Popular Science Monthly/Volume 44/November 1893/Electricity at the World's Fair II
|ELECTRICITY AT THE WORLD'S FAIR.|
By CHARLES M. LUNGREN.
THE facility with which a high temperature may be obtained with electricity, and the heat controlled and located just where it is wanted, makes this agent peculiarly well adapted to the heating of metals for welding and forging purposes. This was early recognized by Prof. Elihu Thomson, to whom the development of the art is chiefly due, and who has devised a great variety of apparatus capable of performing all classes of work, from the simple welding of two wires to the making of large and complicated joints.
The principle involved is very simple. If a current be passed through a rod or wire, heat will be developed in it if the current be of sufficient volume. If this circuit, instead of being formed of a continuous conductor, be a broken one, such as would be furnished by two rods whose ends abut, the heat will be developed first at the surface of con'act, as this is the point of greatest resistance, and then spread along the rods. And if, while the rods are in a heated condition, they be pressed together, they will become strongly united and form a perfect joint. On account of the radiation of heat from the surface and the cooling effect of the air, the rods become hotter at the center than at the surface, which is the reverse of what happens with a forge-heated bar, where the heating begins at the outside and gradually extends to the interior. This feature of the electric welding process has an important advantage in producing a firmer and more perfect joint, and 40 THE POPULAR SCIENCE MONTHLY.
in diminisliing the formation of surface scale. Tests sliow that the electric weld is much stronger than that made in the ordi- nary way in a forge, and, indeed, is in some cases stronger than other parts of the bar.
The machines designed by Prof. Thomson for carrying out this method of welding are extremely simple, the mechanical part consisting essentially of one or more pairs of clamps to hold the pieces to be united, and means for pressing them together while in a heated condition. In operating the machines the cur- rent is turned on by the workman by means of a switch ; but Prof. Thomson has taken advantage of the movement of the pieces toward each other while the weld is being made to break the circuit, thus rendering the operation automatic and insuring the equal heating of the welded pieces. In machines for wire and small rod the welded wires and rods are pressed together by means of springs, but in those for larger work the necessary pressure is applied by hydraulic apparatus. The necessity for this will be appreciated when it is stated that the pressure requisite for steel is 1,800 pounds to the square inch, that for iron 1,200 pounds, and for copper GOO pounds.
Electrically the apparatus is as simple as it is mechanically. The alternating current, which has shown itself so flexible in the hands of the engineer in other departments of electrical work, is here called into requisition. Through the medium of converters the high potential machine current is transformed into others of great volume and low voltage suitable for this class of work. Currents of this character are rendered necessary by reason of the fact that all metals are very good conductors of electricity, and can therefore be heated only by currents of great amount. These currents range, in fact, from a few hundred amperes to eight and ten thousand. The voltage, however, is very low, rarely being more than four or five volts, and in large and heavy work some- times not more than a single volt. On account of this very low electrical pressure all danger from the current is eliminated and the apparatus may be handled with the same freedom as any ordi- nary metal- working machine. In the distribution of the electrical appliances the current is usually generated by a machine conven- iently located with reference to the source of power, and the cur- rent carried by wires to the welders, where the transformation takes place, each welder being provided with its own converter, proportioned so as to supply the character of current best suited to the special work of the machine. The current is under perfect con- trol by means of regulating devices operated by the workman, the usual device employed being a reactive coil. The range of work possible with this method of welding is very great. It not only may be used in forming all ordinary welds with iron and steel, but
�� � has been found capable of welding metals which have heretofore resisted all attempts to unite them direct, and which have therefore had to be brazed or soldered. Wrought iron, copper, brass,
Fig. 8.—Carriage-axle Welder.
silver, platinum, gold, aluminum, and even cast iron may not only be welded together, but may also be welded to one another in many different combinations. In addition to welding, all sorts of brazing may be done by this method, as the same heat which will soften a metal sufficiently to allow it to be welded will, of course, render the various solders fluid.
