Marconi, however, made the important discovery that if his sensitive tube or coherer had one terminal attached to a metal plate lying on the earth, or buried in it, and the other to an insulated plate elevated at a height above the ground, it could detect the presence of very feeble electric waves of a certain kind originating at a great distance.
He therefore saw that it was a mistake to insert a potential-affected detector such as a coherer in between the base of the antenna and the earth because it was then subject to very small variations of potential between its ends.
If we consider the lines of magnetic force in the neighbourhood of the receiving antenna wire we shall see that they move across it, and thus create in it an electromotive force which acts upon the coherer or other sensitive device associated with it.
The ordinary forms of metallic filings coherer of the Branly type require tapping to bring them back to the high resistance or sensitive condition.
Lodge had previously suggested the use of transformed oscillations for acting on the coherer (see British Patent Spec., No.
On the other hand, the coherer or loose contact detectors are chiefly affected by the initial value of the electromotive force acting on the junction during the train of oscillations, and the magnetic detectors by the initial value of the current and also to a considerable extent by the number of oscillations during the train.
Tommasina 8 tried carbon in various forms. The theory of the action of the coherer has occupied the attention of T.
The oscillations set up in the vertical antenna excited sympathetic ones in the lateral circuit provided this was of the proper length; and the coherer was acted upon by the maximum potential variations possible.
The secondary circuit of this transformer is cut in the middle and has a condenser inserted in it, and its ends are connected to the sensitive metallic filings tube or coherer as shown in fig.
He constructed one form of his coherer of a glass tube a few inches long filled with iron borings or brass filings, having contact plates or pins at the end.