He left school at the age of 11 to pursue his interest to be part of a modern business.
While working with Western Union, he encountered a group of rushing messenger boys which inspired him to improve exchange communications.
These young men called runners had to dash from the Stock Exchange to their offices to report the changes in share values.
Calahan realized that the prices could be sent directly via telegraph to each broker's office much more quickly and efficiently in a permanent stream of information.
[4] After many years of work he started the company ADT security, which still stands today.