The process is, moreover, on account of the extreme rapidity with which welds may be made, and the ability to concentrate all the heat at the point of union, a very economical one. Practical commercial work has shown that the cost of the coal burned under the boiler to produce the electricity is just about that of the coal used in a forge to do the work in the old way, and that the saving in time, and hence labor cost, is clear gain, to say nothing of the cleanliness of the process, the freedom from deleterious materials in contact with the metal, such as sulphur and ash, and the advantage of having the work always in clear view. How great the saving in time is may be appreciated by the following statement of actual work vouched for by Mr. Frederick P. Royce and cited in a paper read by him before the National Association of Carriage Builders:
The process, though only introduced into commercial work in 1888, has gone largely into use, and electric welders now form a part of the regular equipment of the carriage and bicycle factory, the boiler and tool shop, the wire mill, the yard of the shipbuilder, and the thousand and one establishments which have to do with the working and shaping of metals. It has been applied with marked success to the joining of the parts of railway frogs, of chairs to rails, and other heavy work, and in ordnance work, to the manufacture of shell and shrapnel. One of the most novel uses to which the process has been put is now to be seen in Boston on a section of the West End street railroad. This is the welding of the ends of the rails together without removing them from their places in the track, the object being to render the line of rails efficient return conductors for the current used with the trolley cars. To accomplish this the necessary apparatus is mounted upon a car provided with driving motors to enable it to be moved along the track as the work proceeds. The current to supply both the motors and the converters is taken from the line wire by the ordinary trolley arm. In making the weld the earth is removed from about the joint, clamps applied, and the current sent through the rail ends until these are brought to a welding heat.
Another method of utilizing the electric current in the working of metals shown by the Thomson Company is due to M. de
Fig. 9.—Tire Welder.
Meritens, a well-known French inventor and constructor of dynamos. This consists in forming an arc between the object to be heated and a movable electrode carried in the hand of the operator. In this case the work forms one terminal of the electric circuit and the hand tool the other. This hand tool consists simply of a stick of ordinary arc-light carbon mounted in a suitable holder, and connected with the circuit by a flexible cable.
The workman is by this simple device provided with a tool of remarkable range and flexibility. He has at his command the enormous temperature of the electric arc, yet in such a form that he can vary it from the heat of a taper to full intensity, and reach with it parts of his work that would otherwise be inaccessible. 44 THE POPULAR SCIENCE MONTHLY.
It is chiefly used at present in forming the longitudinal seams of tubes and vessels, and in filling in blow boles and other imper- fections in castings. In this latter operation additional pieces of metal are fused into the openings, rendering the castings as sound and good as though they had come from the mold in perfect con- dition.
A method of using the electric current, substantially the same as that of Thomson, is employed by Mr. George D. Burton, and shown in operation at the exhibition by the Electrical Forging Company. Mr. Burton^s object is not, however, to heat the metal simply at the line of juncture and then complete the union of the parts in one operation, but to heat a piece of metal either throughout its entire length or any particular part, and then forge it into shape by the hammer or special machines designed to produce particular forms. He uses, as in the Thomson appa- ratus, the alternating current transformed to one of great volume and low voltage ; but instead of employing a number of converters, each adapted to the special work in hand, he makes one large one suffice, tapping this at as many points as desired. The holding device for the bars to be heated consists merely of a massive pair of copper clamps easily manipulated by the workman, and from which the work may be quickly transferred to the anvil or shap- ing machine.
The economy in time of electric heating is very strikingly shown where long bars and rods are heated. For instance, a round bar of tool steel, seven eighths of an inch in diameter and eleven inches long between the clamps of the machine, may be brought to a welding heat in one minute by the expenditure of thirty -two horse power. A bar of the same material, half an inch in diameter and five inches long, requires but twenty-seven and a half horse power for half a minute ; while one an inch square and twelve inches long is raised to a white heat by thirty-six horse power in two minutes and a half. Generally speaking, the elec- tric heating may be done in a tenth of the time required by the forge or furnace, and the power required is between three and four horse power per cubic inch of metal heated. The feature of electric heating already noticed of a bar becoming more highly heated at the center than at the surface when exposed freely to the air, is shown in a very convincing manner at this exhibit by fusing the core of an inch bar without it losing its shape. A consequence of this internal heating of a bar is the holding of its heat much longer than a forge-heated one, permitting of forging operations with one heat which would require two or three by the old method.
