Thomas Edison began serious research into developing a practical incandescent lamp in 1878. Edison filed his first patent application for 'Improvement In Electric Lights' on 14 October 1878. After many experiments, first with carbon in the early 1880s and then with platinum and other metals, in the end Edison returned to a carbon filament. The first successful test was on 22 October 1879, and lasted 13.5 hours. Edison continued to improve this design and by 4 November 1879, filed for a US patent for an electric lamp using 'a carbon filament or strip coiled and connected to platinum contact wires.' Although the patent described several ways of creating the carbon filament including using 'cotton and linen thread, wood splints, papers coiled in various ways,' Edison and his team later discovered that a carbonized bamboo filament could last more than 1200 hours.