Robeson and Birchfield delayed money pack trigger until robber left

By the 1960s, banks had spent three decades trying to booby-trap stolen cash. A 1933 device discharged dye when a robber squeezed the pack; a later patent triggered on tilt, the instant the pack was lifted from the drawer; another required a teller to press a hidden button as the money was handed over. Every one of these designs activated the marking charge on the spot, in the lobby, in the robber's hand, with tellers and customers standing in the blast radius of the tear gas, dye or smoke the device released.

That was also their fatal flaw: as the patent Harold Robeson and Jerry Birchfield later filed put it plainly, existing devices made "it possible for the disabling charge to be activated on the robbed premises and in the presence of any individuals that may be held at bay by the robber." A robber who felt or saw the pack activate while still standing over a hostage had every reason to panic, and a device meant to protect bank staff was instead the thing most likely to get one of them hurt.

Moved the trigger to a transmitter at the exit, so the pack only starts a countdown after the robber carries it past, releasing tear gas and dye when no hostage is near.

The trigger was placed at the exit, a point the robber had to pass, so the device activated only after he left the danger zone.

No teller had to act under duress, eliminating the risk of a premature activation in the lobby.

The delay ensured the marking charge discharged when no hostages were nearby, protecting staff and customers.

The marked cash became worthless after the robber left, deterring the crime without endangering bystanders.

When a safety device risks harming bystanders, move its trigger to a point after the wrongdoer has left the danger zone, rather than making it faster or more sensitive.

Because the marked pack stayed inert through the robbery itself and only began its countdown once carried past the exit field, the tear gas and dye discharged after the robber had already left the building — protecting hostages while still permanently marking both the stolen cash and the robber. By the time of inventor Harold Robeson's death in 1998, the design he and Birchfield patented was in use, in descendant form, at roughly 5,000 U.S. financial institutions.