266

THE COLLIERY GUARDIAN.

February 11, 1916.

MINE TELEPHONE EQUIPMENT*

By Walter C. Freeman.

The many advantages and economies accruing from
the installation of telephone systems in.mines have been
frequently discussed, but little attention has been
devoted to the. question of equipment and its arrange-
ment to give the most efficient operation.

Telephone systems in’general are classified into two
main groups—namely, the “ common battery ” system
and the “local battery” system. Each has its
particular advantages for the class of.service for which
it was designed, though the speech-transmit.ing effici-
encies under the same operating conditions are nearly
equal in value. The main point of difference between
the two systems is the method of supplying electrical
energy to the telephone instruments.

In common-battery operation, as the name implies, the
current for all of the telephones in the system is supplied
by one main battery, usually of the storage type, at a
potential of approximately 20 volts. This battery not
only furnishes the current for talking purposes, but for
the operation of . the various switchboard signalling
devices as well.

It will be readily understood that this method of
power supply is objectionable for mining service, because
any interruption of the current would totally disable
the system. In addition to this serious drawback, the
common battery system does not provide a means of
signalling between any two telephones without the inter-
position of a switchboard and the assistance of the
switchboard operator in establishing a connection.
Furthermore, this system, when installed to meet the
service requirements of mines of average size, does not
compare favourably with the local-battery system in the
factors of first cost, maintenance and operating expense.
For these reasons the common battery is seldom recom-
mended or used for service underground.

The Local Battery System.

The telephones used with the local-battery system are
equipped ’and wired to consume current for talking pur-
poses, furnished by either two or three dry-battery cells
placed within the housing of each telephone set. The
alternating current required for ringing other telephones
on.the same line, and also for signalling the switchboard
in large systems, is generated by a hand generator,
which likewise is included in the equipment of each
instrument. Each telephone is thus independent of an
outside source of power, and consequently may be used
either for systems comprising only two telephones or
for larger systems with many instruments connected.

Fig. 1.—Essential Parts of Mine Telephone
and Circuit Connections.

Ge/v/Tz/o/'

Receiver

Hock

Switch

BaRenes
—ifir

The local-battery system, when the electrical pro-
perties of the various pieces of apparatus in the tele-
phones are properly proportioned, is quite flexible.
Any number of telephones up to 15 may be connected in
multiple to one pair of wires, so that each telephone
station may call and talk with any other on the same
circuit without the aid of a switchboard and operator’s
service. The signalling between the various stations is
accomplished by means of a system of code rings which
are sounded on all telephones simultaneously; therefore
the number of telephones per line should be limited to
about 15, so that the ringing code will not become unduly
complex and difficult to transmit correctly. It is
possible to operate as many as 40 instruments on a line,
but the continual ringing of the bells and the complex
code of signals would be distracting.

In mines where a greater number of telephones are
required, the telephones are separated into groups of
convenient number served by one line. The several
lines are brought to the surface and centralised on a
switchboard, so that a connection may be established
between any two- telephones in different groups. This
connection to the switchboard does not affect the ability
of any station to call and converse directly and inde-
pendently with any other telephones connected to the
same line. An injury to a telephone or an open circuit
in any section of the line wire will not, in the majority of
cases, interfere with the operation of the rest of the
system as a separate unit. The local-battery system is
simple to instal and maintain and, in addition, is
generally less expensive in first cost and annual charges
than the common-battery system.

The three principal elements of a mine-telephone
system are the telephone sets, the line construction, and
the switchboard. The electrical properties of telephones
intended for mining service are largely the same as for
local-battery telephones used in the smaller towns and
villages, but the mechanical construction of mine tele-
phones is quite different, owing to the severe hazards
to which telephones are exposed underground..

Mine Telephones Should be of Strong Construction.

In mining service the instruments are frequently
exposed to mechanical injuries, such as falls of rock and
ore, rough handling by careless workmen, concussion of
blasting and other self-evident conditions. Besides these
hazards, .‘there are many localities in which the tele-

* Coal Age.

phones are subject to excessive, moisture and the
corrosive action of acid mine waters and gases. These
conditions have been taken into consideration, by tele-
phone manufacturers who have developed and now sell
telephones designed especially for. service in mines.