One of the most striking things in the exhibition remarkable on account of being so entirely out of harmony with all our ideas
�� � ELECTRICITY AT THE WORLD'S FAIR. 45
of the conditions under which we expect to see heat generated is the apparatus to be seen in this exhibit which may be appropri- ately termed the " water-pail forge." This consists of an ordinary wooden pail filled with water into which dips a metal plate con- nected with one terminal of the electric circuit. The other termi- nal is attached to a pair of blacksmith's tongs, with which the op- erator picks up and holds the piece of metal to be heated. Imme- diately upon his plunging this into the water the liquid begins to sputter and the metal to glow, until in a few seconds it is brought to a welding heat and is then speedily melted. The heating is so rapid that neither the water nor the metal a few inches away are more than slightly warmed. This curious phenomenon appears to be due to the localization of the resistance of the circuit at the surface of the heated metal by the interposition of a layer of hy- drogen between the metal and the liquid. This is the explanation offered by two Belgian engineers who recently brought out the process abroad with apparently no knowledge of its prior use in this country. In their apparatus they used a glass jar lined with lead which formed the positive pole. The water was acidulated to render it conducting. When the circuit is completed by the immersion of the metal to be heated the current decomposes the liquid, the oxygen going to the lead plate and the hydrogen to the iron or other immersed metal and preventing any direct contact of the metal and the liquid. As hydrogen is a very poor conductor of electricity, the resistance would then be localized at the surface of the metal plate, with the result of heat being rapidly developed at this point. An American investigator, Mr. Jules Neher, who has experimented with the process, regards the heating as being due to the formation of an arc between the heated metal and the liquid, as he has observed that the heating does not take place if the metal be immersed before the current is turned on, the energy of the current then being spent in the electrolysis of the liquid. His explanation is that immediately the metal touches the liquid hydrogen begins to be liberated and, interposing itself between the metal and the liquid, draws an arc in the act of pushing the two asunder. This arc formed under water quickly raises the metal to a high temperature. Whatever the precise explanation, it certainly is a most astonishing thing to see x^ieces of iron and steel glowing at a white heat and running away in melted glob- ules while surrounded by water. The capabilities of the appara- tus would appear to be almost unlimited, and it is not too much to say that it is destined to find wide application in the arts. The operator has at his command the practically unlimited energy of electricity, and should be able to reach temperatures with it here- tofore unattainable. The Belgian experimenters are reported to have succeeded in fusing carbon, and it has been suggested that
�� � it is within the range of possibility that we may in this way reach the solution of the problem of the artificial production of diamonds.
Another and very different use of the heating power of the electric current is its application to cooking and house heating. Attempts have been made for some years now to adapt the current to these purposes, and what has been accomplished in this
Fig. 10.—An Electric Kitchen.
direction is to be seen at the exposition. After considerable experimenting the final form which has been given to the apparatus for such use as hot plates, broilers, and water heaters is that of a wire imbedded in enamel such as is at present used upon kitchen utensils. The wire most commonly employed is German silver, though in some cases platinum has been used. In applying this construction to a flatiron, for instance, the base of the iron is made in the form of a shallow tray into which the enamel is poured. The wire in the shape of a zigzag forms a fiat coil completely surrounded by this insulating compound. A hot plate suitable for heating a kettle of water or baking griddle cakes is made in the same way, and a grid or frame with gutter-shaped bars filled in with the enamel serves as an oven heater, a sufficient number of these grids being disposed at various parts of the oven. Operations such as the broiling of steak are performed on a modified form of broiler in which the ordinary wires give place to narrow inverted U-shaped bars. The heating wires are carried through the hollow space of these bars and imbedded in enamel. For the heating of water in special vessels, such as the ordinary kitchen boiler, the vessel is made with a bottom in the form of a hot plate. In all the utensils shown at the exhibition the enamel used is of the ordinary gray variety which requires firing, but an enamel for this purpose has been introduced in England which needs no baking. When it comes to heating either by direct radiation or through the medium of hot air, the form finally adopted is that of a coil of wire wound over a pottery or porcelain center and partially inclosed in an iron case. For car warming, heaters are placed under the seats, and located so that they can radiate directly into the car, wire guards being placed in front of them to protect the clothing of passengers. Such heaters have been introduced quite extensively into trolley cars, and are said to have been found economical when everything is taken
Fig. 11.—Electric Hot Plate.
into consideration. They require no attention, and take up no room which would otherwise be occupied by passengers, both items of economic advantage in such a use. Heaters designed to take the place of the hot-air furnace are constructed in the same general manner as those for car use. The plan is to place a large primary heater in the cold-air box of the ordinary furnace, and then subsidiary heaters just inside the grating of registers, by means of which additional heat may be obtained when the main heater is insufficient. All classes of apparatus are made to be used with either an alternating or continuous currrent, and adapted to be attached directly to the ordinary electric supply circuits.
