![[IMAGE]](contacts/fig32-5.gif)
Insulators of glass and porcelain being conductors of heat, a change of temperature from cold to warm causes a condensation of moisture upon their surfaces, including the portion protected from the direct action of rain, and from this arises the principal objection to the use of these substances in the construction of an insulator.
Hard rubber is in itself a better insulator than glass ; but its surface, from exposure to atmospheric influences, soon loses its property of repelling moisture, and becomes rough and porous.
A surface which repels watery accumulations will cause them to flow disconnectedly in drops, instead of forming a continuous conducting film. This property is therefore one of great value for the purposes under consideration.
92. The Glass Insulator.--- The insulator most commonly employed in this country is the glass. This is generally made in the form represented by Fig. 32, which is a sectional view of the insulator fixed upon a wooden bracket, the latter being securely spiked to the side of the pole. The line wire passes alongside the groove surrounding the insulator, and is fastened with a tie-wire encircling the insulator, both ends of which are wrapped around the line wire. The concavity of the under side of the glass keeps it dry, in some measure preventing the current form escaping to the wet bracket and pole through the medium of a continuous stream of water.
93. The Wade Insulator.--- This is largely used in the Western States. Its construction is shown in Fig. 33.
A glass insulator, somewhat similar in shape to that last
described, is covered with a wooden shield, to prevent fracture
from stones and other causes, the wood being thoroughly
saturated with hot coal tar, to preserve it from decay. The line
wire is tied to the outside of the shield, in the same manner as
when the glass insulator is used.
This insulator is usually mounted upon an oak bracket, as in
Fig. 33, secured by spikes to the side of the pole or other
support. When it is intended to be mounted upon a horizontal
cross-arm it is placed upon a straight wooden pin, instead of a
bracket. The pin or bracket is usually saturated with hot coal
tar, in the same manner as the insulator shield.
94. Farmer's Hard Rubber Insulator.---
This is shown in Fig. 34. It is a good insulator when
new, but by exposure to the weather its surface becomes rough
and spongy, and retentive of moisture. It is screwed to the
under side of the cross-arm or wooden block, which is secured
to the pole. The best form is that which is made with a drip or
shed, as shown in the figure. If exposed to the direct action
of rain it ought always to be placed in a perpendicular
position. It will be noticed that this insulator holds the line
wire by suspension.
95. The Lefferts Insulator.---
This is composed of a suspension hook fixed in a socket
of glass, of the form represented in Fig. 35. This is inserted
into a hole bored in the under side of a block or cross-arm,
and fastened with a wooden pin. In painting the arm or blocks
the paint must not be allowed to get on the surface of the
glass.
96. The Brooks Insulator.---
Figs. 36 and 37 show the construction of this insulator,
which consists of a suspension hook cemented into an inverted
blown glass bottle, which is again cemented into a cast iron
shell, provided with an arm which screws into the pole, as in
Fig. 36. Another form is made, designed for attachment to a
cross-arm, as in Fig. 38. The remarkable insulating properties
of this arrangement are mostly due to the use of paraffine, with
which the cementing material (sulphur) is saturated. It has
also been discovered that blown glass possesses extraordinary
properties of repelling moisture. Additional advantage of this
fact has been taken in the construction of this insulator, as
may be seen by reference to the cut.
97. Some important improvements
have quite recently been made in the mechanical construction of
the Brooks insulator, which are shown in Fig. 39. In the old
form of hook, shown in Fig. 37, the wire has three bearings. To
hold the wire securely, it is necessary that these bearings
should be so direct as to make it difficult to place the wire in
it, and the latter is often weakened by being bent. The new
hook, shown in Fig. 39, has five bearings for the wire, but not
so direct as to injure or weaken it by bending. The wire can be
placed in this hook without labor or difficulty, and a strain
cannot be applied in any direction by means of which the wire
can be removed or released.
98. Mode of Testing Insulators.---
The proper way to test the comparative value of
insulators is to fix them upon frames or standards, in sets of
ten or more, and place them where they will be fully exposed to
the weather. The test should be made when the weather is very
wet, by means of a wire attached to all of them in the usual
manner, and leading to the testing instrument, battery and
ground. By this means the relative resistances of either of the
insulators above described, and their consequent value in the
construction of a line, may be readily ascertained.
99. Escape.--- When the
insulation is defective, or the wire comes in contact with the
branches of trees, a wet wall, or other partial conductor, a
portion of the current passes to the ground, forming what is
technically known as an escape.
100. Weather Cross.--- The
escape of the current from one wire to another one upon the same
poles, owing to defective insulation, is sometimes wrongly
called ``induction,'' or ``sympathetic currents.'' Weather cross
is a much more appropriate term.
