of sending currents, more or less broken, through the telephones,
which transform them into sound vibrations, and since these are
mechanically transmitted by the board to the microphone, they maintain
and even amplify its action, and produce fresh vibrations on the
telephones. Thus a fresh action is exerted on the microphone, and so
on indefinitely. Again, if a second microphone, in connection with
another telephonic circuit, be placed upon the same board, we have an
instrument which acts as a telephonic relay, that is, it transmits
to a distance the sounds communicated to the board, and these sounds
may serve either as a call, or as the elements of a message in the
Morse code, if a Morse manipulator is placed in the circuit of the
first microphone. Mr. Hughes adds that he has made several very
successful experiments with this system of instruments, although he
only employed a Daniell battery of six cells without any induction
coil. By fastening a pasteboard tube, 40 centimètres in length, to
the receiving telephone, he was able to hear in all parts of a large
room the continuous sound of the relay, the ticking of a watch, and
the scratching of a pen upon paper. He did not try to transmit speech,
since it would not have been reproduced with sufficient distinctness
under such conditions.
Since this first attempt, Mr. Hughes has arranged another and still
more curious system of microphonic relays, for which two microphones
with vertical carbons are required. He places two microphones
of this description on a board, and connects one of them with a
third microphone, which acts as a sender, while the second is in
communication with a telephone and a second battery: in this way the
words uttered before the sender are heard in the telephone, without
employing any electro-magnetic organ for the telephonic relay.
In August 1878, Messrs. Houston and Thomson likewise arranged a system
of telephonic relays which only differs from that of Mr. Hughes in
the particular of having the microphone fixed on the diaphragm of the
telephone, and not on the board beside it. The system consists of three
vertical microphones, which can be combined for tension or quantity,
according to the conditions for which they are required. The model of
this instrument was represented in the ‘Telegraphic Journal’ of August
15, 1878, to which we must refer our readers, if they wish for further
information on the subject.
_Its application to Medicine and Surgery._--The extreme sensitiveness
of the microphone suggested its use for the observation of sounds
produced within the human body, so that it might serve as a stethoscope
for listening to the action of the lungs and heart. Dr. Richardson and
Mr. Hughes are now busy in the attempt to carry out this idea, but so
far the result is not very satisfactory, although they still hope to
succeed. Meanwhile, M. Ducretet has made a very sensitive stethoscopic
microphone, which we represent in fig. 51. It consists of a carbon
microphone C P, with a simple contact, of which the lower carbon P is
fitted to one of M. Marais’ tambourines with a vibrating membrane T.
This tambourine is connected with another T′, by a caoutchouc tube,
which is to be applied to the different parts of the body which demand
auscultation, and which is therefore termed the _tambour explorateur_.
The sensitiveness of the instrument is regulated by means of a
counterpoise P O, which is screwed upon the arm of a bent lever, and
to this the second carbon C is fixed. The extreme sensitiveness of
M. Marais’ tambourines in transmitting vibrations is well known, and
since their sensitiveness is further increased by the microphone, the
instrument becomes almost too impressionable, since it reveals all
sorts of sounds, which it is difficult to distinguish from each other.
Such an instrument can only be of use when entrusted to experienced
hands, and a special education of the organ of hearing is needful, in
order to turn it to account.
[Illustration: FIG. 51.]
In a work lately published by M. Giboux on the application of the
microphone to medicine, this stethoscopic system is rather severely
criticised, and not without reason if, as M. Giboux asserts, it is only
sensitive to the movements which take place on the surface of the body,
and those which are internal are either lost or altogether changed
in character. But without pronouncing on the improvements which may
ultimately be made in the instrument, M. Giboux thinks that its most
important use in medical practice consists in its allowing a certain
number of students to observe with the professor the different sounds
of the body, to study them with him in their different phases, and thus
to profit more readily by his teaching. A microphonic circuit might
bifurcate between several telephones, so that each person might hear
for himself what is heard by others.
The most important application of the instrument to surgical purposes
has lately been made by Sir Henry Thompson, aided by Mr. Hughes, for
the examination of the bladder in cases of stone. It enables him to
ascertain the presence and precise position of calculi, however small
they may be. For the purpose of research, he uses a sound, made of a
Maillechort rod, a little bent at the end, and placed in communication
with a sensitive carbon microphone. When the sound is moved about in
the bladder, the rod comes in contact with stony particles, even if
they are no larger than a pin’s head, and friction ensues, producing
in the telephone vibrations which can be easily distinguished from
those caused by the simple friction of the rod on the soft tissues of
the sides of the bladder. The arrangement of the instrument is shown
in fig. 52. The microphone is placed in the handle which contains the
sound, and is the same as that given in fig. 42, but of smaller size,
and the two conducting wires _e_ which lead to the telephone, issue
from the handle by the end _a_ opposite to that _bb_ to which the
sound _dd_ is screwed. As this instrument is not intended to reproduce
speech, retort carbons instead of wood carbons may be used.
