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The relative strength of this wire is more than 50 per cent. greater than that of iron wire of equal weight, and its conductivity is also largely in excess of the latter. If we take, for example, a No. 8 galvanized iron wire, the gauge now usually employed in this country in the construction of the best lines, and compare it with a compound wire of nearly similar electrical capacity, the superiority of the latter will be manifest.
====================================================================== ------------- | Weight | Tensile|Conductivity| Poles |per mile|Strength| |per mile ------------------------------|--------|--------|------------|-------- Galvanized Iron Wire (No. 8)..| 375 | 1091 | 1 | 35 American Compound Wire (No. 8)| 112 | 514 | 1 07 | 23 ----------------------------------------------------------------------In the above table the average conductivity of a mile of No. 8 galvanized wire is taken as 1, as a standard of comparison. The last column shows the number of poles per mile which will give the same percentage of strain upon the ultimate strength of the wire. In practice, however, it is safe to reduce the proportionate number of poles used for the compound wire, as the steel core is much more homogeneous and less liable to fracture on account of flaws, than the iron wire.
The advantages which arise from increased conductivity of the line wire and the diminution of the number of points of insulation and support are fully treated upon in another part of this work. The mechanical advantages of the compound wire are also very great. The labor of handling and stringing a light wire is much less than when a heavy one is employed. In running the wires over buildings, a mode of construction which has become very common in all large cities, stretches may safely be made double the length of those taken with the ordinary wire, and yet with less strain upon the insulators. Another important point in favor of this wire is the imperishable nature of the copper, which is the exposed metal. It is well known that the zinc coating of galvanized iron wire is soon destroyed near the sea-coast, and from the effects of carbonic acid arising from the combustion of coal in cities (137). Copper, under the same conditions, remains wholly unimpaired. Many cases occur in the construction of lines in which transportation is an item of great expense. In such cases, wire of the same or greater conductivity than galvanized iron, weighs materially less, with no disadvantage whatever arising from its lightness.
161. The following table exhibits the weight, size, and relative strength of compound wires, equivalent in conducting power to the ordinary sizes of iron wire used in telegraphic construction.
========================================================================= S | Galvanized Iron Wire. || Compound Steel and Copper Wire. i |-----------------------||--------------------------------------------- z | Weight | Relative || Weight | Relative | Size of | Size of e | per mile.| Strength. || per mile.| Strength.| Steel Core.| Compound. --|----------|------------||----------|----------|------------|---------- 9 | 313 | 2.9 || 99 | 4.9 | 16 | 14 8 | 375 | 2.9 || 112 | 4.6 | 16 | 14+ 7 | 449 | 2.9 || 121 | 4.4 | 16 | 13- 6 | 525 | 2.9 || 147 | 4.5 | 16 | 12- -------------------------------------------------------------------------The term relative strength, used in the preceding table, is the quotient obtained by dividing the strain which would break the wire by its weight per mile.
In constructing lines with the compound wire, much care should be used in making the joints so as not to separate the copper sheathing from the steel core, thus allowing moisture to penetrate to the steel and oxidize it. This may, however, be guarded against by carefully soldering the joints.
162. The Gravity Battery.--- Several modifications of the Daniell battery (19), especially adapted to telegraphic use, are finding much favor within the past few years. The most economical and generally useful of these improved forms is the gravity battery. The best arrangement is that known as the Callaud. Another combination very closely resembling it, and giving nearly as favorable results, is known in this country as the Hill battery. In these elements the porous cup of the Daniell battery is entirely dispensed with, the two solutions being prevented from mingling by the difference of their respective specific gravities. The zinc plate of the Callaud element, in the form of a short hollow cylinder, open at both ends, is suspended in the upper portion of the containing jar, as shown in Fig. 56, by means of three hooks projecting from its upper edge, resting upon the jar. A strip of copper rolled into a spiral form is soldered to a copper wire covered with gutta-percha, forming the positive pole and connecting it to the zinc of the next element.