Ideal this method of cooking and heating certainly is, and ideal it is likely to remain. There are many things electricity can do—many things it is doing which were without the bounds of our expectation of even yesterday—but supplying heat in
|Fig. 12.—Electric Coil Heater.|
economic contrast with coal and gas for the ordinary operations of the household is not one of them. This is, of course, upon the condition that the current is generated by the combustion of fuel. In situations in which the current is produced by water power, and in which fuel is scarce and dear, the unit of heat furnished by electricity may very well bear comparison with that by direct combustion; but that you can not start with combustion, suffer the tremendous loss of the steam engine, the various losses of the electrical apparatus, pay a profit to the electric supply company, and still compete in point of economy with the primary process of combustion, would seem to be a proposition too clear to need demonstration. Looked at from the point of view of percentages, the steam engine makes a return of but ten per cent of the heat energy of the fuel, the dynamo can hardly be depended upon in practice for more than ninety per cent, and the converter, when this is used, may be counted to absorb ten per cent of the energy delivered to it. This leaves in the one case but nine per cent and in the other a little over eight per cent of the original energy at the disposal of the consumer. Some of this must inevitably be lost in the final heating operation, for, though the apparatus be designed never so well, it can not have an efficiency ELECTRICITY AT THE WORLD'S FAIR. 49
of a hundred per cent. The consumer, therefore, can have in an available form not more than ten per cent and probably not over seven per cent of the heat in the fuel with which the cycle of operations started. This is an eflBciency much below that obtain- able from the direct combustion of the fuel by even the most wasteful methods, and at no price at which electrical energy can be furnished could the two forms of heating be brought on the plane of economic equality. A direct comparison of the actual number of heat units (pound-degree Fahr.) present in each in- stance will show with perhaps greater clearness the economic re- lations of the two methods of heating. A horse power of elec- trical energy is equivalent to 2,5G5 heat units per hour. A pound of coal contains 13,000 heat units, and costs, with coal at five dol- lars per ton of 2,000 pounds, a quarter of a cent. If we give to the coal an efficiency of but ten per cent, it will require two pounds to equal the available heating power of the electrical horse power, allowing that all the heat in the latter case is utilized. This will cost the user half a cent, and making due allowance for the collateral expenses of coal as a fuel, such as kindling, removal of ashes, and cost of handling, it is very evi- dent that electricity can not hope to offer any economical com- petition. The commercial promoters of electrical heating count upon a charge to the consumer of five cents per horse power per hour for cooking purposes, and a cent and a half for heating pur- poses. This is very much under the figures at which electric power is now being furnished for lighting purposes the charge for this being at the rate of from twelve to fifteen cents per horse power-hour but it is proposed to make the same discrimination between light and heat that the gas companies have instituted. At the lower figure electric heating is nearly three times, and at the higher nearly ten times, as expensive as that by coal, allot- ting to coal the above very low duty. But coal has no such low efficiency. The radiant heat from hard coal is fully twenty-five per cent of the total heat generated, and of this fully one half is utilized in a grate fire, which is the most wasteful of the heating devices in use. In the best forms of grates which have been de- vised, in which the surplus heat is used to warm the air supply of the room, as much as thirty-five per cent of the heat may be made available, while in close stoves of the best patterns the effi- ciency will not fall below seventy per cent.
With gas the comparison is of course much more favorable, as here the cost of a unit of heat is much greater than in the case of coal. Illuminating gas has a heating value of six hundred and fifty to eight hundred heat units per foot, according to the qual- ity of the gas. At the lower figures it requires a trifle under four feet to equal the heat value of an electrical horse power.
�� � 50 THE POPULAR SCIENCE MONTHLY.