[Illustration: FIG. 52.]
Some deaf people, whose sense of hearing is not completely destroyed,
have been able to hear by an expedient based upon the principle of the
microphone. For this purpose two telephones, connected by a metallic
crown, which is placed on the temples, are applied to the ears of
the deaf person, and the telephones are placed in communication with
a battery microphone, which hangs to the end of a double conducting
wire. The deaf man keeps the microphone in his pocket, and presents it
as an acoustic tube to the person who wishes to converse with him. Mr.
Hughes’s speaker, represented fig. 42, is the one used.
_Various Applications._--The microphone may be used in many other ways,
some of which are suggested in the ‘English Mechanic’ of June 21,
1878. The article states that by means of this instrument, engineers
will be able to estimate the effects of the vibrations caused on
old and new buildings by the passage of heavy loads; a soldier will
be able to discover the enemy’s approach when he is several miles
off, and may even ascertain whether he has to do with artillery or
cavalry; the approach of ships to the neighbourhood of torpedoes may be
automatically heralded on the coast by this means, so that an explosion
may be produced at the right moment.
It has also been proposed to use the microphone to give notice of an
escape of gas in coal-mines. The gas, in escaping from between the
seams of coal, makes a whistling noise, which might, with the aid
of the microphone and telephone, be heard at the top of the shaft.
Again, it has been suggested that the microphone might be used as a
seismograph to reveal the subterranean noises which generally precede
earthquakes and volcanic eruptions, and which would be much intensified
by this instrument. It might even be of use to Signor Palmieri for his
observations in the Vesuvius Observatory.
The microphone has also been used by Mr. Chandler Roberts to render the
diffusion of gaseous molecules through a porous membrane sensible to
the ear.
As might have been expected, the acclamation with which Mr. Hughes’s
invention was received led to the assertion of other claims to
priority, and in addition to that of Mr. Edison, on which we have
already given our opinion, there are several others, showing that if
some microphonic effects were discovered at different times before the
date of Mr. Hughes’s discovery, they could not have been considered
important, since they were not even announced. Among the number was
that of Mr. Wentworth Lascelles Scott, specified in the ‘Electrician’
of May 25, 1878, and that of M. Weyher, presented to the Société de
Physique, Paris, in June 1878. Another, made by M. Dutertre, is of
somewhat greater importance, for his experiments were reported in
the Rouen papers in February of the same year: yet there is no just
ground for such claims, since the earliest date of his experiments is
subsequent to the experiments first made by Mr. Hughes. These began
early in December 1877, and in January 1878 they were exhibited to
officials of the Submarine Telegraph Company, as Mr. Preece declared in
a letter addressed to the several scientific men.
EXTERNAL INFLUENCE ON TELEPHONIC TRANSMISSIONS.
The obstacles which occur in telephonic transmissions proceed from
three causes: 1. The intensity of sound is diminished by the loss of
current in transmission--a loss which is much greater in the case of
induced currents than in those received from a battery. 2. Confusion
is caused by the influence of adjacent currents. 3. The induction
from one wire to another. This last influence is much greater than is
usually supposed. If two perfectly insulated wires are placed side
by side, one in communication with the circuit of an electric bell,
and the other with the circuit of a telephone, the latter will repeat
the sounds of the bell with an intensity often great enough to act as
a call without applying the instrument to the ear. MM. Pollard and
Garnier, in their interesting experiments with the induced currents
of the Ruhmkorff coil, have ascertained that in this way not merely
sounds may be obtained which correspond with the induced currents
resulting from the action of the primary current, but also those which
result from the action of the secondary current on other helices, which
are termed currents of the second order. These different reactions
frequently cause the telephonic transmissions made on telegraphic
lines to be disturbed by irregular sounds, arising from the electric
transmissions on adjoining lines; but it does not appear that these
influences altogether neutralise each other, so that conversation held
in the ordinary way and a message sent in the Morse code may be heard
simultaneously.
At the Artillery School, Clermont, a telephonic communication has
been established, for the sake of experiments, between the school
and the butts, which are at a distance of about eight miles. Another
communication of the same kind has been established between the
Clermont Observatory and the one at Puy-de-Dôme, which is nearly nine
miles from the former. These two lines are carried on the same posts
for a course of six miles, together with an ordinary telegraphic wire,
and for a distance of 330 yards there are seven other such wires.
The two telephonic wires are separated from each other by a space of
85 centimètres. The following facts have been observed under these
conditions.
1. The school telephone is perfectly able to read off from their sound
the Morse messages which pass through the two adjacent telegraph wires,
and the ticking of the instrument does not at all interfere with the
vocal communication of the telephone, nor render it inaudible.