In order to understand thoroughly how the instru-
ments operate reference may be made to fig. 1, which
shows diagrammatically, the essential parts of a mine
telephone and the circuit connections. This circuit
arrangement is known as a “ bridging ’ ’ telephone circuit
because of the position of the polarised, ringer and the
hand generator, which, are connected in multiple to, or
bridged across, the line wires. In this circuit the
ringer operates whenever the alternating ringing current
traverses the line. The hand generator is equipped with
an automatic circuit closer, so that it is connected in the
circuit only, when the crank is turned to call another
telephone station. The object of this switch is to
remove the low resistance path, through the generator
armature, which would seriously impair the ringing and
talking qualities of. the circuit. The permanent con-
nection of the ringers is necessary for the reception of
the signals, and does not reduce the talking efficiency
to any appreciable extent, because the coils have a total
resistance of about 1,000 ohms, and a high choking
effect on the passage of the voice currents via the ringers.

The talking circuit includes the transmitter, induction
coil, receiver, and hook switch. The latter is operated
by taking the receiver from between the jaws of the
hook. In some telephones the hook switch is controlled
by the weight of the receiver, and is operated by the
upward thrust of a spring when the receiver is lifted
from the hook. The hook switch in its operated position
closes two pairs of contacts. One pair closes the circuit
from one line terminal through the receiver and
secondary winding of the induction coil back to the
other line terminal. The other pair of contacts controls
the battery supply and closes a circuit including the
transmitter, the primary winding of the induction coil
and the battery.

How the Voice is Transmitted.

When the user speaks into the transmitter, the sound
waves impinge upon the transmitter diaphragm and
impart to it a vibratory motion. The vibrations are
carried, by means of a small piston, to a polished carbon
electrode, which is separated from a similar electrode by
small carbon fragments of uniform size. The electrical
resistance between the two electrodes, which are con-
nected to the transmitter terminals, is variable, and
depends upon the area of contact between the various
fragments of carbon and the electrodes. Under the
varying degrees of pressure on the piston the resistance
of the transmitter is raised or lowered, and passes <a
current of ever-changing value, which corresponds in
amplitude to the sound waves of the speaker’s voice.
The undulating current flows in the primary winding of
the induction coil (in reality a small transformer), which
not only steps up the voltage, but also allows the trans-
mitter to operate in a local circuit where it is not affected
by the resistance of the line circuit.

When the incoming voice currents are impressed upon
the receiving telephone they do not pass through the
ringer coils, because of their high impedance or choking
effect of the windings, but continue on through the
secondary winding of the induction coil and the receiver.
The receiver contains a U-shaped permanent magnet,
whose pole extremities are each surrounded by small
electro-magnet coils connected in series. A thin soft iron
diaphragm is clamped at right angles to the pole faces
of the magnet so as to lie within its magnetic field.
The voice currents are alternating in character, and part
of each wave augments the field of the permanent
magnet through the action of the electro-magnet coils,
and increases the pull on the diaphragm, causing it to
approach nearer the poles of the magnet. Conversely,
the other and opposite portion of the wave diminishes
the tractive effect of the magnet, because the current
opposes and counteracts a part of the permanent field
and allows the diaphragm to recede. The rapid recur-
rence of this action results in a vibrating movement of
the diaphragm, which gives motion to the surrounding
air, and reproduces the sounds spoken into the trans-
mitter at the distance station.

When the receiver is replaced upon the hook, the
battery circuit is opened so that no energy will be con-
sumed, while the telephone is not in use. The receiver
circuit is also opened, in order to remove the low
resistance path, in multiple to the ringers, which would
seriously impair the operation of the latter apparatus.
Some telephones are wired for and equipped with a
condenser in the receiver circuit, such condenser forming
a practical bar to the passage of the alternating-current
of low frequency used for ringing, but offering little
resistance to the high-frequency voice current. This is
a valuable device in that it eliminates the ringing trouble
frequently experienced on local-battery telephone lines,
owing to the failure of the last used to hang up the
receiver at the end of a conversation.

Specifying Telephone Equipment.

In ordering equipment for a mine telephone system
it is well to specify that the instruments be equipped
with ringers wound to a resistance of about 1,600 ohms.
This resistance value is low enough to insure ample
current flow in the ringer coils and a clear, loud signal,
yet high enough to allow the installation of 15 or 20
telephones on one line without unduly overloading the
hand generators. The resistance of the ringers should be
equal in the various telephones connected on the same
line, otherwise the telephones equipped with low-
resistance ringers will consume the greater share of the
ringing current, and cause the more highly wound ringers
to ring weakly, if at all.