As electric currents always move in the direction of the least
resistance, their tendency is to escape from a long circuit to a
shorter one. This mixing of the currents from different wires
is a much more serious evil than a simple escape to ground, for
the latter may in most cases be overcome by increased battery
power ; but when cross connection exists between different wires
upon the same poles, an increase of battery upon one wire gives
it an advantage over the others, but necessarily at their
expense.
The effects of weather crosses usually manifest themselves upon
the occurrence of a shower sooner than the escape to ground,
because the horizontal arms become wet sooner than the vertical
pole.
On the English lines this difficulty is obviated by means of an
earth wire attached to each pole, and wrapped around the center
of the arms, thus cutting off the currents passing from wire to
wire, and conveying them to the ground. The battery can then be
increased at will on one wire, without interference with the
others. A much more economical and effective method of obtaining
this result is that of improving the insulation.
101. Effect of Escapes and Grounds upon
the Circuit.--- If the wire touches a conductor
communicating with the earth, or the earth itself, in a moist or
wet place, so that the point of contact offers little or no
resistance compared with the wire beyond, the fault is called a
ground. The effect of a ground or escape is to increase
the strength of the current going out to the line, and to
exhaust the batteries more rapidly. Therefore, in working with
a continuous current, as is the case on American lines, the line
current increases in strength in wet weather, but the
variation or difference in the current at one station,
when the line is opened and closed at another, decreases,
and the effective signals are therefore weakened.
102. The laws of the Electric
Current.--- The laws which govern the propagation and
distribution of electric currents are so simple, and at the same
time so important, that every telegrapher should be familiar
with them. By their aid the phenomena above referred to may be
readily comprehended. The most important of these laws was
first enunciated by Ohm, in 1827, and is known as Ohm's
law. It may be briefly stated as follows :
103. Practical Application of Ohm's
Law.--- First Case.---To illustrate the application of
this law to circumstances occurring in practical telegraphy,
take the case of an ordinary telegraph line (Fig. 40), extending
from A to B, and perfectly insulated, having a resistance of 100
Ohms. Let the main batteries, E and E' have each an electro-
motive force of 1,000, and a resistance of 5 ohms, and let the
resistance of the instruments I and I' be equal to 10 ohms each.
The total resistance of such a circuit will be :
As the effective strength of the current in any circuit
If, however, A is sending to B, his key is alternately open and
closed. When open, the circuit of the battery E (Fig. 41) is
entirely broken. There will still, however, be a circuit from
the battery E', through I' and the line to the fault F, and
thence to the ground.
By Ohm's law we find the strength of this current to be as
follows:
104. Second Case.--- Suppose
the same fault to be located near A (see Fig. 42).
The current from the battery E will divide at F, part going to
the ground through the fault, and the remainder over the line to
B, and through the instrument and battery to ground. The
current from E' will divide in the same manner between the fault
and the route through I and E. Taking the battery E alone, and
considering the other battery E' simply as a conductor, the two
circuits beyond the fault give the following resistance :
* The joint resistance of any two circuits is found by
dividing the product of the two resistances by their sum.
When there are three circuits, first find the joint resistance
of two circuits as above, and treat it as a single circuit,
again applying the same rule. In the same manner the joint
resistance of any number of circuits may be calculated
(175).
// end footnote
This current will divide at the fault between the two circuits,
whose resistances are respectively 115 and 50, or in the
proportion of 23 to 10. Therefore 23 parts of the current will
go to the ground at F, and 10 parts,
105. Third Case.--- Let the
battery at A be doubled, the fault remaining as in the last
case. The electro-motive force and internal resistance of E
are both doubled, as in Fig. 43. The current from E will now
be :
When the batteries and instruments are equal at each end of a
line, a given fault will interfere most with the working of the
circuit when in the centre.
When the fault is near one end of the line, the station farthest
from it will receive the weakest signals, and the station
nearest it the strongest signals.
In increasing the battery power for working over an escape, the
addition should be made to the battery nearest the fault.
109. Distribution of Battery Power.---
If the insulation of a line was perfect at all times,
the position of the battery in the circuit would be a matter of
indifference. As all lines, however, are subject to more or
less leakage or escape throughout their entire length, the whole
battery should not be located at one end of a long line, for in
this case signals would be received much better at one end of
the line than the other. The usual arrangement is to place half
the battery at each end of the line, although if the escape be
uniform throughout the entire length of the line, the effect
upon its working will be the same, whether all the battery is
placed in the centre of the line or a portion of it in the
centre and the remainder divided equally between the two
ends.
