It has been found possible to send several messages simultaneously over a single line. To effect this a _distributer_ is used to put a number of transmitters at one end of the line in communication with an equal number of receivers at the other end, fed by a second distributer keeping perfect time with the first. Instead of a signal coming as a whole to any one instrument it arrives in little bits, but these follow one another so closely as to be practically continuous. By working a number of automatic transmitters through a distributer, a thousand words or more per minute are easily dispatched over a single wire.
The Pollak Virag system employs a punched ribbon, and the receiver traces out the message in alphabetical characters on a moving strip of sensitized photographic paper. A mirror attached to a vibrating diaphragm reflects light from a lamp on to the strip, which is automatically developed and fixed in chemical baths. The method of moving the mirror so as to make the rays trace out words is extremely ingenious. Messages have been transmitted by this system at the rate of 180,000 words per hour.
Chapter VII.
WIRELESS TELEGRAPHY.
The transmitting apparatus--The receiving apparatus--Syntonic transmission--The advance of wireless telegraphy.
In our last chapter we reviewed briefly some systems of sending telegraphic messages from one point of the earth"s surface to another through a circuit consisting partly of an insulated wire and partly of the earth itself. The metallic portion of a long circuit, especially if it be a submarine cable, is costly to install, so that in quite the early days of telegraphy efforts were made to use the ether in the place of wire as one conductor.
When a hammer strikes an anvil the air around is violently disturbed.
This disturbance spreads through the molecules of the air in much the same way as ripples spread from the splash of a stone thrown into a pond. When the sound waves reach the ear they agitate the tympanum, or drum membrane, and we "hear a noise." The hammer is here the transmitter, the air the conductor, the ear the receiver.
In wireless telegraphy we use the ether as the conductor of electrical disturbances.[13] Marconi, Slaby, Branly, Lodge, De Forest, Popoff, and others have invented apparatus for causing disturbances of the requisite kind, and for detecting their presence.
The main features of a wireless telegraphy outfit are shown in Figs. 59 and 61.
THE TRANSMITTER APPARATUS.
We will first consider the transmitting outfit (Fig. 59). It includes a battery, dispatching key, and an induction coil having its secondary circuit terminals connected with two wires, the one leading to an earth-plate, the other carried aloft on poles or suspended from a kite.
In the large station at Poldhu, Cornwall, for transatlantic signalling, there are special wooden towers 215 feet high, between which the aerial wires hang. At their upper and lower ends respectively the earth and aerial wires terminate in bra.s.s b.a.l.l.s separated by a gap. When the operator depresses the key the induction coil charges these b.a.l.l.s and the wires attached thereto with high-tension electricity. As soon as the quant.i.ty collected exceeds the resistance of the air-gap, a discharge takes place between the b.a.l.l.s, and the ether round the aerial wire is violently disturbed, and waves of electrical energy are propagated through it. The rapidity with which the discharges follow one another, and their travelling power, depends on the strength of the induction coil, the length of the air-gap, and the capacity of the wires.[14]
[Ill.u.s.tration: FIG. 59.--Sketch of the transmitter of a wireless telegraphy outfit.]
[Ill.u.s.tration: FIG. 60.--A Marconi coherer.]
RECEIVING APPARATUS.
The human body is quite insensitive to these etheric waves. We cannot feel, hear, or see them. But at the receiving station there is what may be called an "electric eye." Technically it is named a _coherer_. A Marconi coherer is seen in Fig. 60. Inside a small gla.s.s tube exhausted of air are two silver plugs, P P, carrying terminals, T T, projecting through the gla.s.s at both ends. A small gap separates the plugs at the centre, and this gap is partly filled with nickel-silver powder. If the terminals of the coherer are attached to those of a battery, practically no current will pa.s.s under ordinary conditions, as the particles of nickel-silver touch each other very lightly and make a "bad contact."
But if the coherer is also attached to wires leading into the earth and air, and ether waves strike those wires, at every impact the particles will cohere--that is, pack tightly together--and allow battery current to pa.s.s. The property of cohesion of small conductive bodies when influenced by Hertzian waves was first noticed in 1874 by Professor D.E.
Hughes while experimenting with a telephone.
[Ill.u.s.tration: FIG. 61.--Sketch of the receiving apparatus in a wireless telegraphy outfit.]
We are now in a position to examine the apparatus of which a coherer forms part (Fig. 61). First, we notice the aerial and earth wires, to which are attached other wires from battery A. This battery circuit pa.s.ses round the relay magnet R and through two choking coils, whose function is to prevent the Hertzian waves entering the battery. The relay, when energized, brings contact D against E and closes the circuit of battery B, which is much more powerful than battery A, and operates the magnet M as well as the _tapper_, which is practically an electric bell minus the gong. (The tapper circuit is indicated by the dotted lines.)
We will suppose the transmitter of a distant station to be at work. The electric waves strike the aerial wire of the receiving station, and cause the coherer to cohere and pa.s.s current. The relay is closed, and both tapper and Morse inker begin to work. The tapper keeps striking the coherer and shakes the particles loose after every cohesion. If this were not done the current of A would pa.s.s continuously after cohesion had once taken place. When the key of the transmitter is pressed down, the waves follow one another very quickly, and the acquired conductivity of the coherer is only momentarily destroyed by the tap of the hammer.
