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The coherer, which is the result of the work of many men - Hughes, Lodge, Branley and Popoff among others - consists essentially of a small quantity of metal filings lying loosely between metallic electrodes. The first practical form of the device for telegraphic purposes was brought out by Marconi. and consisted of a very small quantity of nickel filings, to which were added a small percentage of silver filings, lying between silver electrodes having bevelled ends so that the space between them, in which were the filings, was wedge-shaped. The purpose of thus bevelling the plugs is to enable the sensitiveness of the coherer to be adjusted. The most sensitive position is when the nose of the wedge is pointing downward and reverse position is that of least sensitiveness. The plugs and fillings are enclosed in a glass tube, which is exhausted to a patial vacuum, and the wires connected to the plugs pass out through the ends of the tube (fig.41). The coherer depends for its action on the fact that, if its teminals are subjected to a potential difference above a certain value, the resistance due to the loose contact between the fillings and plugs suddenly falls to a much lower value; some investigators think that ordinary electro-static attraction is a sufficient explanation of its behaviour, others hold that microscopic sparks pass between the fillings and slightly weld them together; however this may be, the fact remains that, after being subjected to potential differences set up by the oscillations, the resistance falls enormously, and if the coherer is joined up with a relay and cell, and the relay contacts joined up with a Morse writer and battery, the passage of electrical oscillations will be made evident by the closing of the relay circuit and consequent recording of signals.

As, however, the coherer will not of itself resume its former high resistance a small electro-magnetic hammer is provided to tap gently on its underside, and by shaking the filings loose it restores the coherer to its high resistance and again renders it sensitive to oscillations. (fig.42) shows the circuits of the Marconi coherer receiver. The antennae circuit consists of turning inductance and primary of oscillation transformer joined in series and connected to antennae and earth. The secondary winding of the oscillation transformer is cut in the middle but its continuity for electrical oscillations is preserved by the insertion of a condenser. To the ends of the secondary winding is connected a variable condenser for turning it to the primary and across this latter is the coherer. The relay with a single dry cell in series is connected across the condenser inserted in the break of the secondary winding. To the contact terminals of the relay are joined a battery of the cells in the series with the Morse printer, and in parallel with the printer is the tapper, the function of which is to shake loose the filings in the coherer after it has been actuated by the oscillations. Owing to the high self-induction of the relay, printer, and tapper coils, it is essential that they and also the contacts of the relay and tapper should be shunted by high non-inductive resistance to eliminate the sparking which would otherwise occur and which, though small, would be sufficient to actuate the coherer. The adjustment of the various circuits and pieces of apparatus comprised in the above-described set is usually thought to be a difficult matter, but if it is systematically done it will be found fairly simple.The operator should proceed as follows:-first, by means of the adjusting screw set the magnet of the tapper as far away from its armature as it is possible, then adjust the knob of the tapper so that if is at the distance of about one millimetre from the coherer.

The next step is to turn the adjusting screw of the relay till the local circuit closes and then to slowly turn it in the reverse direction till it just opens. Test letters should now be sent on the buzzer (the buzzer is a small trembler movement worked by a dry cell and constitutes a generator of feelbe electrical oscillations), and at the same time the magnet of the tapper made to gradually approach its armature till the strenght of the beat is sufficient to give good sharp signals on the Morse pointer. If the beat is too weak the signals will tend to run together, and if it is too strong they will be cut up-that is to say, the dashes will appear as a series of dots. The whole of the apparatus above descibed, with the exeption of the printer, is enclosed in a metallic box to prevent damage to the goherer from the powerful oscillations which would be set up in the circuits when the transmitter was in use.

This coherer, which may be used either with a telephone or with a syphon recorder, is constructed as follows: -a small metallic cup A (fig.44) contains a globule of mercury on which is placed a small drop of oil, which forms an infinitely thin insulating film over it; above the globule of mercury is a small iron disc with a sharp edge and which is slowly rotated. By means of an adjusting screw the lower edge of the disc is made to touch the oil-covered mercury, but the pressure is not so great as to puncture the film of oil. In series with the coherer is joined a dry cell and telephone receiver, or syphon recorder, as the case may be, and the passage of electrical oscillations, by breaking down the insulating film of oil, allows the cell to operate the receiving instrument. This form of coherer is self-resorting and needs no tapping arrangement.

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