If a certain portion of the line is especially defective in its
insulation, the distribution of battery power may sometimes be
varied in accordance with the principles laid down, with
manifest advantage.
The insulation of the batteries themselves is a matter of great
importance, and should never be neglected.
(29).
110. Working several Lines from One
Battery.--- It has been for many years the practice in
this country to work a considerable number of lines at the same
time from a single battery. The number of wires that can be
worked in this manner without interference depends entirely upon
the proportion between the internal resistance of the battery
employed and the joint resistance of all the circuits connected
with it. If the resistance of the battery itself is
inappreciably small in comparison with that of the lines
connected with it, the current on any given circuit will vary
but little, whether the others be open or closed. With the
Grove battery of, say, 50 cups, it is possible to work as many
as 40 or 50 well insulated lines, of 300 miles or more in
length, without appreciable interference. The great objection
to this system is that, in wet weather, the resistance of the
lines is enormously diminished, and the interference on one
circuit with another, as a necessary consequence, greatly
increased.
It is a common practice when this occurs to increase the number
of cups in the battery, which in most cases has a tendency to
aggravate the very evil it is sought to remedy ; for with every
such addition the resistance of the battery becomes greater in
proportion to that of the lines, and the currents more unsteady
and fluctuating. No small part of the trouble experienced in
working lines in wet weather arises from this cause, although
usually attributed entirely to defective insulation. It is
true, however, that the latter indirectly causes the difficulty,
by lessening the resistance of the wires.
111. Experiments made on a very
wet day, upon a number of circuits of nearly the same length
(100 miles), leading out of New York city, proved that when one
such wire was attached to a carbon battery of 60 cups the
addition of three other similar wires reduced the current on the
first one 12 per cent. It is a common practice to attach as
many as eight wires to such a battery, which in the above case
would have reduced the current about 25 per cent.
112. It is the opinion of many
scientific experts in practical telegraphy that increased
efficiency, as well as economy, would result from working
telegraph lines with a single series of Daniell's battery, in
its most approved form, upon each circuit. The objections urged
against this battery is the increased amount of room it takes
up, as well at its somewhat greater original cost.
113. As long as the present
system remains in vogue, care ought to be taken that the
different circuits leading from the same battery are as nearly
as possible equal in resistance ; and it must not be forgotten
that the interference caused by attaching too many wires to a
battery cannot be remedied by the addition of more cups for
intensity. The electro-motive force of a carbon battery is
exhausted with a rapidity nearly in proportion to the number of
circuits supplied from it. In the case of the Grove battery
this effect is not so apparent.![[IMAGE]](contacts/fig33-5.gif)
![[IMAGE]](contacts/fig34-5.gif)
![[IMAGE]](contacts/fig35-5.gif)
![[IMAGE]](contacts/fig36-5.gif)
![[IMAGE]](contacts/fig37-5.gif)
![[IMAGE]](contacts/fig38-5.gif)
![[IMAGE]](contacts/fig39-5.gif)
Call the sum of the electro-motive forces ...E
`` `` internal resistance of the battery...R
`` `` resistance of line and instruments...L
`` `` the effective strength of current ...C
E
Then C= ---------
R + L
That is : The effective strength of the electric current in
any given circuit is equal to the sum of the electro-motive
forces divided by the sum of the resistances
(174).![[IMAGE]](contacts/fig40-5.gif)
100 ohms, line, \
20 `` instruments, > = L
10 `` batteries, = R
---
130 `` = R + L
The line being perfectly insulated, the whole current from the
batteries will necessarily act upon both instruments.
E
is, by Ohm's law, equal to -------, in this case it will
R + L
be
2000
---- = 15.4
130
With key open at A or B.......... = 00.0
Difference, or effective working strength. = 15.4
If, on the above line, an escape occurs between the stations A
and B, offering a resistance of 50 ohms, the effect will be the
same as if a wire having a resistance of 50 ohms were connected
from the centre of the line to the ground. The current from
each battery has a tendency to divide at the fault between the
two routes open to it, in proportion to their relative
conductivity ; or what is the same thing, in inverse ratio to
their respective resistances. But in this case the electro-
motive forces and resistances are exactly the same on each side
of the fault ; and the positive current from one battery, and
negative from the other, have an equal tendency to escape to
ground at the fault. These opposite tendencies consequently
neutralize each other, and no effect whatever is produced upon
the circuit by the fault as long as the line remains closed both
at A and B.![[IMAGE]](contacts/fig41-5.gif)
5 ohms resistance of battery,...... = R
10 `` `` `` instrument, \
50 `` `` `` 1/2 line, > = L
50 `` `` `` fault, /
---
115 = R + L
E 1000
C = ------- = ------ = 8.7
R + L 115
With the key closed at A, the strength of the current in
the instrument at B was found to be
15.4
With key open at A, as above.............. 8.7
----
Difference, or effective working force ... 6.7
In this case the latter will obviously be the same, whether A
sends to B or B to A.![[IMAGE]](contacts/fig42-5.gif)
