In 1863, Edward A. Calahan invented a stock ticker and formed the Gold and Stock Telegraph Company in 1867 to exploit the technology. Gold and Stock also developed a messenger system that sent instructions to and from the stock exchange floor. Three years later, the president of Gold and Stock Telegraph Company woke up to a burglar in his home, which inspired him to create a telegraph-based alert system.
This system eventually connected 50 of his neighbors to a central station where all the alert boxes were monitored. There were many small telegraph delivery companies in the United States in the 19th century.
In 1874, 57 district telegraph delivery companies affiliated and became "American District Telegraph". With the increase in telephone usage in the late 19th century, ADT's messenger business slowly declined in popularity. ADT tried branching out and developing their signaling business, while still maintaining their telegraph business as primary income source. ADT incorporated into Western Union in 1901 and separated its messenger business from its main signaling business at that time. In 1909, Western Union and ADT came under the control of American Telephone & Telegraph Company (AT&T). ADT began to expand into new areas, such as fire alarms and security alarms between 1910 and 1930, but was kept separate from AT&T's Holmes alarm business. ADT became a publicly traded company in the 1960s.
According to wikipedia