Gas may be counted upon for a duty of seventy-five per cent ; so that the amount necessary to do the heating work of the electri- cal horse power will be five and a third feet. This, with gas at a dollar and a half per thousand will cost '078 of a cent, and with gas at a dollar a thousand a not uncommon price at present in the United States will cost but little more than half a cent. For cooking purposes the two methods of heating are on an equality in the matter of ease of manipulation, absence of collateral expenses, and limitation of the use of the fuel to the exact time required to perform the operation in hand. Their value to the householder is, therefore, in direct ratio to their cost. Gas clearly has the advan- tage of being from five to ten times the cheaper source of heat, an advantage so great as to make its supremacy secure. With the cheaper forms of fuel gas which have grown up, and will doubtless come into larger and larger use as the lighting field of gas dwindles, electricity can have even less chance of competing. This method of heating will doubtless find a field of its own, in which its use will be determined by other conditions than those of economy, but it can never hope to take over to itself any con- siderable part of the heating domain, so long as fuel remains at anything like the present prices.
The Centennial left us in the telephone a new method of com- munication, which in the time since then has grown into one of the necessities of business life. The Columbian will leave us, in the telautograph of Prof. Elisha Gray, another method of commu- nication which promises to rival the telephone in utility. This new method is not exclusive of the earlier one, but rather supple- mentary to it. The telephone has endowed us with the power of talking at a distance ; the telautograph will confer upon us the ability to write in the same way. It supplies an essential feature lacking in the telephone a record and hence becomes available for many uses for which the telephone is unfitted. Mistakes so liable with speech transmission are here impossible, as the receiv- ing instrument reproduces faithfully all the movements of the transmitting pencil, and only a blunder upon the part of the send- ing operator can cause misunderstanding or confusion. With telautograph exchanges established in cities after the manner of those of the telephone, it will be possible for subscribers to do by means of it much of the correspondence now carried on by mail ; and when the system is extended to provide communication be- tween cities, the business man will have at his disposal a method of letter transmission incomparably more swift than the most rapid of fast mails. The extent to which such a system may be used in substitution of mail service will, of course, depend upon the expense attending it, and as this must always be greatly in
�� � ELECTRICITY AT THE WORLD'S FAIR. 51
excess of letter carriage, it can apply only in cases in which im- portant interests are involved and dispatch is of moment. Sucli instances are, however, growing increasingly frequent in the modern business world, so that the telautograph, if it prove as successful in actual commercial work as it has in experimental tests, will not lack for a large and profitable field.
The attempts to realize facsimile transmission go back almost to the beginning of telegraphy. As early as 1846 Alexander Bain attempted such reproduction by means of trailing contacts pass- ing over the face of metallic letters at the transmitting end of the circuit, and like contacts sweeping over a chemically prepared paper at the receiving end. When the contacts were on the faces of the letters a current was sent to line ; and these current im- pulses, decomposing the chemical preparation of the receiving paper, made brown or blue marks, according to the nature of the chemical solution, which reproduced in broken outline the origi- nal letters. This method of operation was ten years later much improved by Caselli, who transcribed the message or sketch to be sent on a metallic-faced paper, and caused a stylus actuated by a pendulum to traverse in succession all parts of the design. A similar stylus reproduced the drawing or writing on chemically prepared paper at the receiving end. Many attempts have been made by subsequent inventors to adapt this method of transmis- sion to commercial work, but without success. All systems of this kind, it will be observed, depend upon the establishment of exact synchronism between the transmitting and receiving instru- ments, and this is a condition very difficult to realize in practice. Moreover, the message must first be written either in a special ink or on a special paper, and afterward transmitted, which ren- ders the process slow and necessitates expert knowledge to oper- ate it.
The telautographic method proceeds upon entirely different lines. In this the movement of the transmitting pencil in the hand of the operator causes electrical impulses to be sent over the line, which impulses, through the medium of appropriate mechanism, act upon the receiving pen and cause it to duplicate the movement of the sending one. The possibility of doing this depends upon the geometric principle that the movement of a point in describing a plane curve, no matter how intricate, may be resolved into two rectilinear movements at right angles to each other. In order, therefore, to have the pen at the receiving end of the line follow all the motions of the transmitting one, it is only necessary to resolve the movement of this latter into its right line components and reproduce them at the further end. A point situated at the focus of these lines of movement will then describe the exact motions of the original one. Simple as this
�� � conception is, it has been found by no means an easy one to realize in practice. The first one to attempt the application of this principle to autographic transmission appears to have been Mr. E. A. Cowper. In his apparatus, constructed in 1874, the receiving pen was mounted upon a light armature located between the poles of two electro-magnets placed at right angles to each other. These magnets were included in separate line circuits, and when energized by currents from the transmitting end of the line, attracted the pen armature, causing it to describe a line or curve which was at every instant the resultant of the two right-angled magnetic attractions.