1. By the line instrument and battery at B.. 115 ohms.
2. `` fault F........................... 50 ``
115 x 50
Their joint resistance will be * ....... ---------- = 34.8 ohms
115 + 50
Add resistance of battery itself, 5 ohms, and instru-
ment, I, 10 ohms................................ 15 ``
------
The total resistance will be........................ 49.8
1000
And the current leaving the battery, E, = ------ = 20
49.8
// footnote
20 x 10
= --------- = 6.1, will go over the line to B.
33
The current from the other battery, E', in like man-
ner divides at F, between the fault and the circuit
through the instrument and battery at A. The joint
resistance of the two circuits is
15 x 50
--------- = 11.5
15 + 50
Add the resistance of the battery E,
5 ohms, instrument I, 10 ohms,
and line, 100 ohms................. 115.0
-------
Total resistance...................... 126.5
The current leaving the battery E will 1000
therefore be........................ ------- = 7.9
126.5
The resistance of the two circuits beyond the fault
being 15 and 50, or as 3 to 10, 3 parts will go to
7.9 x 10
ground and 10 parts, or ---------- = 6.1, through I.
13
When A sends to B, the current in the instrument
at B will be :
Key closed at A.
From battery E'...................... 7.9
`` `` E ...................... 6.1
--------------
Total strength in I'................. 14.0
Key open at A.
From battery E'............ 1000/165 = 6.1
`` `` E ...................... 0.0 6.1
--------------
Difference, or available working current at B, 7.9
Now let B send to A. The current at A will be :
Key closed at B.
From battery E ...................... 20.0
`` `` E'...................... 6.1
--------------
Total strength in I ................. 26.1
Key open at B.
From battery E ............ 1000/65 = 15.4
`` `` E'...................... 0.0
--------------
Total strength in I.................. 15.4
------
Difference, or available working current at A, 10.7
![[IMAGE]](contacts/fig43-5.gif)
2000
-------------------
50 x 115 = 36.5
20 + ----------
50 + 115
which will divide at the fault in the same proportion
36.5 x 10
as before, the part going to B being ----------- = 11.0.
33
The current from E' will be
1000
----------------
20 x 50 = 7.7
115 + ---------
20 + 50 7.7 x 5
and the portion reaching A --------- = 5.5.
7
When A sends to B the signals will be as follows :
Key closed at A.
Current at B = 7.7 + 11.0 = 18.7
Key open at A.
1000
Current at B..... = ------ = 6.1
165
-------------
Effective strength at B ...... 12.6
Now let B send to A :
Key closed at B.
Current at A = 36.5 + 5.5 = 42.0
Key open at B.
2000
Current at A..... = ------ = 28.6
70
-------------
Effective strength at A ...... 13.4
![[IMAGE]](contacts/fig44-5.gif)
106. Fourth Case.--- Double the battery at B, the
fault remaining unchanged. See Fig 44.
1000
Current from E = ------------------
50 x 120 = 19.9
15 + ----------
50 + 120
19.9 x 5
Portion going to B ... = ---------- = 5.8
17
2000
Current from E' = ------------------
50 x 15 = 15.2
120 + ----------
50 + 15
15.2 x 10
Portion going to A ... = ----------- = 11.7
13
A sending to B :
Key closed at A.
Current at B = 15.2 + 5.8 = 21.0
Key open at A.
2000
Current at B... = ------ = 11.8
170
------
Effective strength at B... 9.2
B sending to A :
Key closed at B.
Current at A = 19.9 + 11.7 = 21.0
Key open at B.
1000
Current at A... = ------ = 15.4
65
------
Effective strength at A... 16.2
107. Thus we find that on a circuit consisting of
Line wire resistance.............. 100 ohms.
2 batteries `` .............. 10 ``
2 instruments `` .............. 10 ``
each battery having an electro-motive force of 1000, the
signals received will be as follows :
Signals at A. Signals at B.
When the line is perfect.................... 15.4 15.4
With escape 50 ohms in centre............... 6.7 6.7
Same fault at A............................. 10.7 7.9
Same fault at A, with battery doubled at A.. 13.4 12.6
Same fault at A, with battery doubled at B.. 16.2 9.2
108. The results of this
investigation may be summed up as follows :