Art. LXIV.—On, Duplex Telegraphy.
Plates XXVI. and XXVII.
[Read before the Wellington Philosophical Society, 21st November, 1874.]
My attention was drawn to the system of duplex telegraphy by an extract from a paper read by Mr. Culley before the Society of Telegraph Engineers in London, and which was published in the “Telegraph Journal.”
The system at present in use in England is based on the differential principle. The system that has been adopted in New Zealand is based on the Wheatstone Bridge principle. The differential principle is arrived at by producing an equality of currents. The Wheatstone Bridge principle is based upon an equality of tensions (now called potential). Tension or potential may be considered as a term analogous to pressure, as applied to water.
Before proceeding to describe the details on which duplex telegraphy is worked in New Zealand, it will be as well perhaps to give the following illustration as to the distribution of tensions in a split or derived circuit:—Figure 1 (Plate XXVI.) represents two pipes running parallel to one another. If at the point x a stream of water is made to enter both pipes, the pressure in each will be precisely the same. Now turning to figure 2. Supposing a connecting pipe, z w, be made between the two on the same level, at a point in the centre of the connecting pipe, there will be no motion, for the pressure on each end of the branch will be precisely the same; for where two forces meet one another, both travelling with equal and sustained velocities, at that point of juncture there is no motion, the two forces having equal and opposite effects, and thus neutralizing the action of each other. The case is different, however, if we make the connection at a lower level in the one pipe than in the other, as in figure 3. Although the pressure in each pipe still remains the same, the flow from g to the point m is to a lower level, therefore the pressure in the connecting pipe is all the one way, or nearly so.
Now, the principle on which duplex telegraphy is applied in New Zealand is on precisely the same conditions as shown in figure 2; for if we consider A B and C D to be two electric circuits branching from one conductor at x and joining again at y, the proportionate tension or potential is the same at every equal proportional part of the resistance of the two circuits; and at any point in either circuit, provided those conditions be observed, connection between the two circuits may be made, and there will be no tendency for the current circulating in either circuit to pass across from one circuit to the other, for although there will be a tendency for the current circulating in A B to pass to C D by way of z w, still as there is the same tendency for the current circulating in C D to pass to A B by way of w z, the two opposites neutralize one another;
hence there is no current moving in z w, for the potential is equal at the points z and w in either circuit; a galvanometer or relay placed in this cross connection would show no sign of a current passing, for in both cases the currents will have equal and opposite effects on the instruments so placed in the cross connection. This condition will hold good even though the resistance of one circuit be double that of the other, for at definite proportional points equal potentials exist, and no currents can therefore pass from one point to the other in the branch connection.
In making the foregoing statement, I wish it to be understood that, although no current passes from z to w or from w to z when the currents are circulating in the two circuits A B and C D, still I am of the opinion that the coils of the instrument placed in the cross connection z w are charged from either end at the same instant of time, and remain so charged so long as the current is circulating in both the circuits of A B and C D; and at the moment the current ceases to circulate in both circuits (which will be at one and the same time), the charges that have rushed in from either side make their exit the way they entered, and through their having the same potential and the same velocity, any effect the outgoing current on the one side might have on the instrument so placed in the cross connection is counteracted by the outgoing current on the other side. This can be easily proved by placing a galvanometer exactly in the middle of a circuit, and sending the same current, equal as regards quantity and potential, from either end. When the one current is sent, the needle lies over to the one side, when the other current is sent the needle lies over to the other side; but when both currents are sent on the line at one and the same time the needle keeps its normal or upright position, for the simple reason that both currents are alike in their potential, and consequently have equal and opposite effects on the needle, or, in other words, the two forces being equal, they apparently neutralize the effects of each other. But let the potential of one current (the quantity on both sides being still the same) be more than the other, then the effect of the strongest current will be just weakened by an amount equal to the potential of the opposite and weaker current, for the pressure on the one side will be greater than the other. It is by being able to balance these potentials at two definite proportional points opposite to each other, that duplex telegraphy on the Wheatstone Bridge principle is made practicable.
