A PREAMBLE ON ENERGY 59 



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Fig. 17. — Diagram of a Hertzian Oscillator. 



When the charges become too great they '* spark across " (2), 

 and then the inertia of the current causes the polarity to be 

 reversed (3), again the charge sparks (4), and again the polarity 

 becomes reversed (5). These alternations may occur at the rate 

 of several millions of times per second. Every time they occur 

 the electro-magnetic field round the oscillator changes and it 

 changes sooner after an oscillation between A and B at some 

 place near the latter than at some place far away. A change 

 occurring at A and B would be followed a second later by a change 

 in the electro-magnetic field 300,000 kilometres distant, at a 

 place 600,000 kilometres away two seconds later and so on. 



Cycles of changes at the centre of the field affect all the field, 

 but these cycles in the field are perceived later in time than they 

 occur at the centre. Everywhere in the field the cycle of changes 

 is similar in form to the cycle at the centre, but the amplitude of 

 the cyclic change falls off as we pass out from the centre. If 

 we plot graphically the regular increase and decrease of energy 

 at some place in the field it is represented by a " wave," as 

 shown in Fig 18 on page 60, and we say, for convenience, that 

 pulses of energy occur in the field. 



1 8c. Radiant Energy. Such pulses of energy appear to be 

 emitted from a source. If we represent them graphically (as in 

 the above diagram) a train of waves, each with a certain " wave- 

 length " appears to radiate out from the oscillating system. Thus 

 waves of light are said to be emitted by a luminous substance ; 

 electro-magnetic waves from an oscillating thermionic valve and 

 so on. The lengths of the waves vary (io~^ cm. from an X-ray 

 tube and 1,554 rnetres from the wireless station called 5 XX). 

 The frequencies of occurrence per second of these waves is very 

 variable, but the velocity with which they appear to be emitted 