These magnetic attractions were varied in exact accordance with the movements of the transmitting pen by augmenting and diminishing the strength of the current flowing through the magnet coils, and this variation of current strength was in turn accomplished by causing contacts attached to the transmitting pen to pass over the terminals of resistance coils and successively cut out or introduce resistance in the line circuits. On account of the very limited movements which could be given to both the transmitting and receiving pens, the writing had to be done upon
|Fig. 13.—Telautograph Transmitter.|
a strip of paper which was moved under the pen. The writing with the transmitting pen was done through a square hole about an inch on a side, and the characters had therefore to be made practically one over the other. There was thus but little opportunity for the operator to follow the work and see clearly what he was doing, and only an expert could make an intelligible writing. The details of this method were subsequently much improved by two American inventors, and the apparatus employed for a time in commercial work; but the essential limitations of the method proved too serious a handicap, and the system soon fell into disuse.
In taking the subject up experimentally Prof. Gray at first used the method of a variable resistance, but he speedily abandoned it as impracticable, and adopted the step-by-step method of operation, which he now uses. This consists in causing the transmitting pen to send to the line a succession of distinct electrical impulses, the number of which is governed by the extent of the pen's movement, which are employed, not in affecting the receiving pen directly, but in controlling the mechanism which actuates it. As the extent of movement of the pens is determined only by the number of electrical impulses, these may be given any desired range, and it becomes possible to use the transmitting pen with almost the same freedom as a pen or pencil in ordinary writing, and to write in the same way—that is, in successive lines extending across a page.
In the final form given to the instruments by Prof. Gray and shown at the exposition the transmitter consists of a box provided
|Fig. 14.—Telautograph Receiver.|
with a leaf upon which the paper rests. The paper is drawn continuously from a roll, and is shifted mechanically from time to time by the operator. The writing pen consists of a pencil lead mounted in a holder, to the lower end of which two silk threads are attached. These threads are at right angles to each other, and lead from
the pencil to two drums, upon which they are wound in such a manner as to cause the drums to rotate backward and forward as the threads follow the movement of the pencil point. The drums, therefore, move in exact accordance with the rectilinear components of the pen's motion, and it is only necessary to reproduce their motions at the other end to cause the receiving pen to duplicate the movements of the transmitting one. In an earlier form of the transmitter each drum carried an arm, which was swept by its movement over a series of radial electrical contacts, and thus sent a succession of electrical impulses to line. The friction of this moving arm was, however, found to be objectionable, and this arrangement has therefore been replaced by a magnetic device in which a toothed iron disk acting magnetically upon a soft iron lever keeps this in vibration. This lever plays between two contact points, and according as it is upon one or the other of the contacts a positive or negative current impulse is sent by a battery through the line circuit. These current impulses of alternating polarity serve to operate at the receiver polarized relays, which control by means of escapements drums similar to those in the transmitter, which drums actuate the receiving pen.
This pen consists of a glass tube drawn to a capillary bore at the end and supplied with a free-flowing ink from a reservoir by means of a rubber tube. It is mounted upon and at the junction of two light aluminum arms, making a right angle with each other. Each of these arms is attached to its operating drum by means of a cord passing around the drum, so that the rotation of this moves the arm to and fro in the direction of its length after the manner of the ordinary bow drill. The drums are given a tendency to rotate by a small electric motor located in the case of the receiver, and as this rotation is controlled by the polarized relays, which are in turn operated by the current impulses sent out by the transmitting pen, it will be seen that the movement of this latter determines that of the receiving pen, both in amount and direction, and that hence the two pens must move in exact accordance
Fig. 15.—Sample Work of Telautograph.
with each other. The mechanism of the receiving instrument is at present a little intricate, and some of the operations to be performed, as the lifting the pen from the paper, shifting the paper, and reversing the motion of the operating drums, require in the present construction two additional line wires, but these, it is expected, can by contemplated improvements be dispensed with, leaving only two line wires for the performance of all the necessary operations. The system has so far been operated over a distance of only thirteen miles; but from the character of the currents used—distinct successive electric impulses—there would seem to be no reason why it should not be capable of operation over as long distances as the ordinary telegraphic instruments.