The term “current,” I would here remark, is only used for convenience sake. What electricity is, is not known as yet. Some writers seem to infer that it is another form of heat. All writers agree, however, in treating it as a force; and in its utilization, by applying the laws that govern forces, results looked for can always be obtained, provided due care is taken as regards its peculiar nature whilst conducting experiments by its agency.
I have endeavoured, in the foregoing, to render intelligible how two currents circulating in two conductors parallel to each other can continue to do so without interfering with each other when connected, provided certain conditions are observed. The attached diagram (Pl. XXVI., fig. 4) shows the exact proportions in a duplex circuit on the principle of the Wheatstone Bridge, and is also a fac-simple of the plan, both as regards the relative values of the different resistances and the position of the apparatus, upon which duplex telegraphy is conducted in one of the wires in the Cook Strait cable, now in successful operation since the 18th June last.
It will be seen on reference to the diagram that the same proportions, as regards units of resistance, are employed at either end of the duplex circuit, the letters A and B representing the two ends of the cable and the apparatus employed at each end. If the arrangement at one station (A, for instance) is explained, the same may be considered as representing the conditions that exist at B.
The battery at A consists of ten cells (modified Daniell's Gravity Battery) of an internal resistance equal to 100 units or thereabouts. The copper pole is connected to the key, and the zinc pole is placed direct to earth. When the key is depressed the copper current flows, one portion passing to line and cable through the resistance of 800 units, and the other to the artificial resistance (520) through the resistance (400). The proportions in the bridge are 800 on the one side and 400 on the other, so that when the current arrives at H it divides in proportional parts, two-thirds of the current passing by way of H D through the artificial resistance to earth, and the remaining one-third by way of H C to the line, and after traversing the line and cable arrives at E, and then splits again, two-thirds passing by way of E F to earth, and the remaining one-third by way of E B to earth. The two-thirds that passes through E F works the relay, and thus records the signal that is sent from station A. Turning again to station A, it will be seen that the out-going currents, as I said before, divide proportionally at H, two-thirds passing by way of H D, and one-third by way of H C; and as these proportions are still maintained beyond the points C and D on either side of the relay, there is no temptation for the current which traverses H D to pass through C D, nor is there any temptation for the current which traverses H D to pass through D C; for as the currents in each of the above circuits are proportional throughout, and the potential of each current the same at C and D, consequently they exercise equal and opposite effects on the relay in C and D, and therefore when the currents are circulating in H C and H D, their potentials being the same, there is no current in C D; thus the first step towards effecting duplex has been accomplished—viz., we are enabled to send the home current to line without affecting the home-receiving apparatus. But supposing, instead of the artificial resistance (520 units), we
place only 200 there, what would be the result? Through our lowering the potential at D, the current arriving at C, instead of passing to line, a portion of it would rush through C D. The effect of this would be similar to the effect I described in figure 3, when two water-pipes have a connection made between them from a higher to a lower level; so, in like manner, a greater portion of the current arriving at C would find its shortest road to earth to be through C D, as the potential at the points C and D would have been disturbed by altering the artificial resistance; thus the first step towards accomplishing duplex would be defeated.
It will be seen, on turning to the diagram, that between the back contact of the key and the earth, 100 units resistance is inserted. This is placed there to prevent the balance being disturbed when the key is at rest; for as the battery in itself represents an internal resistance of 100 units, by inserting the 100 units resistance on the back connection of the key, it follows that, whether the key is up or down, the same resistance is always in circuit. I may remark, however, that there is an instant of time when the key is on the hang—that is to say, when neither back nor front contact is made—when both these resistances are cut out, but, as far as my experience has led me, I have found no inconvenience from it. I have, however, in order to reduce the interval between the two contacts, devised a key which practically removes the difficulty, as near as it is possible to do so. This resistance in the back connection of the key will vary as the resistance of the battery varies; for instance, on a duplex circuit of 200 miles, by experiment, I find it requires thirty cells to work it efficiently; and as thirty cells are equal in their internal resistance to about 300 units, it takes 300 units in the back connection of the key to preserve the required conditions.
