. Cohesion occurred when a wireless wave, passing, broke down the film of oil, and immediately following, when the film of oil was replaced by the disk, decohension occurred.

 

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