A glass harmonica was invented by Benjamin Franklin and is known as the Franklin harmonica. It consists of glass bowls of varying sizes filled with different amounts of water. This instrument was popular in the 1800s. Which detail is least relevant to someone researching Benjamin Franklin’s genius? The instrument was popular in the 1800s. The bowls varied in size and were filled with different amounts of water. The harmonica consisted of glass bowls and water. Franklin invented a glass harmonica.