It will be observed that the line measures 776 units resistance, and that the artificial resistance measures 520, which is equal to one-half the resistance of the line, and one-half the joint resistance (268 units) at the B side. The same applies to the A side when telegraphing from B. This joint resistance, 268 units, is the actual resistance that the current measures when arriving at E or C, as the case may be, and is arrived at in the first instance by testing with a galvanometer at E or C, the circuit being through E H, E F direct to earth, and from H F direct to earth, and from H to earth through the 100 units in back contact of key. It may appear strange that whilst the resistances that the current arriving at E has in its path to travel should in their sum represent 800 + 400 + 400 + 100 = 1,700 units, that the actual resistance in circuit is only 268 units; but it must be borne in mind that instead of one path for the current there are no less than four for it to distribute itself over, consequently the resistance in the aggregate is lessened thereby.
The condensers, which are placed at F and D respectively, are used to balance the static induction of the line. On long lines they are most valuable adjuncts, but on short lines do not appear to be necessary. Duplex telegraphy has to thank Mr. Stearns, of America, for this addition. Doubtless, so far as we know at present, and with the present appliances, it would not be possible to work duplex circuits of any length without the aid of the condenser. The condenser is made of a proportional capacity equal to the capacity of the real line. The exact action of the condenser within itself is not known, but I am of the opinion that it acts as a sort of storehouse for the current, receiving a portion of the charge when the current is sent on to the line, and, when the current ceases to flow, gradually discharges itself, and by so doing neutralizes the static discharge of the real line, that is the second flow of the current that is induced in the line in the opposite direction, or the residue of the charge which still clings to the conductor after the first rush of the current has passed.
I have now described all the conditions under which duplex is worked by the Wheatstone Bridge principle. I have shown how A can telegraph to B without disturbing his own apparatus, and how B can do the same.
The next step is to describe the effect when both A and B send their currents simultaneously to line, and how it is that the one current does not interfere with the other when both are on the line at once. The question is, do they pass one another? If not, how does A's signal record itself at B, and vice versâ, without the one interfering with the other, or mixing up, or clashing with each other.
It has been stated “that the two currents do not pass one another as has been imagined, but that, when both stations signal at the same time, the current sent by either station acts upon the distant instrument by determining whether the line or the rheostat (the artificial resistance) shall offer the easier path for the currents originating there.”
The results of an experiment I made by the aid of two induction coils (each coil giving a spark of 1 1/2 inch in air), have led me, however, to a somewhat different conclusion, and I am inclined to think that the currents sent from either end do actually pass one another, or at any rate exchange impulses, and so record the opposite signal. If the currents, instead of passing one another, simply exchange impulses, they only obey one of the laws of motion; for if we treat the two currents as two elastic bodies moving with equal velocities, and in opposite directions, at the point of contact they are each acted upon in their turn, and in the opposite direction, for as the motion of two bodies when started from a state of rest is in the direction given them by their first impulse, any change in direction after collision from that first taken must therefore be due to an exchange of force or impulse at the time of such collision.
My experiment with the two induction coils was as shown in Plate XXVII. The two vacuum spheres, 1 1/2 inch in diameter, had platinum wire points (x) at a distance of about half an inch above and below a platinum wire (z), that went right through from one side of each sphere to the other at right angles, and also axial, as shown in diagram. The sparking poles A and C of the two induction coils were joined to F and E of the spheres, and the receiving poles B and D of the coils joined to H and G; thus the spark from No. 1 coil would pass from A to F, then through the No. 1 sphere, then across the vacuum tube, then on to G by way of z, and back to its own receiving pole B. The spark from coil No. 2 would pass from C to E, then spark through the No. 2 sphere, then through the vacuum tube, then on to H by way of z, and back to its own receiving pole D; thus, by these means, the phenomena of two currents of electricity passing through the same space and in opposite directions were rendered visible.
