Image caption Lord Reith was involved in the creation of the SOS messages

When somebody is critically ill, people pick up their mobile phones and let their relatives know. But in 1923 the BBC started broadcasting directly to individuals letting them know their loved ones were "dangerously ill". These SOS messages lasted for 70 years.

In 1954 my grandfather found out his father was dying through a BBC SOS message. He didn't hear it himself, but a man at the building site he was working on in the East Riding of Yorkshire, did. "Please head to Dublin where your father is dangerously ill," it instructed.

It's remarkable that the message even made it. It was heard by somebody who deduced it was him even though they only knew him by his nickname, Joe.

My grandfather had left his home in Ireland almost seven years before, and the relationship between him and his father had broken down.

Image caption My great-grandfather, the subject of an SOS

Despite this, on hearing he was the subject of an SOS, he headed straight to Dublin, trusting that the short message was correct. It was - but he was too late to see his father again before he died.

The impact of that 30-second message was life-changing, both for him and the rest of the family. He re-connected with his mother and secured for my family our link with the Emerald Isle.

Image caption My grandfather (left) with his mother (centre) and my father (bottom left) on their first trip to Ireland after the SOS

I tried to find that message in the BBC's archives, without success. But I listened to many others. They are a snapshot of some of the most intense, personal moments in people's lives.

They're very short, and clinical. "Will Mr and Mrs Little, last heard of eight months ago in the Birmingham area, head to Leeds General Infirmary where Mrs Little's mother is dangerously ill," for example. Listening to them leaves more questions than answers - did the subject make it there in time? What happened afterwards? How had they lost contact with their family in the first place?

These questions were never going to be answered. John Reith, the BBC's first director-general was personally involved in the creation of the SOS messages and was clear that they were a public service and not entertainment.

The words were never sensationalised, a few facts such as names, addresses, car registrations, caravan descriptions and phone numbers.

The first SOS message played out in March 1923, a 30-second appeal to help find a missing six-year-old boy. Listeners got in touch, and the boy was found safe and well.

Media playback is unsupported on your device Media caption An SOS message

This success encouraged the BBC to keep producing them, and an eclectic mixture of "emergencies" followed - a Pelican escaping at St James' Park, and a callout for a wet nurse for twins born at a hospital in Norfolk in 1937.

Another message to a bird-lover urged him to contact a Birmingham pet shop after buying parrots there because the seller had subsequently died from what was believed to be Psittacosis or "parrot fever".

By the 1930s they were part of the radio furniture and decisions had to be made on how to stem the flow of messages. The BBC arranged to have research done to find out which SOS messages were working.

The results were in by 1931. For missing persons about 45 out of 190 were successful, but for illness the success rate was higher at about a half. The BBC stopped broadcasting appeals for missing persons.

Jack Thompson remembers hearing his name on an SOS message and says it was terrifying to suddenly have the presenter talking directly to you, especially after the news bulletin had focused on serious world news stories.

He was on a camping trip with his wife and three sons in the early 70s and was enjoying "whatever food it is you eat on camping trips" when they heard the radio message: "Will the Thompson family, believed to be travelling somewhere in Surrey contact Chesterfield Hospital as Mrs Thompson's mother is dangerously ill." The words are burnt into his memory.

"We just weren't expecting it to be for us," 84-year-old Jack says. "It put us into a bit of a panic and, of course, at that time we didn't have any phone numbers to call apart from the hospital so it was stressful as they couldn't give us much information."

They were a long way from Chesterfield but headed straight to the hospital. Jack remembers the journey vividly, his wife was scared for the whole way having been startled at the broadcast, and having no details about her mother's situation. After a long car journey they arrived in time to see Mrs Thompson, who recovered slightly but died soon after. The SOS message had given them the chance to spend time with her at the end.

While the Thompson family heard the message themselves, often the subjects would be informed by complete strangers - a neighbour down the road, a manager at a hotel they were staying at, somebody recognising a car registration plate.

Media playback is unsupported on your device Media caption Linda Miller told her SOS story to the BBC's Eddie Mair © Getty Images

It seems unbelievable that a stranger would recognise a car registration plate from a 30-second clip but this is exactly what happened to Linda Miller. She told her story to the BBC's Eddie Mair for a Radio 4 documentary in 2011.

Linda was six when she became very ill with suspected osteomyelitis, inflammation of the bone. She was staying with her aunt while her parents were on holiday in London.

An SOS message went out but they didn't hear it. Remarkably a stranger had, and knocked on their car window to alert them to the fact their daughter was ill in a hospital in Sunderland. He had heard the message and recognised their registration plate.

Miller's parents rushed to Sunderland. She recovered and was fine, but the stranger kept in touch sending letters to find out how she was.

A stranger, knocking on a car window to alert parents to the news their daughter was ill. It seems as though the BBC's 1931 year book summed up the SOS messages perfectly when it wrote that they show the "inherent kindliness of human nature".

It is not known when the messages ended, but it was at some point during the 90s, although BBC World Service continued the practice into the 21st Century. Mobile phones made them redundant, but for those who were directly affected by the SOS messages, my family included, they had a huge impact that lives on today.

Follow Kathleen Hawkins on Twitter @kat_b_hawkins

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