let us describe some of these detectors of the self-restoring contact type. We
shall begin with a rather poor representative, the carbon microphone.
Microphonic Detector. — In 1879, Professor D. E. Hughes, the
inventor of the microphone, accidentally found that the contact
of a piece of carbon, with bright steel, when used with a telephone receiver, 
was responsive to the inductive effect produced by the
make and break of the primary current of an induction coil .
Hughes did not publish his results until interviewed on the subject
by Mr. Fahie in 1899. He then wrote Mr. Fahie a letter, which
was published in the London Electrician , May 5, 1899. In look
ing over the description of Professor Hughes's experiments we now see that 
in 1879 he was producing and receiving electric waves,
and had discovered in the microphone a self- restoring contact
detector. A diagram of one ' form of Hughes's microphonic detector, is shown 
in Fig. 110, which is redrawn from a sketch in
DETECTORS —CRYSTAL RECTIFIERS 159
Mr. Fahie's History . In this diagram , C is a carbon pencil
touching a steel needle N ; S is a brass spring by which the pres sure of the 
contact can be regulated. The adjustment of the spring is regulated by means of
the disc D.
Professor Hughes used the microphone with or without a battery in the local 
circuit ; and when the battery was omitted, he attributed the sound in the
telephone to the thermoelectromotive force developed at the Fig . 110. 
Hughes's microphonic steel- carbon detector.
carbon -steel junction. The de tector was more sensitive with a battery in the 
local circuit than without it .
Various modifications of this microphonic detector of Hughes have been employed 
in practical wireless telegraphy. One modi fication, which had a considerable 
application a few years ago ,
S
D
?
Steel Needle
G
Fig. 111. Steel- carbon detector. Fig. 112. Detector of carbon gran ule between metallic plugs.
is obtained by placing a steel needle across two blocks of carbon,
as shown in Fig. 111. Another is made by placing a granule of
carbon between metallic plugs in a tube, as shown in Fig. 112.
The microphone is more sensitive than the filings coherers.
It is , however, somewhat troublesome on account of sensitiveness
to mechanical vibrations and on account of liability to cohere
under strong signals, and it is surpassed in sensitiveness to electric
1 Fabie, History of Wireless Telegraphy, 1902, Dodd, Mead & Co.
160 WIRELESS TELEGRAPHY
waves by the crystal detectors, in which the carbon of Hughes's microphone is replaced by certain crystalline mineral substances.