Bell had used a disc of fragile gold-beaters' skin with a patch of sheet-iron glued to the centre.

He could not believe, for a time, that a disc of all-iron would vibrate under the slight influence of a spoken word.

But he and Watson noticed that when the patch was bigger the talking was better, and presently they threw away the gold-beaters' skin and used the iron alone.
Also, it was Watson who spent months experimenting with all sorts and sizes of iron discs, so as to get the one that would best convey the sound.

If the iron was too thick, he discovered, the voice was shrilled into a Punch-and-Judy squeal; and if it was too thin, the voice became a hollow and sepulchral groan, as if the speaker had his head in a barrel.
Other months, too, were spent in finding out the proper size and shape for the air cavity in front of the disc.

And so, after the telephone had been perfected, IN PRINCIPLE, a full year was required to lift it out of the class of scientific toys, and another year or two to present it properly to the business world.
Until 1878 all Bell telephone apparatus was made by Watson in Charles Williams's little shop in Court Street, Boston--a building long since transformed into a five-cent theatre.