Midnight
Tim steadily advanced in his chosen vocation , and the boys heard from him frequently . No one rejoiced more than they when they learned that he was at last in the big-time circuit . During all these events the boys had been busy at developing the receiving set , and at last it was finished to their satisfaction . In the course of their work they gathered a large amount of familiarity with radio which proved of immense value later on , as will be seen in the next volume of this series , entitled : `` The Radio Boys at Mountain Pass ; Or , The Midnight Call For Assistance . '' The special set that represented the advance they had made in radio reception included the regenerative principle . This feature added immensely to the sensitiveness of the set . It consisted of a coil , variously known as the tickler , the intensity coil , and the regeneration coil .