278 



COMMUNICATION 



the adjustment can be made by means of the nut in front of 

 it. 



When the set-up is complete, have it carefully checked 

 by the instructor before you close the switch. Dots and 

 dashes may be made by pressing the key which closes and 

 opens the circuit at the will of the operator. When the key 

 is closed a spark should jump across the spark gap in the 

 secondary coil if the coil is properly connected and adjusted. 

 Record your notes on the experiment and complete the fol- 

 lowing statements. 



nun 



CUD- 



FIG. 455 



The voltage in the circuit is furnished by the dry 



cells. To make the electric current jump the spark gap a 



voltage is necessary. The coil makes this from the 



voltage of the dry cells in the primary circuit. The 



key and the primary circuit. A dot is made by 



while a dash is made by One side of the 



circuit is connected to the while the other is connected 



to the ground. 



Experiment 169. How may wireless messages be re- 

 ceived? 



At the other end of the room from the sending set, 

 or in some other part of the building, place another single 

 wire aerial similar to the one used with the sending set. 

 Lead a wire down from this to a table. 



TO GROUND, 

 GAS OR WATER P\PE 



FIG. 456 



A crystal may be secured at any radio store or one can 

 be made from galena (lead sulphide). Connect one of the 

 binding posts on the crystal to the aerial and the other 

 to the ground. Connect the ear phones across the two bind- 

 ing posts of the crystal as shown in Figure 456. 



Have some one send dots and dashes with the sending 

 set and adjust the "cat whisker" on the crystal until you 

 find a sensitive spot where you can hear the sending. 



Summarize your conclusions in a carefully written para- 

 graph. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What common experiences have you had with 

 waves? If you have been to the sea shore you are 

 familiar with the regularity with which the waves 

 roll in upon the beach. You have no doubt read of 

 messages placed in bottles and carried along for 

 thousands of miles by ocean currents. In an earlier 

 unit you learned how sound is carried through air, 

 water, and other substances by waves set up by some 

 disturbance such as a bell, a whistle, or a vibrating 

 string. Have you ever made a string telephone and 

 noticed how the sound waves travel along the thread? 

 In these sound devices the human ear is a receiving 

 set while the spoken word, the bell, the whistle, or 

 the vibrating string may be broadcasting. The sound 

 is carried from the sending device to the receiver by 

 waves which travel through air, water, string, or 

 some other material substance. 



The beacon fires and heliographs of the ancients 

 were used to send messages. The human eye was the 

 receiving set and the messages were carried with the 

 tremendous speed of light, 186,264 miles per second. 

 Just think, seven and one-half times around the earth 

 in the "tick of a watch." Every source of light is a 

 broadcasting station sending out messages which may 

 be received by the eye. As we stand in front of a fire- 

 place the fire sends out waves which are too long 

 for the human eye to see, yet our sense of feeling be- 

 comes a receiving station and picks them up. These 

 are known as infra-red waves. Have you ever been 

 sunburned? Then you have picked up the waves sent 

 out by the sun which are too short for the eye to see, 

 but which will cause irritation to the skin. These are 

 known as ultra-violet waves. 



Sound waves are carried by any solid, liquid, or gas. 

 Water waves are carried by water. It is impossible to 

 have a wave motion unless there is some medium in 

 which to wave. Because scientists have never been 

 able fully to understand what medium transmits ultra- 

 violet waves, visible light waves, and infra-red heat 

 waves, they assumed one and called it the "ether." 

 The ether is supposed to fill all of the space of the uni- 

 verse, even between the molecules of solids, liquids, 

 and gases, and the space of a vacuum. In this ether, 



