The clang factor means that it doesn't hurt so much when you are being dragged along a corridor towards your next beating. It renders your despair less acute and the demons that visit you during your endless nights are, thanks to the clang factor, friendlier than those that haunt your fellow prisoners.
The clang factor means simply that you have been to prison before and so, by extension, you are less affected by the metallic closing of your cell door every night than are your fellow inmates. Even if you are innocent.
For Paddy Hill, one of the Birmingham Six, the clang factor cost pounds 25,000. That was the amount Sir David deducted from his offer of compensation for 16 years' wrongful imprisonment because Mr Hill had been to prison before, during the Sixties, for assaulting a nightclub bouncer.
In his explanation for offering Mr Hill, 52, pounds 200,000 for wrongful imprisonment, while four of the Six were offered pounds 225,000 and one - inexplicably - was offered pounds 235,000, Sir David wrote that the "clang factor" was absent for the others. It was just one of the anomalies - which the Six feel amount to insults - that have resulted in six years of legal argument during which all the men have suffered financially and mentally in a nightmare that seems to have no ending.
"I saw Paddy Hill being kicked along a landing by prison officers," says Billy Power, 51, another of the Six. "I saw them kicking and dragging him along for a beating, and I saw his fear. They kicked his teeth out. And then they offered him less compensation than us because he had been to prison before, as though it hurt less.
"We sued over our beatings, and the Home Office admitted that prison officers had been responsible, but we haven't been offered any compensation whatsoever for that. Everything they are doing - the level of their offer, the dragging of the feet - simply reinforces the whispering campaigns that maybe we did it.
"We don't care so much about the money, but it should be high enough to send out the message loud and clear: these men are innocent, and we made a mistake."
In the past six years, since the Court of Appeal released them, the men have been given two interim payments totalling pounds 200,000. Lawyers are still arguing over the final settlement, which should include elements for loss of past and future earnings and damages for the men's appalling treatment. Earlier this year, the Home Office made a "final offer" but Sir David said it was still open for negotiation when it was challenged in the courts by Gerry Hunter.
And there the matter rested, apparently becalmed, until yesterday when the men pledged to camp out on the steps of Jack Straw's office until the Home Secretary agrees to meet them.
The interim payments sound like a lot of money, but they were gone in the blink of an eye. The men dared not resettle in Birmingham, so most of them had to splash out pounds 100,000 immediately for somewhere to live in London.
Then there were the binges, the admitted squandering. "I kept buying presents for my family," says Paddy Hill. "I had these enormous feelings of guilt. I had been away from my children for 16 years. What do you do when your grandchild runs away because she doesn't know you? I suppose I tried to buy their love."
In the meantime, Hugh Callaghan, Paddy Hill, Billy Power, Gerry Hunter, Richard McIlkenny, and Johnny Walker have all suffered anxiety, depression, panic attacks and financial hardship. All their relationships have been put under enormous strain. Only Richard McIlkenny remained living with and married to his original wife. Billy Power divorced his wife and, happily, remarried; Hugh Callaghan, 67, is still married to Eileen, but they live separately.
"We aren't formally separated or anything," he says. "But she has lived in Birmingham since 1949 and needs to be close to her doctor and family. [She has undergone major heart surgery.] I would feel anxious for her and for my daughter, Geraldine, and my two grandchildren if I moved back. People look at you when you walk down the street or in the park, and you think: `Oh, no. The Birmingham Six. They recognise me ...'"
When the men were released, they were offered no counselling, no help to resettle, no support from the state apart from their interim payments. As their money ran out, they have subsequently discovered that they are not entitled to benefits, because no National Insurance contributions were paid during the 16 years they were in prison.
"We're all terribly scarred," says Callaghan. "My neighbours are very patient with me, and they knock on my door in the middle of the night to see if I'm OK. I scream a lot during my nightmares and wake them up."
All the men's families have suffered through their absence - or their presence. Sons and daughters have endured abuse and violence from those who have not accepted the men's innocence. One son was badly beaten. One daughter broke down, and had to be sectioned only this week.
"It hasn't been easy for them," says Paddy Hill. "When I came out, I'd be on buses and just panic. I'd scream at the bus driver to let me out in between stops. My problems were worse on the Tube.
"They [the state] gave us no help ... And we've all had problems with relationships. Living with me is like walking around with a hand-grenade in your pocket. You just never know when I'm going to go off."
When finally they were given help, clinical psychologists, led by Dr Adrian Grounds of the Institute of Criminology in Cambridge, said that the Six's trauma was comparable to that suffered by people who have been in serious car crashes.
"Research showed signs of irreversible psychological damage," says their solicitor, Gareth Peirce. "Until they came out, no one knew what to expect. There had never been a case like this before, so the psychologists are only now gathering information about the effects of long-term wrongful imprisonment."
When a Birmingham newspaper recently reported that a whispering campaign was circulating, criticising the men, Hugh Callaghan's psychologist, Dr Ray Aldridge, wrote to the newspaper staking his reputation on Mr Callaghan's innocence. It was a welcome gesture, given that no government minister has had the decency to do the same thing.
The Six are now hoping that Jack Straw will be courageous enough to lay the matter to rest, and to say "Sorry" on behalf of us all.
"I don't think money would solve most of their difficulties," says Ms Peirce. "Money would not compensate them for the lost time and the problems associated with their imprisonment, but the fact that so small an award has been offered represents another rejection and has caused great pain in its own right. It is not sending the message that needs to be sent - that these men are innocent - and it has not been accompanied by an apology"n