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— [Illustration: THE MORSE ALPHABET. A — ——— B ——— — — — C ——— — ——— — D ——— — — E — F — — ——— — G ——— ——— — H — — — — I — — J — ——— ——— ——— K ——— — ——— L — ——— — — M ——— ——— N ——— — O ——— ——— ——— P — ——— ——— — Q ——— ——— — ——— R — ——— — S — — — T ——— U — — ——— V — — — ——— W — ——— ——— X ——— — — ——— Y ——— — ——— ——— Z ——— ——— — — ] [Illustration: THE MORSE NUMERALS.