The local operating conditions and service require-
ments in mines are so varied that it is impracticable to
attempt to recommend any standard practice covering
the number and locations of the telephones. It is the
usual practice to instal one telephone within convenient

distance of each group of miners, so that reports and
orders may be quickly and easily transmitted.

Fig. 2 shows a portion of the plan of a mine and the
locations of the instruments. In this diagram the
squares represent the telephones, and the circles the
loud-ringing extension bells connected by temporary
wiring to each telephone. The telephones are located in
the main passageways, so as to take advantage of favour-
able air currents for carrying the sound of the gongs in
the proper direction as far as possible. The extension
bells are used to- avoid the necessity of moving the tele-
phones as the work advances, and so that the signals will
be heard clearly above the noise .of running machinery.

Although local-battery mine telephones will operate on
a circuit consisting of one wire and the earth for the
so-called return, it is the best practice to employ a
full metallic circuit consisting of two wires connecting all
instruments. Ground-return circuits are objectionable
in that they are extremely liable to become noisy owing
to foreign currents, especially in mines where electricity
is used for haulage and the operation of drills and
cutters. Furthermore the ground connection is in many
cases difficult to perfect, and there is liability of “ cross-
talk ” or interference between simultaneous conversa-
tions on separate lines of this kind.

Best Type of Circuit.

The metallic circuit is more expensive to instal owing
to the additional amounts of material and labour, but
this difference is offset by the gain in reliability and
efficiency. The kind of line wire to be used is a matter
of choice, depending upon the local conditions. Iron,
copper and copper-clad steel are the conductors generally
used, and in all cases these wires should have a heavy
insulation and a substantial weatherproof exterior braid.
No. 12 B.w.g. iron wire, insulated in the manner

Fig. 2.—Plan of Mine, showing Location of
Instruments.

o Extension Betts

nr

described, is entirely satisfactory for mine systems, in
electrical properties and mechanical strength, but should
be used only in dry mines. In localities where there is
much seepage, especially of waters having an acid
reaction, lead covered and rubber insulated twin copper
wire should be employed. Insulated copper clad steel
wire of the No. 14 B. and S. gauge has been used to.
some extent in place of iron wire with good results.
This wire has the tensile strength of steel, and a con-
ductivity about 40 per cent, of that of pure copper wire
of the same diameter.

The wires should be supported much in the same
manner as electric light and signal wires, on either
porcelain knobs or glass insulators fastened to the roof
timbering. The same precautions for insulating electric
light circuits should also be observed in the construction
of telephone lines. The clearance between the telephone
lines and such circuits should be kept at the maximum
distance, to avoid accidental contact and. to reduce the
electro-magnetic induction between the two systems.
In some places where lighting and power circuits parallel
rhe telephone lines for a considerable distance, the
foreign currents caused by induction make the telephone
lines so noisy that conversation is nearly impossible.
To diminish this interference, it is necessary to use
either twisted-pair telephone wires on such parallel runs,
or to transpose the two wires of the telephone circuit at
frequent intervals. Either method exposes each wire
of the pair equally to the disturbing influence and
balances the circuit so that the induced currents in each
tend to neutralise. Under this condition there is no
flow of induced current; therefore no extraneous noise is
heard in the telephones.

All splices should be made with wire connectors, or
else well soldered without the aid of acid soldering
pastes or compounds. Bad splices are intermittent
sources of trouble and are extremely hard to locate.
Where wires pass under or over pipes or through holes
liable to abrade the insulation they should be covered
with “ circular loom,” “ flexduct,” or porcelain tubes.

As before stated, it is necessary to use a switchboard
for connecting the various lines when the number of
telephones installed does not permit the connection of
all instruments to one circuit.

One switchboard scheme used by a West Virginia
mining company seems to be ideal for a small system.
The company in question used a small switchboard,
and had it equipped with a small double-pole double-
throw switch in connection with each of the ten line
equipments installed therein. The purpose of these
switches was to disconnect the lines from the switch-
board, and to connect them to a pair of bus wires at
night, so that any telephone might call any other without
the necessity of keeping the operator on duty when the
volume of traffic was light. As each line carried but
three telephones, each station could easily signal any one
of the remaining 29 stations. This instance is cited to
show that, with a little study, telephone systems can
be worked out so as to give maximum convenience and
economy.