During the impression of a dot by the Morse inker, contact is made and broken repeatedly; but as the armature of the inker is heavy and slow to move it does not vibrate in time with the relay and tapper. Therefore the Morse instrument reproduces in dots and dashes the short and long depressions of the key at the transmitting station, while the tapper works rapidly in time with the relay. The Morse inker is shown diagrammatically. While current pa.s.ses through M the armature is pulled towards it, the end P, carrying an inked wheel, rises, and a mark is made on the tape W, which is moved continuously being drawn forward off reel R by the clockwork--or electrically-driven rollers R^1 R^2.
SYNTONIC TRANSMISSION.
If a number of transmitting stations are sending out messages simultaneously, a jumble of signals would affect all the receivers round, unless some method were employed for rendering a receiver sensitive only to the waves intended to influence it. Also, if distinction were impossible, even with one transmitter in action its message might go to undesired stations.
There are various ways of "tuning" receivers and transmitters, but the principle underlying them all is a.n.a.logous to that of mechanical vibration. If a weight is suspended from the end of a spiral spring, and given an upward blow, it bobs up and down a certain number of times per minute, every movement from start to finish having exactly the same duration as the rest. The resistance of the air and the internal friction of the spring gradually lessen the amplitude of the movements, and the weight finally comes to rest. Suppose that the weight scales 30 lbs., and that it naturally bobs twenty times a minute. If you now take a feather and give it a push every three seconds you can coax it into vigorous motion, a.s.suming that every push catches it exactly on the rebound. The same effect would be produced more slowly if 6 or 9 second intervals were subst.i.tuted. But if you strike it at 4, 5, or 7 second intervals it will gradually cease to oscillate, as the effect of one blow neutralizes that of another. The same phenomenon is witnessed when two tuning-forks of equal pitch are mounted near one another, and one is struck. The other soon picks up the note. But a fork of unequal pitch would remain dumb.
Now, every electrical circuit has a "natural period of oscillation" in which its electric charge vibrates. It is found possible to "tune," or "syntonize," the aerial rod or wire of a receiving station with a transmitter. A vertical wire about 200 feet in length, says Professor J.A. Fleming,[15] has a natural time period of electrical oscillation of about one-millionth of a second. Therefore if waves strike this wire a million times a second they will reinforce one another and influence the coherer; whereas a less or greater frequency will leave it practically unaffected. By adjusting the receiving circuit to the transmitter, or _vice versa_, selective wireless telegraphy becomes possible.
ADVANCE OF WIRELESS TELEGRAPHY.
The history of wireless telegraphy may be summed up as follows:--
1842.--Professor Morse sent aerial messages across the Susquehanna River. A line containing a battery and transmitter was carried on posts along one bank and "earthed" in the river at each end. On the other bank was a second wire attached to a receiver and similarly earthed. Whenever contact was made and broken on the battery side, the receiver on the other was affected. Distance about 1 mile.
1859.--James Bowman Lindsay transmitted messages across the Tay at Glenca.r.s.e in a somewhat similar way. Distance about 1/2 mile.
1885.--Sir William Preece signalled from Lavernock Point, near Cardiff, to Steep Holm, an island in the Bristol Channel. Distance about 5-1/2 miles.
In all these electrical _induction_ of current was employed.
1886.--Hertzian waves discovered.
1895.--Professor A. Popoff sent Hertzian wave messages over a distance of 3 miles.
1897.--Marconi signalled from the Needles Hotel, Isle of Wight, to Swanage; 17-1/2 miles.
1901.--Messages sent at sea for 380 miles.
1901, Dec. 17.--Messages transmitted from Poldhu, Cornwall, to Hospital Point, Newfoundland; 2,099 miles.
Mr. Marconi has so perfected tuning devices that his transatlantic messages do not affect receivers placed on board ships crossing the ocean, unless they are purposely tuned. Atlantic liners now publish daily small newspapers containing the latest news, flashed through s.p.a.ce from land stations. In the United States the De Forest and Fessenden systems are being rapidly extended to embrace the most out-of-the-way districts. Every navy of importance has adopted wireless telegraphy, which, as was proved during the Russo-j.a.panese War, can be of the greatest help in directing operations.
[13] Named after their first discoverer, Dr. Hertz of Carlsruhe, "Hertzian waves."
[14] For long-distance transmission powerful dynamos take the place of the induction coil and battery.
[15] "Technics," vol. ii. p. 566.
Chapter VIII.
THE TELEPHONE.
The Bell telephone--The Edison transmitter--The granular carbon transmitter--General arrangement of a telephone circuit--Double-line circuits--Telephone exchanges--Submarine telephony.
For the purposes of everyday life the telephone is even more useful than the telegraph. Telephones now connect one room of a building with another, house with house, town with town, country with country. An infinitely greater number of words pa.s.s over the telephonic circuits of the world in a year than are transmitted by telegraph operators. The telephone has become an important adjunct to the transaction of business of all sorts. Its wires penetrate everywhere. Without moving from his desk, the London citizen may hold easy converse with a Parisian, a New Yorker with a dweller in Chicago.