This series comprises the records of private telephone companies which were taken over by the Post Office in 1912. It consists of items relating to individual companies, National Telephone Company rules and instructions, staff related records, agreements, judicial proceedings, valuation of assets and a collection of early telephone directories.

Papers of Professor Silvanus Phillips Thompson, 1828-1951, comprising correspondence, 1828-1919, notably with William Edward Ayrton, 1883-1899; Sir William Fletcher Barrett, [1878-1896]; Antoine Henri Becquerel, 1902-1908; Alexander Graham Bell, 1879-1880, concerning his experiments with the telephone; Sir Wiliam Crookes, 1876-1916; Michael Faraday, 1830-1835 [written to Richard Phillips]; Sir William Huggins, 1879-1907; David Edward Hughes, 1884-[1912], concerning magnetism; Sir Joseph Larmor, 1902-1916; Sir Oliver Joseph Lodge, 1897-1919; Sir James Augustus Henry Murray, 1904-1913; Augusto Righi, [1902]-1916; William Thomson, 1882-1907; notes and printed material, [1907]-1951; photographs and prints, 1870-1910, notably photographs of friends and family.