WIRELESS TELEGRAPH 



63M 



WIRELESS TELEGRAPH 



denser is connected across the spark gap. Glass 

 is thr substance most commonly used as the 

 dielectric between the metal plates of thr con- 

 denser. The most common form of condenser 

 in which glass is used is the Ley den jar. An- 

 other form consists of glass plates coated with 

 tin foil and placed in a suitable rack. 



Tuning. Every I ; -irk sends out 



waves of a definite length; the length of the 

 waves depends on the size of the spark coil 

 used and on the amount of capacity in the cir- 

 cuit. A tuned receiving set ivtvives \vavrs of 

 one length, but is not affected by waves of 

 other lent: 



l;n may be used as an illustration of 

 what is meant by tuning. If two violins are 

 fastened firmly on a table near to each other 

 and a string of one is vigorously bowed, the 

 string of the same pitch on the other violin 

 will be set in vibration and give out a sound. 

 The sound vibrations traveling through the 

 table set in vibration one string, and only one, 

 on the second violin, because only one of those 

 strings is in tune with the string that is being 

 bowed. If the second violin has no string in 

 tune with the string of the first it will not re- 

 ceive the message, and there will be no re- 

 sponding sound. It is the same in wireless 

 telegraphy. The receiving instrument must be 

 in tune with the sending instrument; that is, 

 it must be of the same pitch, or respond to 

 waves of the same length as those of the send- 

 ing instrument. 



There are "untuned" receiving circuits. These 

 can be illustrated by a tuning fork and a table. 



ON RAILWAY TRAIN'S 

 At left, the operator at work ; at right, the aerial. 



If a tuning fork is struck and placed on a table 

 whilr- it is vibrating, the table will give out a 

 loud sound of the same pitch as that of the 

 fork, no matter what pitch it may be. The 



table does not have to be tuned. The vibra- 

 tions of the table are called forced vibrations, 

 because the timing fork forces the table to vi- 

 brate in unison with itself. Some receiving sets 

 are like the table; they do not require tuning, 

 and their vibrations are forced vibrations. 



A tuning coil is used for the purpose of set- 

 ting the receiving instruments to the same 

 wave length as the sending instruments. A 

 tuning coil is used also in connection with the 

 sending device, to regulate the length of wave 

 sent out. The tuning coil for receiving con- 

 sists of many turns of fine wire; the one for 

 sending consists of a few turns of very heavy 

 wire, and is usually called simply a helix. Slid- 

 ing contacts are used to throw more or fewer 

 turns of the coil into the circuit. The length 

 of wave is increased by throwing more turns 

 of wire into the circuit. This increases the 

 self-induction of the circuit. 



TW T O circuits are in tune when they are 

 adapted to send and receive waves of the same 

 length. If a wireless operator discovers that 

 the signals which he receives from a certain 

 station are weak, he may find that by length- 

 ening or shortening his aerial the signals will 

 be stronger; with this length of aerial his re- 

 ceiving circuit would be in tune with the cir- 

 cuit of the sending station. It would be incon- 

 venient to lengthen or shorten the aerial every 

 time he wished to tune his circuit with that 

 of a different station. To avoid this difficulty 

 the tuning coil is used. Introducing more turns 

 of the tuning coil into the circuit has the same 

 effect as lengthening the aerial so that longer 

 waves can be received. This effect is due to 

 self-induction. Increasing the self-induction 

 increases the wave length. The aerial also has 

 a certain capacity, like the capacity of a con- 

 denser. The length of the waves depends on 

 hoi h the self-induction and the capacity of the 

 circuit. If the capacity is increased then to re- 

 tain the same wave length the self-induction 

 must be correspondingly reduced. To produce 

 the same wave length the product of self-induc- 

 tion and capacity must be kept the same. 



Codes. Two codes are in use in wireless 

 telegraphy, the Morse and the Continental. 

 The Continental code is used in Great Britain 

 and its colonies and by all ocean liners except 

 those that ply the American coast. The Morse 

 code is used in the United States. The two 

 codes are given in the article TELEGRAPH. 



Electric Waves and Their Discovery. Light 

 and electric waves are alike, except that elec- 

 tric waves are longer than light waves. The 



