Fig. 82.—Transmitter for wireless telegraphy.
The vertical rod, or antenna, A, is not insulated, but is connected by its lower end with one end of a coil of insulated wire, S, wound on a wooden frame ([see Fig. 82]). The other end of this last coil is connected to a metal plate, e, buried in the earth. Around the wooden frame is wound a second insulated wire, P, one end of which is connected to one spark-ball of the induction coil, and the other end to the outside of a Leyden jar, L, or collection of jars. This double coil on a frame is called an oscillation transformer. The inside of this condenser is connected to the second spark-ball of the induction coil I. When these spark-balls S are placed a short distance apart, and the coil set in action, we have a torrent of oscillatory electric sparks between these balls, and powerful oscillations set up in one circuit of the oscillation-transformer. These oscillations induce other oscillations in the second circuit of the oscillation-transformer, viz. in the one connected to the aerial. The oscillations produced in the air-wire, or aerial, are therefore induced, or secondary oscillations. The aerial wire, or antenna, has therefore a much larger store of electric energy to draw upon, viz. that stored up in the Leyden jars, than if it was itself directly charged by the coil.
In order, however, to obtain the best results certain adjustments have to be made. It has already been explained that every open electrical circuit has a certain natural time-period for the electrical oscillations which can be set up in it. This is technically called its tune.
If we administer a blow to a suspended pendulum we have seen that, if left to itself, it vibrates in a definite period of time, called its natural period. In the same manner, if we have a condenser or Leyden jar having electrical capacity which is joined in series with a coil of wire having electrical inertia or inductance, and apply to the circuit so formed a sudden electromotive force or impulse, and then leave the circuit to itself, the electric charge in it vibrates in a certain definite period, called its natural electrical periodic time.
The aerial, or antenna, is simply a rod connected to the earth, but it has a certain inductance, and also a certain electrical capacity, and hence any metal rod merely stuck at one end in the earth has a perfectly definite periodic time for the electrical oscillations which can be produced in it. We may compare the rod in this respect with a piece of steel spring held at one end in a vice. If we pull the spring on one side, and let it vibrate, it does so in accordance with its natural time-period for mechanical vibrations. The sound waves given out by it have a wave-length equal to four times the length of the spring. In the same manner the fundamental wave-length of the electric waves emitted by an “earthed aerial,” or rod stuck in the earth, when an electric impulse is applied to its lower end, and electrical oscillations are set up in it, have a wave-length equal to four times that of the rod. Hence to obtain the best result the circuit, including the aerial A, must be “tuned” electrically to the circuit including the Leyden jar L.[27]
A consideration of these arrangements will show you that if the hand-key in the primary circuit of the induction coil is pressed for a long or short time, we have long or short torrents of sparks produced between the secondary balls, and long or short trains of electric waves emitted from the aerial, or earthed vertical wire.
Whenever we have any two different signals, we can always make an alphabet with them by suitable combinations of the two. In the well-known Morse alphabet, with which every telegraphist is as familiar as we all are with the printed alphabet, the sign for each of the letters of the alphabet is composed of groups of long and short symbols, called dots and dashes, as follows: Each letter is made by selecting some arrangements of dots or dashes, these being the technical names for the two signs. The Morse code, as used all over the world, is given in the table below—
| The Morse Alphabet. | |||||
| A | – ––– | J | – ––– ––– ––– | S | – – – |
| B | ––– – – – | K | ––– – ––– | T | ––– |
| C | ––– – ––– – | L | – ––– – – | U | – – ––– |
| D | ––– – – | M | ––– ––– | V | – – – ––– |
| E | – | N | ––– – | W | – ––– ––– |
| F | – – ––– – | O | ––– ––– ––– | X | ––– – – ––– |
| G | ––– ––– – | P | – ––– ––– – | Y | ––– – ––– ––– |
| H | – – – – | Q | ––– ––– – ––– | Z | ––– ––– – – |
| I | – – | R | – ––– – | ||
| The Morse Numerals. | |
| 1 – ––– ––– ––– ––– | 6 ––– – – – – |
| 2 – – ––– ––– ––– | 7 ––– ––– – – – |
| 3 – – – ––– ––– | 8 ––– ––– ––– – – |
| 4 – – – – ––– | 9 ––– ––– ––– ––– – |
| 5 – – – – – | 0 ––– ––– ––– ––– ––– |
| Full Stop – ––– – ––– – ––– | |
| Signal for calling up – – – ––– – – – ––– | |
The process of sending a wireless message consists in so manipulating the key in the primary circuit of the induction coils that a rapid stream of sparks passes between the secondary balls for a shorter or for a longer time. This gives rise to a corresponding series of electric waves, radiated from the aerial. The dash is equal in duration to about three dots, and a space equal to three dots is left between each letter, and one equal to five dots between each word. Thus, in Morse alphabet the sentence “How are you?” is written—