By 1874, Bell's initial work on the harmonic telegraph had entered a formative stage, with progress made both at his new Boston "laboratory" (a rented facility) and at his family home in Canada a big success.[N 14] While working that summer in Brantford, Bell experimented with a "phonautograph", a pen-like machine that could draw shapes of <answer>sound</answer> waves on smoked glass by tracing their vibrations. Bell thought it might be possible to generate undulating electrical currents that corresponded to <answer>sound</answer> waves. Bell also thought that multiple metal reeds tuned to different frequencies like a harp would be able to convert the undulating currents back into <answer>sound</answer>. But he had no working model to demonstrate the feasibility of these ideas.
What did Bell believe the reeds would turn the signal into?
sound