Scarcely a word was spoken as hour after hour Craig sat with the receiver to his ear, connected with the coils down in the storeroom.  “You might call this an electric detective,” he had explained to Spencer.  “For example, if you suspected that anything out of the way was going on in a room anywhere this would report much to you even if you were miles away.  It is the discovery of a student of Thorne Baker, the English electrical expert.  He was experimenting with high-frequency electric currents, investigating the nature of the discharges used for electrifying certain things.  Quite by accident he found that when the room on which he was experimenting was occupied by some person his measuring-instruments indicated that fact.  He tested the degree of variation by passing the current first through the room and then through a sensitive crystal to a delicate telephone receiver.  There was a distinct change in the buzzing sound heard through the telephone when the room was occupied or unoccupied.  What I have done is to wind single loops of plain wire on each side of that room down there, as well as to wind around the room a few turns of concealed copper wire.  These collectors are fitted to a crystal of carborundum and a telephone receiver.”

We had each tried the thing and could hear a distinct buzzing in the receiver.

“The presence of a man or woman in that room would be evident to a person listening miles away,” he went on.  “A high-frequency current is constantly passing through that storeroom.  That is what causes that normal buzzing.”

It was verging on midnight when Kennedy suddenly cried:  “Here, Walter, take this receiver.  You remember how the buzzing sounded.  Listen.  Tell me if you, too, can detect the change.”

I clapped the receiver quickly to my ear.  Indeed I could tell the difference.  In place of the load buzzing there was only a mild sound.  It was slower and lower.

“That means,” he said excitedly, “that some one has entered that pitch-dark storeroom by the broken window.  Let me take the receiver back again.  Ah, the buzzing is coming back.  He is leaving the room.  I suppose he has found the electric light cane and the pistol where he left them.  Now, Walter, since you have become accustomed to this thing take it and tell me what you hear.”

Craig had already seized the other apparatus connected with the art-gallery and had the wireless receiver over his head.  He was listening with rapt attention, talking while he waited.

“This is an apparatus,” he was saying, “that was devised by Dr. Fournier d’Albe, lecturer on physics at Birmingham University, to aid the blind.  It is known as the optophone.  What I am literally doing now is to hear light.  The optophone translates light into sound by means of that wonderful little element, selenium, which in darkness is a poor conductor of electricity, but in light is a good conductor.  This property is used in the optophone in transmitting an electric current which is interrupted by a special clockwork interrupter.  It makes light and darkness audible in the telephone.  This thing over my head is like a wireless telephone receiver, capable of detecting a current of even a quarter of a microampere.”