163. The manner of setting up this battery is as follows:
A sufficient quantity of soft water is poured into each jar to fill it to a point above the upper surface of the zinc. The battery should now be placed in the position which it is to permanently occupy, unless this has already been done. After the connections are made and everything in readiness, about three-quarters of a pound of sulphate of copper in lumps of the size of a hickory nut or larger, is dropped in, taking care that it does not lodge upon the zinc. The solution of sulphate of copper being of greater specific gravity, will remain at the bottom of the jar. The battery, after it is set up, should be kept on a closed circuit for about twelve hours, when its resistance will have become reduced so that the force will be available. As the battery continues in action, the sulphate of copper solution gradually becomes weaker and the zinc solution stronger. It is therefore necessary from time to time to add crystals of sulphate of copper, and to remove a portion of the zinc solution and replace by water. A good practical rule for maintaining this battery is to always see that the stratum of liquid around and in contact with the copper is kept of a blue color. The formation of transparent crystals upon the zinc indicates that the point of saturation of the zinc solution has been reached and that it should be diluted with water. A Baume hydrometer is very convenient for determining the density of the zinc solution. The latter should be maintained at from 20° to 30° in a main battery, and from 15° to 25° in a local.
It often occurs in using this battery that stalactites of copper attach themselves to the lower edge of the zinc and hang suspended in the solution, slowly but constantly increasing in length. These are first produced by a deposit of copper upon the zinc, which sets up a local action followed by a rapid decomposition of the solution and a further deposit of copper. These should be removed by means of a bent wire and allowed to fall to the bottom of the jar, as they occasion a useless expenditure of sulphate.
Absolute quietude is essential to the proper performance of this battery. A slight jar will cause the solutions to mingle, and this effect will be followed by a rapid deposition of metallic copper upon the zinc. When the zincs are removed for cleansing, care must be taken not to agitate the solution.
Prof. Hough, of the Dudley Observatory, has suggested the use of sheet lead in the place of the copper spiral, as it is cheaper and more readily cut and formed into proper shape. There is no perceptible difference in the electro-motive force or in the resistance of the battery when lead plates are substituted for copper in this way.
The electro-motive force of the gravity battery is the same as the Daniell, and the average resistance when in good working condition about three units.
164. Siemens' Universal Galvanometer.--- The apparatus employed for the measurement of electrical resistances consists essentially of a standard resistance, which is used for the purpose of comparison, a galvanometer, by which the result is indicated, and a galvanic battery. In the different methods of testing, these appliances are arranged in various ways, as particular circumstances may render convenient or desirable. The various methods of testing in use may be classified, however, under three heads, viz.:
1. By the angles of deflection of a galvanometer needle.
2. By the differential galvanometer.
3. By the Wheatstone bridge, or electrical balance.
The first-named method is the simplest in principle, and, with proper care, gives very accurate results. It is not so convenient as the other two methods for ordinary use, but is applicable more especially for the measurement of very high resistances, such as insulators, etc. It is also employed in measuring the internal resistance of batteries. As the strength of the current passing through the coils of a galvanometer is always proportionate to the sine or tangent of the angle of deflection of the needle, and is also inversely proportional to the resistance in circuit, it follows that if we find the deflection with a certain known resistance in circuit to be only 22°, and we then substitute for this known resistance an unknown one, which gives us a deflection of 39°, the tangent of the latter will be twice that of the former, and the unknown resistance is consequently found to be half that of the known resistance. (170.)
The second method is very convenient and is much used, although not equal in strict accuracy to the third method. The galvanometer coils are wound with two wires of the same length and resistance, insulated from each other with the utmost care. The needle is therefore surrounded by an equal number of convolutions of each wire, which are also equidistant from it. One end of each wire is connected to the battery, but in such a manner that the current flows in opposite directions through the wires. When, therefore, the two currents are of equal strength, one tends to deflect the needle to the right and the other to the left with equal power, and the needle remains at rest. If we insert an unknown resistance into the circuit of one of these wires the current is weakened, as is also its effect on the needle, which no longer remains balanced and at rest, but is deflected to one side. If we now insert a series of known resistances into the circuit of the other wire, until the needle is again brought into equilibrium, we are certain that the unknown resistance in one circuit is exactly equal to the known resistance in the other. (122.)
The third method is susceptible of the greatest accuracy of measurement, when proper precautions are observed. The connections of the ``bridge'' are arranged as follows:
We will suppose the wires A B C D (Fig. 57), arranged in the form of a parallelogram, to be of exactly equal resistance. If we attach the two poles of a battery, E, to the points 1 and 2, its current will divide at 1, half of it going through A B, and the other half going through C D, to the point 2, and thence to the other pole of the battery. The galvanometer G, placed on a wire connected across from 3 to 4, will not be affected as long as A B is equal to C D, no matter what the absolute resistance may be.
