and remained so until the filings were shaken, thus giving time for
the fact to be observed in an ordinary telegraphic instrument.

The tube of filings through which the electric current is made to pass
in wireless telegraphy is called a coherer signifying that the filings
cohere or cling together under the influence of the electric waves.
Almost any metal will do for the filings but it is found that a
combination of ninety per cent. nickel and ten per cent. silver answers
the purpose best.

The tube of the coherer is generally of glass but any insulating
substance will do; a wire enters at each end and is attached to little
blocks of metal which are separated by a very small space. It is into
this space the filings are loosely filled.

Another form of coherer consists of a glass tube with small carbon
blocks or plugs attached to the ends of the wires and instead of the
metal filings there is a globule of mercury between the plugs. When
electric waves fall upon this coherer, the mercury coheres to the
carbon blocks, and thus forms a bridge for the battery current.

Marconi and several others have from time to time invented many other
kinds of detectors for the electrical waves. Nearly all have to serve
the same purpose, viz., to close a local battery circuit when the
electric waves fall upon the detector.

There are other inventions on which the action is the reverse. These
are called anti-coherers. One of the best known of these is a tube
arranged in a somewhat similar manner to the filings tube but with two
small blocks of tin, between which is placed a paste made up of alcohol,