The transition to battery-powered hearing aids occurred during the early twentieth century. Initially the battery packs were cumbersome and had to be carried in separate boxes or strapped to the individual's leg. The early aids were carbon type, followed by vacuum tube aids introduced in 1939. In 1944, the first vacuum tube hearing aid was developed which contained the battery inside the aid. The first transistor hearing aid was introduced in the 1950s. With the development of the transistor, the aids were able to become smaller and more powerful. The use of microchips for programming hearing aids was introduced in 1985 to better meet individual needs.