Although Bell was, and still is, accused of stealing the telephone from Gray, Bell used Gray's water transmitter design only after Bell's patent had been granted, and only as a proof of concept scientific experiment, to prove to his own satisfaction that intelligible "articulate speech" (Bell's words) could be electrically transmitted. After <answer>March</answer> 1876, Bell focused on improving the electromagnetic telephone and never used Gray's liquid transmitter in public demonstrations or commercial use.
Bell stopped using the liquid-based method after what month?
March