2. The two adjacent telegraphic lines, although not in contact, confuse
their messages together, and it has sometimes been possible to hear
messages from Puy-de-Dôme at the school through the wire which runs to
the butts, although the distance between the two lines is nowhere less
than 85 centimètres.
These inconveniences have been in some degree remedied by inserting
strong resistances in the circuit, or by putting the current to earth
at some distance from the telephonic stations.
M. Izarn, Professor of Physics at the Lycée, Clermont, holds that
telephonic electric currents may readily be turned aside by the earth,
especially if in the course of their passage they encounter metallic
conductors, such as gas or water pipes. He writes as follows on the
subject, in a paper addressed to the Académie des Sciences, on May
13, 1878:--‘I set up a telephone in the Clermont Lycée with a single
wire, more than 50 yards in length, which crosses the court-yard of
the Lycée, and goes from the laboratory, where it is suspended to a
gas-burner, to a room near the porter’s lodge, where it is suspended
to another gas-burner. When I applied my ear to the telephone, I could
distinctly hear the telegraphic signals, Morse or otherwise, which came
either from the telegraph office at Clermont, or from the telephone
office which was at work between the School of Artillery and the butts
below Puy-de-Dôme, a distance of eight miles. I could overhear words,
and especially the military orders issued at the butts for the purpose
of being heard at the school. Yet my wire is perfectly independent of
those used for signalling, and is even very remote from them; but as
the wires of the telegraph office and of the School of Artillery go to
earth at a little distance from the gas-pipes, it is probable that this
phenomenon is caused by a diversion of the current produced in my wire,
by means of the earth and the network of metal pipes.’
Mr. Preece made the same remark in his notice of ‘some physical points
connected with the telephone.’ Again, we read in the ‘Telegraphic
Journal’ of June 15, 1878, that in a telephonic concert transmitted
from Buffalo to New York, the singers at Buffalo were heard in an
office placed outside the telegraphic circuit in which the transmission
was effected. On enquiry, it was ascertained that the wire through
which the telephonic transmission took place, was at one point in its
course close to the one which directly transmitted the musical sounds,
but the distance between the two wires was not less than ten feet.
When the circuits are altogether metallic, there is much less risk of
confusion, and M. Zetzche declares that sounds proceeding from other
wires are in this case little heard, and then only momentarily, so that
it is much more easy to hear with this arrangement than with the one in
ordinary use. ‘It is not,’ he says, ‘the resistances of the wire, but
rather the diversions of the current near the posts, which interfere
with telephonic correspondence on long lines above ground. This was
proved by the following experiments:--I connected the telegraphic line
from Dresden to Chemnitz with a line from Chemnitz to Leipzig (54
miles), which made a circuit of 103 miles, going to earth at its two
extremities. There was no communication between Dresden and Leipzig,
but Leipzig and Dresden could communicate with ease, in spite of the
greater extent of line. I broke the connection with earth, first at
Leipzig, then simultaneously at Leipzig and Dresden, and I observed
the following effects. When insulation took place at Leipzig only,
the telephone could be heard at the stations of Dresden, Riesa, and
Wurzen; when the line was insulated at both ends, the communication was
good between the two latter stations, but it was observed that at the
intermediate station the words spoken at Wurzen were more distinctly
heard than the words spoken at Riesa were heard at Wurzen. Since the
distance from Wurzen to Leipzig is little more than half that from
Riesa to Dresden, there are consequently nearly twice as many posts
on the latter line, which carry the currents to earth, and hence I
conclude that these diversions of current explain the possibility
of conversing on an insulated line, and also why sounds are more
distinctly heard at the Riesa station in consequence of the greater
intensity of current still remaining on the line.’
Some vibrations also result from the action of currents of air on
telegraphic wires, which produce the humming sound so well known on
some lines, and these may also react on the telephone; but they are
in this case generally mechanically transmitted, and they may be
distinguished from the others, if the sounds which ensue are heard
after the telephone is excluded from the circuit by a break with a
short circuit and after the communication to earth established behind
the telephone has been broken.
The induced reactions caused by the line wires on each other are not
the only ones which may be observed on a telephonic circuit: every
manifestation of electricity near a telephone may produce sounds of
greater or less force. Of this we have already given a proof in M.
d’Arsonval’s experiments, and others by M. Demoget demonstrate the
fact still more clearly. In fact, if a small bar magnet provided with
a vibrator be placed before one of the telephones of a telephonic
circuit, and the vibrating plate of the telephone be removed, in
order to draw away the sound produced by the vibrator, its humming
noise may be distinctly heard on the second telephone of the circuit;
a noise which attains its maximum when the two extremities of the
electro-magnet are at their nearest point to the telephone without
a diaphragm, and it is at its minimum when this electro-magnet is
presented to it along its neutral line. M. Demoget supposes that the
action which is exerted in this instance is that of a magnet exerting
two inducing actions which are opposite and symmetrical, with a field
limited by a double paraboloid and with an axis, according to his
experiments, which extended 55 centimètres beyond the magnetic core,
and a vertical diameter of 60 centimètres. He believes that in this way
it would be easy to telegraph on the Morse system, and that, in order
to do so, it would only be necessary to apply a key to the inducing
electro-magnet.
