"Perfectly, ma'am, I'm afraid," said the man civilly. From his voice Wimsey took him to be a superior workman of some kind—probably an engineer.
"Is it an electrical device? Do you understand it?"
"Yes, ma'am. It will have a microphone arrangement somewhere, which converts the sound into a series of vibrations controlling an electric needle. When the needle has traced the correct pattern, the circuit is completed and the door opens. The same thing can be done by light vibrations equally easily."
"Couldn't you open it with tools?"
"In time, yes, ma'am. But only by smashing the mechanism, which is probably well protected."
"You may take that for granted," interjected Wimsey reassuringly.
She put her hands to her head.
"I'm afraid we're done in," said the engineer, with a kind of respect in his tone for a good job of work.
"No—wait! Somebody must know—the workmen who made this thing?"
"In Germany," said Wimsey briefly.