Notebook of Gerard Henderson Cowan - Part 10

Notes
The curre
A telephone is an instrument 
which enables two people

to communicate with

each other beyond the

range of their voices.

D III Telephone
down still further and the
current flowing back in the
opposite direction nullifys
their effect thus allowing
the diaphragm to spring
back. This causes the diaphragm 
to vibrate at the same rate
as the armature.
By this means electrical
vibrations are converted into
mechanical vibrations.
The receivers on the D III are
put in parallel. The advantage
of this is if the leads of one
receiver get broken the other
may still be used, also the
wire in receivers being very
fine has a big resistance
and by putting the receivers
in parallel this resistance
is halved 

- D III Telephone -
The Microphone.
Speech does not give a set
number of vibrations like an
armature so some very sensitive
instrument is needed to
convert the vibrations of the air
caused by the voice into
electrical vibrations.
The instrument used is the
Microphone.
This consists of the Capsule
and an outer Casing into
which the Capsule fits
Capsule consists of a metal
xxxxof ^body over which is stretched a mica Disc and
Carbon Diaphragm beneath
this diaphragm is a
carbon block this block
has has circular groves
cut in it which are filled
with Carbon Shot 

Hand drawn diagram - see original

D III Telephone
Microphone
The carbon block is held in
place by a screw this
screw is insulated from the
metal Case of the Capsule
and has a platinum contact
point on its end.
- Action of the Microphone -
When the pressel switch is
pressed the current flows
up the screw into the
carbon block through the
carbon shot to the diaphragm,
which is lightly pressing
against them, from the
diaphragm to the metal
case.
Now the diaphragm is
very sensitive and when
you speak into the microphone
it vibrates this 

How to adjust armature of D III
Release both clamping screws &
withdraw both Contact screws.
Press the key and advance
No 1 contact screw until the
loudest buzz is heard then
clamp up No 1 clamping screw.
Next, with the key still pressed
advance No 2 Clam Contact
screw until the keenest buzz
is heard then clamp up
No 2 clamping screw and
armature is adjusted
D III Telephone
Microphone
making the current better or
less contact which gives a
varying current of electricity.
Every change in this current
effects the magnetic field
which in turn induces an
exactly similiar current
in the secondary winding,
which acting on the metal
diaphragm of the receiver
produces exactly the same
vibrations thus reproducing
the voice 

Tests of the D III
No 1 Press the key and a buzz
should be heard
No 2 Short circuit L and E, press
the key and a loud buzz
should be heard in both receiver
No 3 With L & E still shorted circuited
press the pressel switch in
the microphone and
clicks should be heard in
the receiver
Blow into the microphone
and a whistling sound
should be heard in the
receiver
Tests of the DIII
No 1 Press the If no buzz is heard,
(A) test the batterys (B) & adjust the
armature (C) Clean all contact
points
No 2 test fails, there must be
some fault in the Secondary
(A) See if the two terminals of the
receiver leads have become
short circuited by anything.
If a buzz is heard in one
receiver only the leads of
the other are probably
DIS.-
No 3 If this test fails the fault must
be in the speaking circuit
as the receivers and secondary
circuit have already been tested
Examine the terminals on
microphone case to see if they
are short circuited. Take out the Capsule
and complete circuit between spring ^and case 

Tests of a DIII Cont'd
No 3 Contd
If the pressel switch is now
pressed and clicks are heard
in the receiver the speaking
circuit is all right, therefore
fault lies in Capsule, see that
the Capsule has good contact
if it has and will still not work
the Capsule is no use and
must be replaced by a new one
Notes on a Condenser
A Condenser to used to enable
you to tap in on existing
telegraph lines.
All condensers are formed
by two conductors and
an insulator.
A Steady current will not
act through a condenser
an Alternating Current will 

True North is the true north

meridan

Magnetic North is the North

given by any compass
Hand drawn diagram - see original document.

The difference between the

true and magnetic north

is called the magnetic

variation

All bearings taken by the 

compass are magnetic

Map Reading
Map A map is a collection of conventional

signs, put together on paper, which

represent a certain sector of actual

landscape

Sketch A Field Sketch is a piece of landscape

The most important things to look

for when opening a map

1^st The scale. given in the following 

ways Hand drawn diagramme or 1/20000 or 4 inches to

1 mile

2^nd The true and magnetic north line

which enables you to set your map

correctly

3^rd Various methods of representing

hill features

Methods of finding the approximate true N

Watch Method

Point the hour hand to the

sun then take a line outwards

from the center of the dial

halfway between the hour

hand and 12 o'clock this line

represents due S

Hand drawn diagram - see original document.

The equinox method

at the end of March or Sept

the sun rises due East

The alters of all churches

in England point due East

Also ^ by the pole star which is

Methods of finding N Continued

approximately North.

To find this take five times 

the distance, between the stars

known as pointers, straight

up wards in the direction

they are pointing. The two

stars known as pointers are

part of the Great Bear Constellation

 

Hand drawn diagram - see original document.

GREAT BEAR CONSTELLATION

Setting a map with a compass

Place the compass on the true

North and South line, then

without disturbing the

compass draw this around

until the Magnetic North

on the map coincides with

the magnetic north given by

the Compass
Setting a map by Objects

You can identify your

position on the map

pick another prominent object

that can also be identified on

the map, draw a line between

the two and then swing

the map around until

this line is straight between

the two objects, thus
 Hand drawn diagram - see original document.

Setting Map by Objects Contd
When your position is not 

known pick two prominent

objects that you can identify

on the map and do

exactly as before

The Service Prismatic  Compass

Consists of a magnetic needle

balanced on a pivot and

carrying a dial divided into

figures degrees, contained in

a metal box round which

is a graduated brass ring

marked to show every fifth

degree and the points of the

compass

The metal cover C opens on

a hinge and is fitted with

a glazed window W, on which

is traced a fine black hair

line V, for use as a sighting

vane

The Service Prismatic  Compass

Opposite the hinge of the cover

is fitted a prism R through 

which can be read the

graduled graduated edge of 

the dial, while at the same

time an alignment of the

object and of the sight vane

on the cover is observed through

the slit above it

The Prism should be moved up

or down in its slot until

the figures on the dial

are properly focussed.

A clamping screw S is

provided for clamping the

needle when not in use, and

a "check-spring" A for checking

its oscillations when

observing.

A brass ring B, is attached for

for convenience in holding it