Cornell made his fortune in the telegraph business as an associate of Samuel Morse, having gained his trust by constructing and stringing the telegraph poles between Washington, D.C. and Baltimore, Maryland, the first telegraph line of substance in the U.S. To address the problem of telegraph lines shorting out to the ground, Cornell invented the idea of using glass insulators at the point where telegraph lines are connected to supporting poles. After joining with Morse, Cornell supervised the erection of many telegraph lines, including a portion of the New York, Albany & Buffalo line in 1846 and the Erie and Michigan Telegraph Company connecting Buffalo to Milwaukee with partners John James Speed and Francis Ormand Jonathan Smith. Cornell, Speed and Smith also built the New York and Erie line competing with and paralleling to the south the New York, Albany and Buffalo line in which Morse had a major share. The line was completed in 1849 and Cornell was made president of the company.