necessary. The coherer was the solution. As early as 1870 a Mr. S.A.
Varley, an Englishman, had discovered that when he endeavored to
send a current through a mass of carbon granules the tiny particles
arranged themselves in order under the influence of the electric
current, and offered a free path for the passage of the current. When
shaken apart they again resisted the flow of current until it became
powerful enough to cause them to again arrange themselves into a
sort of bridge for its passage. Thus was the principle of the coherer
discovered.

An Italian scientist, Professor Calzecchi-Onesti, carried these
experiments still further. He used various substances in place of the
carbon granules and showed that some of them will arrange themselves
so as to allow the passage of a current under the influence of the
spark setting up the Hertzian waves. Professor E. Branly, of the
Catholic University of Paris, took up this work in 1890. He arranged
metal filings in a small glass tube six inches long and arranged a
tapper to disarrange the filings after they had been brought together
under the influence of the spark.

With the Branly coherer as the basis Marconi sought to make
improvements which would result in the detector he was seeking. For
his powder he used nickel, mixed with a small proportion of fine
silver filings. This he placed between silver plugs in a small glass
tube. Platinum wires were connected to the silver plugs and brought
out at the opposite ends of the tube. It required long study to
determine just how to adjust the plugs between which the powder was
loosely arranged. If the particles were pressed together too tightly
they would not fall apart readily enough under the influence of the
tapper. If too much space was allowed they would not cohere readily
enough. Marconi also discovered that a larger proportion of silver
in the powder and a smaller amount between the plugs increased the
sensitiveness of the receiver. Yet he found it well not to have it
too sensitive lest it cohere for every stray current and so give false
signals.

Under the influence of the electric waves set up from the spark-gap
those tiny particles so arranged themselves that they would readily
carry a current between the plugs. By placing these plugs with their
platinum terminals in circuit with a local battery the current from
this local battery was given a passage through the coherer by the
action of the electric waves coming thro