Mr. Preece points out three ways of overcoming the difficulty presented
by the induced reactions caused by the wires on each other.
1. By increasing the intensity of the transmitted currents, so as to
make them decidedly stronger than the induced currents, and to reduce
the sensitiveness of the receiving telephone.
2. To place the telephonic wire beyond the range of induction.
3. To neutralise the effects of induction.
The first mode may be effected by Edison’s battery system, and we have
seen that it is very successful.
In order to put the second mode in practice, Mr. Preece says that
it would be necessary to study the two kinds of induction which are
developed on telegraphic lines: electro-static induction, analogous
to that produced on submarine cables, and electro-dynamic induction,
resulting from electricity in motion. In the former case, Mr. Preece
proposes to interpose between the telephone wire and the other wires a
conducting body in communication with the earth, capable of becoming a
screen to the induction by itself absorbing the electro-static effects.
He says that this might be accomplished by surrounding the telegraphic
wires adjacent to the telephonic wire with a metallic envelope, and
then plunging them in water. He adds that the effects of static
induction are not completely destroyed in this way, since the substance
used is a bad conductor, but they are considerably reduced, as he
has proved by experiments between Dublin, Holyhead, Manchester, and
Liverpool. In the second case, Mr. Preece admits that an iron envelope
might paralyse the electro-dynamic effects produced by absorbing them,
so that if insulated wires were employed, covered with an iron case,
and communicating with the earth, the two induced reactions would be
annulled. We will not follow Mr. Preece in his theory as to these
effects--a theory which seems to us open to question, but we content
ourselves with pointing out his proposed mode of attenuation.
In order to carry out the third expedient, it might be thought that it
would be enough to employ a return wire instead of going to earth, for
under such conditions the currents induced on one of the wires would be
neutralised by those resulting from the same induction on the second
wire, which would then act in an opposite direction; but this mode
would only be successful when there is a very small interval between
the two telephone wires, and they are at a considerable distance from
the other wires. When this is not the case, and they are all close
together, as in submarine or subterranean cables, consisting of several
wires, this mode is quite inefficient. A small cable, including two
conductors, insulated with gutta-percha, may be successfully carried
through the air.
The use of two conductors has the further advantage of avoiding the
inconvenience of stray currents on the line and through the earth,
which, when the communications to earth are imperfect, permit the line
current to pass more or less easily into the telephonic line.
In addition to the disturbing causes in telephonic transmission we have
just mentioned, there are others which are also very appreciable, and
among them are the accidental currents which are continually produced
on telegraphic lines. These currents may proceed from several causes,
at one time from atmospheric electricity, at another from terrestrial
magnetism, at another from thermo-electric effects produced upon the
lines, at another from the hydro-electric reactions produced on the
wires and disks in communication with the earth. These currents are
always very unstable, and consequently they are likely, by reacting
on the transmitted currents, to modify them so as to produce sounds
upon the telephone. Mr. Preece asserts that the sound proceeding
from earth currents somewhat resembles that of falling water. The
discharges of atmospheric electricity, even when the storm is remote,
produce a sound which varies with the nature of the discharge. When
it is diffused and the clap takes place near at hand, Dr. Channing,
of Providence, U.S., says that the sound resembles that produced by a
drop of fused metal when it falls into water, or, still more, that of
a rocket discharged at a distance: in this case it might seem that the
sound would be heard before the appearance of the flash, which clearly
shows that the electric discharges of the atmosphere only take place in
consequence of an electric disturbance in the air. Mr. Preece adds that
a wailing sound is sometimes heard, which has been compared to that of
a young bird, and which must proceed from the induced currents which
terrestrial magnetism produces in the metallic wires when placed in
vibration by currents of air.
M. Gressier, in a communication made to the Académie des Sciences on
May 6, 1878, has spoken of some of these sounds, but he is totally
mistaken in the source to which he ascribes them.
‘In addition to the crackling sound caused by the working of telegraph
instruments on the adjacent lines, a confused murmur takes place in the
telephone, a friction so intense that it might sometimes be thought
that the vibrating disk was splitting. This murmur is heard more by
night than by day, and is sometimes intolerable, since it becomes
impossible to understand the telephone, although nothing is going on
in the office to disturb the sound. The same noise is heard when only
one telephone is used. A good galvanometer inserted in the circuit
reveals the presence of sensible currents, sometimes in one direction,
sometimes in another.’