The vacuum tube, in addition to the spiral tube passing through it, was in globular form at each end, when the spark passed out of the sphere from coil No. 1 (its spark being passed though by itself first, and then No. 2 treated in the same manner); in the end of the vacuum tube, next the coil No. 1, the light in it assumed a striped appearance, and, after passing through the tube, assumed in the other end a glowing-like appearance, like teased-out cotton wool. The spark from No. 2 coil, treated in the same manner, showed just the same results in the contrary direction. When only one coil was in operation, the light in the spiral tube was very luminous; but when both were in operation at one and the same time, this light was rendered doubly so, thus proving there was double the quantity of light in the tube when both currents were on at the same time. This was most distinctly visible.
Another experiment I tried was connecting a Morse key between the one pole of the battery and the make and break of each coil:—when the key was depressed the flashes of flame could be distinctly seen discharging at either end of the vacuum, the signal coming from No. 2 could be plainly seen and read off at the H end of the tube in the cotton wool form described, whilst the striped appearance of the spark in the H end (the same end) of the tube from coil No. 1 could also be seen there. Previous to the last experiment above referred to, I made another by detaching the two receiving poles from G and H, and joining them together, so that the spark that passed from No. 1 coil before it could get back to its own receiving pole was forced to travel through the secondary wire of coil No. 2. Coil No. 2 had to perform the same operation as No. 1 before its spark could reach its own receiving pole. When the connections were joined up this way, at intervals, a spark would pass in either coil direct from its own sparking pole to its own receiving pole, and this whilst the light was in the vacuum tube. It would appear, from
this sparking across, as if every now and again the resistance to the passage of the current from either side was in a measure increased, and rather than the whole of it would pass though the vacuum tube a portion preferred passing direct.
My first experiment in this direction resulted in some slight damage to one of the coils (the condenser being destroyed), and on my second experiment of the same nature, through the accident resulting from the first, I was not inclined to prolong the experiment beyond being able to satisfy myself that the current still continued to pass in the manner described in the first experiment, when the receiving poles were joined to their respective spheres at G and H.
It will thus be seen from the foregoing description of the experiments above named, that there is some tangible ground for believing that the currents in duplex telegraphy do pass one another. The subject yet, in my opinion, requires further investigation before that point can be finally settled; possibly it may never be conclusively proved either one way or the other, or, at any rate, not until such time as we are enabled to state what electricity itself is. Looking at it from a common-sense point of view, however, one would naturally come to the conclusion that they must pass, or that the currents, like two bodies meeting each other, exchange impulses, and thus cause the opposite signal, or rather the will of each sender, to be transmitted to its destination.
The foregoing method in working duplex is on the open-circuit principle; that is to say, the signal is made by sending the current to line (charging the wire with electricity from the battery for the time being).
It is possible, and perfectly practicable, however, to work duplex, in accordance with the arrangement described, by the constant current, or closed circuit, by making the following slight alterations in the manner of joining up. The plan is as follows:—The battery is placed at the back connection of the key, with the zinc current to line, and the resistance to balance the battery is placed in the front contact, the earth connections being still maintained in both cases, as shown. The relays are placed so as to receive the opposite current to that sent from the battery, and are so adjusted that when the current is on the line (which will be when the key is at rest) the tongue of the relay is held on the insulated stop by the current entering the relay, weakening the magnetism on the side it enters, and strengthening the magnetism on the side the insulated stop is on. As soon as the key is moved, and contact made on the front anvil, through the current immediately leaving the line, the opposite pole of the relay reasserts its sway and attracts the tongue of the relay, and in doing so closes the local circuit, and thus records the signal.
The relays used in either of the foregoing systems are Siemens' polarized relays, and all the values given are in Siemens' units.
In the plan showing the system on which duplex is worked on the No. 3 wire in the Cook Strait cable, for convenience sake the local circuits are omitted.