834 The Outline of Science 



instruments are in cases, in other makes the instruments are left 

 quite open. 



If the operator wants to send a message, after first seeing 

 that his apparatus is correctly tuned, he starts up the motor- 

 generator, if the charging apparatus consists of a motor-generator 

 and transformer. Now if everything is correctly connected, he 

 can by depressing the key, which completes the charging circuit, 

 cause the condenser to be charged and discharged very rapidly, 

 thereby producing the oscillations which are transferred to the 

 aerial and radiated in the form of waves. If the key is depressed 

 for a long period a long train of waves is radiated, but if the 

 key is depressed for a short period only a short train of waves will 

 be radiated. This division into shorter and longer trains of waves 

 is sufficient for intelligible signalling. The Morse Code, consist- 

 ing of an arrangement of dots and dashes, is universally used for 

 this purpose. The dots correspond, of course, to a short train of 

 waves, and the dashes to the longer trains. By combining these 

 short and long trains, therefore, in the same way as dots and 

 dashes are combined in the Morse Code, messages can be sent as in 

 ordinary telegraphy (see Fig. 7 facing p. 829) . 



We must now consider how the waves sent out are received 

 at a distant station. 



When a wire is made to cut across magnetic lines of force 

 an electric current is induced in the wire. It is also true that 

 when the magnetic lines of force cut across the wire they induce 

 an electric current in it. Now we have seen that the waves sent 

 out by the aerial are waves of electric and magnetic force. If, 

 therefore, these waves strike across a wire in their path they 

 will induce currents in this wire. The oscillating currents so in- 

 duced will be much feebler, however, than the original oscillations 

 in the transmitting aerial. In the receiving circuit there must be 

 some apparatus for making these feeble incoming oscillations per- 

 ceptible. The apparatus now employed is a telephone receiver. 

 However, if these very rapidly oscillating currents were to be 



