STUDIES IN RADIO BROADCAST TRANSMISSION 



167 



cesslvel}' a total of fifteen small condensers across the main condenser 

 of the transmitter oscillator shifts the frequency in steps over an 

 adjustable range. The contactor is rotated at the rate of nine re\-o- 

 lutions a minute, which is sufficiently slow to show definite steps in 

 the oscillograph record. At the receiving end a local oscillator sup- 

 plies a radio-frequency wave for beating the incoming frequencies 

 down to values within the audible range. 



A long oscillograph record of this stepped frequency gives a sort 

 of moving picture of the fading for the entire band covered. A 

 sample of such a record is shown in Fig. 17 with alternate pictures 

 in the series removed to simplify the relations, since by reason of 



Fig. 17 — Sample band fading record 



the two-way traversal of the frequency band successive pictures 

 are reversed. If a series of such built-up pictures as these could be 

 taken rapidly on moving picture film, and projected successively 

 upon a screen we should have before us an animated view of band 

 fading. And according to the results of experimental investigation 

 the subject offers a lively theme for such a presentation. The peaks 

 and depressions glide nervously back and forth across the setting. 

 The successive pictures of Fig. 17 (which, by the way, weie selected 

 for their half-tone reproduction possibilities rather than as first 

 class examples of the records taken) illustrate a rather leisurely 

 movement of this sort. These ten built-up photographs cover a 

 period of slightly more than one minute. In the first seven pictures 

 a depression appears at the left, while in the last three this depression 

 seems to have made an exit followed by the simultaneous entrance of 

 another from the opposite wing of the stage. Evidence of such 



