A careful examination of the opinions of scientific men given in the
telephone cases--before Lord McLaren in Edinburgh and before Mr. Justice
Fry in London--leads me to the conclusion that scientific men, at least
those whose opinions I shall quote, are not agreed as to what is the
action of the carbon microphone.
In the Edinburgh case, Sir Frederick Bramwell said: "The variations of
the currents are effected so as to produce with remarkable fidelity the
varied changes which occur, according as the carbon is compressed or
relieved from compression by the gentle impacts of the air set in motion
by the voice."
"The most prominent quality of carbon is its capability, under the most
minute differences of pressure, to enormously increase or decrease the
resistances of the circuit." "That the varying pressure of the black
tension-regulator (Edison's) is sufficient to cause a change in the
conducting power." Sir Frederick also said "he could not believe that
the resistance was varied by a jolting motion; could not conceive a
jolting motion producing variation and difference of pressure, and such
an instrument could not be relied on, and therefore would be practically
useless."
Sir William Thomson, in the same case, said: "The function of the carbon
is to give rise to diminished resistance by pressure; it possesses the
quality of, under slight degrees of pressure, decreasing the resistance
to the passage of the electric current;" and, also, "the jolting motion
would be a make-and-break, and the articulate sounds would be impaired.
There can be no virtue in a speaking telephone having a jolting motion."
"Delicacy of contact is a virtue; looseness of contact is a vice."
"Looseness of contact is a great virtue in Hughes' microphone;" and "the
elements which work advantages in Hughes' are detrimental to the good
working of the articulating instrument."
[Illustration: Fig. 1.]
Mr. Falconer King said: "There would be no advantage in having a jolting
motion; the jolting motion would break the circuit and be a defect in
the speaking telephone," and "you must have pressure and partially
conducting substances."
Professor Fleeming Jenkin said, "The pressure of the carbons is what
favors the transmission of sound."
All the above named scientific men agree that variations of a current
passing through a carbon microphone are produced by _pressure_ of the
carbons against one another, and they also agree that a jolting motion
could not be relied upon to reproduce articulate speech.
Mr. Conrad Cooke said, "The first and most striking principle of Hughes'
microphone is a shaking and variable contact between the two parts
constituting the microphone." "The shaking and variable contact is
produced by the movable portion being effected by sound." "Under Hughes'
system, where gas carbon was used, the instruments could not possibly
work upon the principle of pressure." "I am satisfied that it is not
pressure in the sense of producing a change of resistance." "I do not
think pressure has anything to do with it."
Professor Blyth said: "The Hughes microphone depends essentially upon
the looseness or delicacy of contact." "I have heard articulate speech
with such an instrument without a diaphragm." "There is no doubt that to
a certain extent there must be a change in the number of points of
surface contact when the pencil is moved." "The action of the Hughes
microphone depends more or less upon the looseness or delicacy of the
contact and upon the changes in the number of points of surface contact
when the pencil is moved."
Mr. Oliver Heaviside, in _The Electrician_ of 10th February last,
writes: "There should be no jolting or scraping." "Contacts, though
light, should not be loose."
[Illustration: Fig. 2.]
A writer, who signs "W.E.H.," in _The Electrician_ of 24th February
last, says: "The variation of current arises from a variation of
conductivity between the electrodes, consequent upon the variation of
the closeness or pressure of contact;" and also, "there must be a
variation of pressure between the electrodes when the transmitter is in
action."
It seems, then, that some scientific men agree that variation of
pressure is required to produce action in a microphone, and some of them
admit that a microphone with loose contacts will transmit articulate
speech, while others deny it, and some admit that a jolting or shaking
motion of the parts of the microphone does not interfere with articulate
speech, while others say such motion would break the circuit, and cannot
be relied on.
I will now describe two microphones in which there is a shaking or
jolting motion, and loose contacts, and no variation of pressure of the
carbons against one another, and both of these microphones when used
with an induction coil and battery give most excellent articulation. One
of these microphones is made as follows: Two flat plates of carbon are
secured to a block of cork, insulated from each other; into a hole of
each carbon a pin of carbon fits loosely, projecting above the carbons;
another flat piece of carbon, having two holes in it, bridges over the
two lower carbons, being kept in its place by the pins of carbon which
fit loosely in the holes in it, the bottom carbons being connected with
the battery; a block of cork has a flat side of it cut out so as when
secured to the lower cork the carbons will not come in contact with it,
yet be close enough to it to keep the carbons from falling apart. The
cork covering the carbons forms a dome.
Any good telephone receiver when used in connection with this
microphone, reproduces articulate speech with remarkable distinctness,
especially hissing sounds, and with a loud and full tone.
A description of this microphone was published in _La Lumiere
Electrique_, of 15th April, 1882, and a drawing thereof on 29th April of
same year.
Another form of microphone is made as follows: Two blocks of gas carbon,
C, B, each about one and a half inches long and one inch square, having
each a circular hole one and a quarter inches deep and half inch in
diameter; these two blocks are embedded in a block of cork, C, about
one-quarter of an inch apart, these holes facing each other, each block
forming a terminal of the battery and induction coil; a pencil of
carbon, C, P, about three-eighths of an inch in diameter, and two inches
long, having a ring of ebonite, V, fixed around its center, is placed in
the holes of the two fixed blocks; the ebonite ring fitting loosely in
between the two blocks so as to prevent the pencil from touching the
bottom of the holes in the blocks. The space between the blocks is
closed with wax, W, to exclude the air, but not to touch the ring on the
pencil. A block of cork fitting close to the carbon blocks on all sides
is then firmly secured to the other block of cork. The microphone should
lie horizontally or at a slight angle.
This microphone produces in any good telephone perfect articulation in a
loud and full tone. In these microphones there is certainly "looseness
and delicacy of contact," and there is a "jolting or shaking motion,"
and it does not seem possible that there can be any "pressure of one
carbon against another."
I repeat the question I asked at the beginning of this communication,
and hope that it may elicit from you, or some of our scientific men, an
explanation of the theory of the action of this form of microphone.