 Originally, the Doctors time machine, the TARDIS, was to have a  different appearance in order to blend in wherever and whenever it  materializes due to its "chameleon circuit." However, it was decided  that this constant changing of a regular prop would be too  expensive. So, it was decided that the circuit would be permanently  disabled due to the TARDIS age, thus retaining the appearance of a  1963 Police Callbox.