Again, when A bears the same proportion to C than B does to D, or when A: C:: B: D, no current will pass from 3 to 4 through the galvanometer. If the resistance of A be made 10, that of B 1, of C 1,000, and of D 100, the total resistance of A B will now be 11, and that of C 1,100 ; but the tension in each branch will have fallen in the same proportion at the points 3 and 4, and no current will pass between those points.
If, therefore, we insert a known standard resistance in the wire B, and an unknown one in D, and divide a given resistance between A and C until we get no effect upon the galvanometer needle, we are then certain that the resistance of A bears the same proportion to that of B as the known resistance does to the unknown one D, which may be readily calculated by proportion or the ``rule of three.''
It is not necessary, of course, that the wires should be arranged in the exact form shown, nor in fact is it often done, but the principle is more easily explained and remembered when thus arranged.
165. The Universal Galvanometer of Dr. Werner Siemens is constructed upon the principle of the Wheatstone bridge, just described, but its connections are so arranged that it may be used when desired for the method of deflections first mentioned.
The galvanometer is mounted upon a disk of slate about six inches in diameter. A groove in the edge of this disk, extending about half way round the circumference, contains a wire of considerable resistance, which corresponds to the wires A and C in the above diagram. A small platina roller, mounted upon a radial arm, is connected to one pole of the battery, and forms the connection with the wire A C, as shown at 1 in the diagram. The wire corresponding to B is supplied with three standard resistances of 10, 100, and 1,000 Siemens' units, respectively, either of which may be placed in circuit at pleasure, by means of contact plugs. The wire D is provided with binding screws for the attachment of the wire, or other resistance which it is required to measure. The galvanometer consists of a pair of very delicate astatic needles, suspended by a fine silk fibre. The coil has a resistance of 100 Siemens' units.
The radial arm carrying the platina roller also carries a pointer or index, moving over a scale upon the circumference of the slate disk, which is divided into 300 degrees, and which may be read to one-fifth of a degree by means of a vernier.
In using the instrument, the standard resistance corresponding most nearly to the unknown resistance which is to be measured is unplugged and placed in circuit at B (Fig. 57), while the unknown resistance itself is inserted at D. The radial arm carrying the platina roller 1 is now moved towards A or C, until the needle is balanced. The proportion of A to C is then read off the scale, from which the proportion of B to D is readily calculated, or is taken from a printed table furnished with the instrument.
This galvanometer may also be employed for comparing electro- motive forces, according to the method of Poggendorff,* and is applicable to almost any purpose for which an apparatus of the kind may be required.
* See Clark's ``Electrical Measurement,'' p. 105. Also Sabine's ``Elect. Tel.,'' p. 320.
// end footnote
This instrument usually has a constant of about four degrees, with one Daniell's cell through 1,000,000 Siemens' units. When used as a Wheatstone bridge, its range of measurement is from 0.17 to 59,000 Siemens' units. Higher resistances, such as insulators, may be measured by the method of deflections. The entire apparatus (except the battery) occupies a space only nine inches in diameter, and the same in height. It is packed in a neat case, and can be carried about with great convenience.
166. Pope & Edison's Printing Telegraph.--- Type-printing telegraph instruments, which were formerly employed for commercial telegraphing, have, within two or three years, been extensively introduced, in a modified and simple form, in the various branches of private telegraphy, with great success. One of the best of these is that of Pope & Edison, which is used on a large number of private lines in New York city.
The different portions of the apparatus, with the exception of the battery, are mounted upon a small table, similar in size and construction to that of an ordinary sewing-machine. At the back of the table are six binding screws to which are attached the line and ground wires, and the wires leading to the main and local batteries. The instrument operates upon what is known as the ``open circuit principle,'' each station transmitting with its own main battery---the line at the receiving station being connected directly through the relay to the ground without the intervention of a second battery.
