Craig continued to tinker tantalisingly with the machine.
"The principle on which it is based," he added, "is that a mass of tempered steel may be impressed with and will retain magnetic fluxes varying in density and in sign in adjacent portions of its mass. There are no indentations on the wire or the steel disk. Instead there is a deposit of magnetic impulse on the wire, which is made by connecting up an ordinary telephone transmitter with the electromagnets and talking through the coil. The disturbance set up in the coils by the vibration of the diaphragm of the transmitter causes a deposit of magnetic impulse on the wire, the coils being connected with dry batteries. When the wire is again run past these coils, with a receiver such as I have here in circuit with the coils, a light vibration is set up in the receiver diaphragm which reproduces the sound of speech."
He turned a switch and placed an ear-piece over his head, giving me another connected with it. We listened eagerly. There were no foreign noises in the machine, no grating or thumping sounds, as he controlled the running off of the steel wire by means of a foot-pedal.
We were listening to everything that had been said over the Willoughby telephone during the day. Several local calls to tradesmen came first, and these we passed over quickly. Finally we heard the following conversation:
"Hello. Is that you, Ella? Yes, this is Maud. Good-morning. How do you feel to-day?"
