In the spring of 1875, Edison decided to expand his laboratory facilities and devote his full time to invention. He took over most of the Ward Street building for the laboratory while Joseph Murray, who had replaced William Unger as his manufacturing partner, continued to manufacture telegraph apparatus on the top floor and half of the third. On May 31, Edison and Charles Batchelor drew up a list of inventions to work on in the new laboratory, including a "copying press that will take 100 copies & system." This turned into Edison’s electric pen, his first successful invention outside of telegraphy and the first one that he marketed himself.
By this time Edison had added a second experimental assistant, James Adams. On 2 October 1875, Edison signed an agreement giving Batchelor and Adams a percentage of his royalties from the electric pen. The agreement also set aside a small portion for the laboratory.