“I can hardly hear now. There is something wrong.”
“Maybe the battery is running out,” Jessie suggested.
“What’s that mean?”
“Goodness, Amy! You know that sometimes a doorbell won’t ring because the battery has failed.”
“But this battery isn’t like a doorbell battery, is it?”
“Exactly. It’s a B battery. And a B battery is merely a collection of dry cells. The voltage is twenty-two and a half. That is because the detector-tube gives best results when the voltage on the plate of the tube does not exceed that figure. The number of electrons flowing between filament and plate depends on the plate potential up to a certain point——”
“Whatever that is!” ejaculated Amy, stopping her. “Don’t be so scientific.”
“Not much science about that,” laughed her chum. “But our B battery, I fear me, needs renewing. You see, any dry battery recuperates considerably when not in use. These signals sounded loud enough when we first listened in. But the sounds soon decreased.”
She took off her earphones. “I’ll go and telephone to Mr. Brill to send up a fresh series of batteries for the set. I have no tester, so I do not know how weak these batteries are.”
“I remember something,” said Amy suddenly. “Once when our doorbell wouldn’t ring Darry opened the battery cases and soaked them in vinegar. Wouldn’t that work in this case?”