The printing apparatus is placed upon a circular iron base in the centre of the table. In front of it is placed a dial containing the letters of the alphabet, arranged in a circle and provided with an index or pointer, mounted upon a horizontal shaft. This shaft also carries a type wheel, with the letters of the alphabet engraved upon the type wheel and dial. An electro-magnet beneath the base is provided with an armature, attached to a vibrating lever, the latter armed with pawls or clicks, so arranged in relation to the scape-wheel that every time the electro-magnet attracts its armature, the wheel is made to revolve a distance of one tooth, and the type wheel and index upon the same shaft a distance of one letter. At the extreme right of the circular base, and partly beneath it, as seen in the engraving, is placed a second electromagnet, whose armature lever passes in a horizontal direction below the type wheel. Directly underneath the type wheel an india-rubber pad is fixed upon the lever, by means of which an impression of the letter which is opposite it upon the type wheel may be taken in the manner hereafter to be described. This lever is also provided with a simple mechanical device for moving the paper forward the proper distance, as the impression of each successive character is imprinted upon it. This may be seen at the left of the printing apparatus. The type wheel is provided with a suitable inking roller, as shown in the engraving.
It will thus be understood that the printing mechanism is operated by two distinct electro-magnets, one of which is so arranged that its successive pulsations may be made to advance the index step by step to any required letter, while the other forces the strip of paper against the inked type upon the wheel, after it has been moved to the proper position by the first magnet. The type wheel is, of course, so arranged in reference to the index upon the same shaft, that when the latter points to any given letter the corresponding letter upon the type wheel is opposite the impression pad.
These two electro-magnets are placed in the circuit of a local battery, which is brought into action by a relay placed in the main line circuit, as in the ordinary Morse apparatus. The relay is shown at the right of the printing mechanism, covered by a small glass shade. It is the same in principle as the ordinary Morse relay, with the addition of a device termed the ``polarized switch,'' which consists of a permanently magnetized steel bar, pivoted between the poles of the relay magnet, and forming a part of the local circuit. This is attracted to the right or left according to the polarity of the relay magnet, which itself, in turn, depends upon the direction of the electrical current in the main circuit. The polarized switch determines the direction of the local circuit, causing it to pass through the magnet for moving the type wheel, or through the impression magnet, as may be required.
Two lever finger keys, with vulcanite knobs, are placed on each side of the printing apparatus, as shown in the engraving ; and it is by means of these that the instrument is operated. They are connected to opposite poles of the main battery in such a manner that, by depressing the right hand key, the positive pole of the battery is connected, through the relay magnet to the line, and the negative to the ground, while the left hand key, on the contrary, sends a negative current through the relay and line in the same manner.
The mode of operating the instrument is exceedingly simple. By depressing the right hand key a sufficient number of times in rapid succession, a series of positive currents is sent through the relays at both ends of the line, which are repeated upon the local circuits of both instruments. The positive currents deflect the polarized switches to the left, so that the local circuit is directed into the type-wheel magnet. The index and type wheel of both instruments, therefore, advance one letter every time the key is depressed, and they may thus be readily brought to any desired letter. When this has been done, the left hand key is depressed, which sends a negative current, reversing the polarized switch, and the local circuit is directed through the printing magnet, producing the impression of that letter upon the strip of paper, and this process may be continued indefinitely.
Suitable arrangements are provided for bringing the type wheels of the two instruments together in case they should accidentally be thrown out of correspondence.
These instruments are entirely automatic in their action, and a dispatch may be printed at the remote end of the line, in the absence of an attendant. In the event of any derangement of the printing apparatus, it may be used as a dial instrument as conveniently as if especially constructed for that purpose.
The battery is always disconnected, except at the moment of working, and therefore is consumed but slowly. Other systems require the battery to be constantly connected to the line whether working or idle. A battery of two carbon cells per mile, and in many cases even less, will work the instrument and remain in action from one to four weeks without renewal, according to the amount of telegraphing done upon the line.
It is impossible for the main circuit to be accidentally left open. Only one adjustment---that of the tension spring of the relay---is required after the instrument is first put in operation, and that but rarely on lines of ordinary length.
(Transcription note: The engraving referred to several times in this section appeared nowhere in my copy of this book, which is a pity, since it must have been a real beauty. In the sequence of figures, number 58 is missing. There is no doubt that the missing figure and the missing plate are one and the same, since this section comes between sections that describe figures 57 and 59. If anyone finds a copy of this book with the missing engraving, it would please me enormously to be able to complete the catalog of figures.)
(Addendum to the transcription note: Alan Barley <firstname.lastname@example.org> was kind enough to let me know that a copy of the patent for the Pope&Edison's printing telegraph referred to on this page was available at http://edison.rutgers.edu/patents/00102320.PDF. I have made a local copy here. While this probably doesn't contain the same illustrations that were intended for the book, the patent has to be useful if you're trying to make sense of the text in the book. The illustrations from the patent are below.)
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