88. A telegraph wire suspended on poles is attached to insulators, to prevent the escape of the current to the earth at the points of support. Insulators should be regarded in the light of conductors, whose value depends upon their resistance to the passage of the current.
89. The insulation of a line is never perfect, even in the dryest weather. There is a leakage at every support, which is greatly increased when the surfaces of the insulators are damp, especially if covered with smoke or dirt. Experiments show that soot will destroy the surface insulation of the best insulators, even when exposed to the cleansing action of the rain. This evil is confined, however, principally to cities, and does not manifest itself to nearly so great an extent in the open country.
90. Insulators, considered as conductors, follow the same law as other conductors. The less the diameter and the greater the length, the more resistance is opposed to the escape of the current. As in this case the resistance is almost entirely a question of surface, the best insulator is that having the smallest diameter and the greatest length between the wire and the support. The latter is accomplished by making the insulator of a cup form, or still better, of two cups, one placed within the other.
91. The material of which the insulator is composed should be a poor conductor of electricity and heat, a non-absorbent of moisture, with a surface repellant of water, and free from pores or cracks. It should also remain unaffected by exposure to the weather, and the effects of heat and cold. Nearly all of the materials ordinarily employed are, however, liable to some of these objections.
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 :
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).
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 :
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.
As the effective strength of the current in any circuit
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.
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:
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.
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 :
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
* 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,
20 x 10 = --------- = 6.1, will go over the line to B. 33
The current from the other battery, E', in like manner 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
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 :
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
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 :
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.