WIRELESS TELEGRAPH 



WIRELESS TELEGRAPH 



ceiving and a large surface from which the 

 waves go out in sending. A common form of 

 aerial is shown in the illustration. This is the 

 horizontal form, and is used on ships and on 

 land when it can be placed on a high elevation, 

 such as the roof of a tall building. 



A COMMON FORM OF AERIAL 



Four or five wires are commonly used for a 

 horizontal aerial. The leading-in wire may be 

 connected either to the middle or to one end 

 of the horizontal wires. Vertical aerials com- 

 posed of a large number of wires supported by 

 high towers are used by some wireless stations 

 for long distance signalling. The greater the 

 height of the aerial the greater is the distance 

 through which the station can operate. Mar- 

 coni in his early experiments thought that 

 doubling the height of the aerial made the 

 sending distance four times as great, but later 

 found that this does not quite hold true for 

 great heights. Aerials are usually made of 

 strands of phosphor-bronze wire, but for ama- 

 teur work No. 12 copper wire can be used. 

 The wires must be insulated at all supporting 

 points. 



Spark Coils. The spark coil used in wireless 

 telegraphy is an induction coil. Induction coils 

 for wireless telegraphy have the secondary 

 winding of 1;. than other induction 



coils, such, for example, as those used in pro- 

 ducing X-rays. The secondary winding, there- 



INSULATION OF AERIAL 



fore, has less resistance than in other induction 

 coils, permitting a stronger flow of 

 with each discharge and producing wh.r 

 known as a "hot spark." This means that for 

 a given length of spark more energy is K 



off than with other kinds of induction coil. 

 The length of spark depends on voltage only. 

 If the reader will recall that electric power 

 equals volts times amperes, he will see that 

 since the spark coil used in wireless telegraphy 

 has higher amperage it has more power for the 

 same voltage, that is, for the same length of 

 spark, than other induction coils. Both high 

 voltage and high amperage are used in coils for 

 wireless telegraphy. 



Transformers. For long distance signalling, 

 for example, signalling across the Atlantic 

 Ocean, the spark coil operated by means of a 

 current from a battery does not furnish suffi- 

 cient power. For such work an alternating 

 current and a transformer are required. Alter- 

 nating current with an electromotive force of 

 about 2,000 volts is supplied to the primary of 

 the transformer, and the secondary gives out 

 an electromotive force of 20,000 volts or more. 

 The terminals of the secondary coil constitute 

 the spark gap. 



Interrupters. The interrupter for wireless 

 telegraphy must work at high speed, and the 

 break must be as nearly instantaneous as pos- 

 sible. The spring interrupter used with the 

 experimental induction coil does not meet re- 

 quirements except for coils under two-inch 

 spark ; a double spring interrupter may be used 

 for somewhat higher power. Generally the in- 

 terrupter is separate from the coil. There are a 

 number of interrupters in which a small eh 

 motor operates the contacts. The electrolytic 

 rruptcr consists of a platinum anode and a 

 lead cathode, dipping in a solution of sulphuric 

 acid in water. As the current flows through 

 the acid, bubbles of hydrogen collect on the 

 platinum, stopping the current. The bubbles 

 then disappear, allowing the current to flow. 

 With high voltage this operation takes place 

 and is repeated very rapidly. 



A number of different forms of mercury tn- 

 t<rrni>(< r* are in use, and these depend on the 

 fact that mercury is a condi; 

 A jet of mercury strikes a metal plat, and tin 

 interruptions are produced by tin metal plate 

 moving out of contact with the j-t of 



Interrupters are used, of course, only wln-n :i 

 *t current is used for the primary coil. 

 When an alternating current is used in th< 

 mary no interrupter is needed, for the alteraa- 

 of currrnt in tho primary take the place 

 of the interrui 



Condensers. Condensers are included in 

 every wireless equipment for the purpose of 

 increasing the strength of the spark. The con- 



