BTH merged with the Metropolitan-Vickers company in 1928 to form Associated Electrical Industries (AEI), but the two brand identities were maintained until 1960.

At this stage Laing, Wharton and Down was renamed as British Thomson-Houston and General Electric became the majority owner of the company.

[3] In 1900 BTH bought Glebe Farm on the west side of Mill Road north of the railway in Rugby for £10,000, from Thos.

In the same year BTH obtained a licence to produce the Curtis steam turbine, which became one of the company's major products.

In 1905 BTH made its first turbo-alternator and in 1911 got licences for all of General Electric's drawn-wire light bulbs, which it produced under the Mazda trademark.

[4] In 1907 BTH started a joint venture with Wolseley Motors to make petrol-electric buses and in 1909 the company supplied major coal-fired steam generators to London to power an electric trolley system that was being set up.

[3] During World War I BTH expanded into naval electrical equipment, supplying the Royal Navy with various lighting, radio and signalling gear.

BTH had been in the process of buying Edison Swan (Ediswan) and Ferguson, Pailin & Co, with AEI completing the purchases in 1929.

The BTH factory in Northern Ireland made the turbo generator and propulsion motor for one of the world's first turbo-electric merchant ships, the banana boat SS San Benito, in 1921.

BTH's directors seemed sceptical of the design and offered little help, and in 1940 decided they were not really interested in making jet engines due to their commitment to electrical equipment.

[5] Rivalry with Metrovick intensified, particularly after BTH won the contract to build the new Buenos Aires Central Costanera S.A. power station, valued at £35 million, in 1957.

The Ediswan trademark appeared on semiconductors in 1956 and the following year British Rail Class 15 diesel-electric locomotives were designed by BTH.

At this point the size of the Rugby site peaked, with all of the company's land west of the Black Path built over.

The firm's clubhouse on Hillmorton Road was demolished in 2007, and the south edge of its surrounding sports field was encroached along for house building.

During post-World War II Britain, AEI established a consolidated research effort at Aldermaston in Berkshire, England.

The research centre was based at Aldermaston Court, a large stately home owned by AEI that had been requisitioned for military use in the war era.

One of the BTH-built batch of New Zealand Railways DSC class Bo-Bo shunters has been preserved and is used in industrial service, complete with original Rolls-Royce engines.