It is a crisp, blustery, autumn afternoon on the South Bank in London. Charlotte Campbell stands with her back to the river playing an acoustic guitar and singing Leonard Cohen’s song Hallelujah, a buskers’ favourite. “Now I’ve heard there was a secret chord, That David played, and it pleased the Lord, But you don’t really care for music, do you?”

People stop, watch a while, film Campbell on their phones, or mouth along. “It goes like this: The fourth, the fifth, The minor fall and the major lift, The baffled king composing Hallelujah …” Some drop coins into her guitar case, or send their kids to do so. Pound coins, silver ones, or just a few coppers.

Campbell is definitely a busker for the 21st century. She has cards, with information on how to find her on YouTube, Facebook, Twitter and Instagram. And as well as the guitar case for coins, she also has something that might make you look twice – a contactless card reader. Because hardly anyone uses cash any more – just like the Queen. (Even two years ago, a survey found that the average Briton carried less than £5 on them.) No cash? No problem; tap Campbell’s card reader, to give a quid.

Although, in the half hour that I am watching and listening – to covers of Fleetwood Mac, Christina Perri, Ed Sheeran, the Beatles, plus some of Campbell’s own material, including a song with the line: “I don’t need your coins, no, just your ear” – no one does tap their card, until I do. It doesn’t help that the sign has blown over. She gets more taps when she busks at train stations, she says.

The project to enable buskers to accept contactless payments, still in its infancy, was launched by the London mayor, Sadiq Khan, and Campbell was one of a select few performers to be given a contactless card reader (by the Swedish financial technology company iZettle). But only a tiny proportion of her earnings comes that way. (Buskers, like everyone else, I learn, are reluctant to divulge exactly what they earn, but Campbell makes enough to live and pay rent in London.) For a while, she had been thinking about how the move towards a cashless society would affect her career, and noticed that more people were saying they didn’t have any cash on them. Her music is on iTunes and Spotify, and she has a website where you can donate by various means.

Is contactless in the spirit of busking, I wonder? “There is a romantic thing about dropping a coin into a hat. That’s what people think they’re going to miss,” Campbell says. “But if people don’t have cash any more, that’s never going to be something people will get to do ever again. There’s only two options here – we either don’t have buskers or we drop a coin into a hat in a different way. We have to romanticise the tap on the screens somehow.” And she laughs.

Campbell has not yet managed to romanticise the tap, or work it successfully into her hat line. (A hat line is a busker’s end-of-set patter, designed to extract maximum cash from audience. “If you could just take £1 or £2 out of your wallet and give me the rest,” is an old favourite.) When Campbell says: “Also, I have a card reader,” she sounds almost apologetic.

Facebook Twitter Pinterest Benjamin Clementine, who was ‘discovered’ while playing on the Paris Metro. Photograph: Moreau Lionel/ABACA Press France

Michael Hennessy, a New Yorker now living and busking in Bath, has also noticed that his hats have been down recently. Over the phone, he tells me he puts this down to multiple factors: global uncertainty, Brexit, Trump, Bath and North East Somerset council’s unhelpful parking restrictions that are putting off the tour operators, and, yes, the fact that people don’t have money in their pockets any more.

Hennessy also has a card reader, although, so far, it has been more effective at generating comedy than income. People point at it and laugh. To which he now responds: “You know you wanna!” or “Come closer and I’ll empty your bank account.”

Sherika Sherard busks on the South Bank and in Bath. She was a bit embarrassed about her card reader at first. “But then I thought, actually, I believe in what I do, and my talents, it’s OK,” she says. “When you think of yourself as a musician and you have music to showcase, it makes sense.”

She makes little jokes about having to keep up with technology, people laugh. She has even had people give their cards to their kids to come up for a beep. “People see that you’re taking yourself seriously. It shows that you mean business.”

Facebook Twitter Pinterest Sherika Sherard busking in London. Photograph: Duncan J Harding

Dr Paul Simpson is about the nearest there is to a professor of busking. He is a professor of human geography at the University of Plymouth, but he has done lots of research into street performance, as well as busking himself. And he is sceptical about contactless busking. “Throughout history, there have been attempts to legislate, regulate and organise, restrict,” he says. But part of a busker’s value “is the informality, the novelty, the excitement, the potential for unpredictability that they can bring to urban life”. Visa debit doesn’t quite capture this mercurial spirit.

Simpson, who used to busk in Glasgow, says the sound of a coin hitting a pile of change helped get him through. “If it was a chilly day on Buchanan Street, drizzle in the air, you’d been there for hours, the sight of the coins inside your case gave you a sense of how you were doing, it was a material manifestation of appreciation. Because people don’t tend to clap or stop, it was the equivalent of a round of applause at the end of a song.” He wonders if the beep of a card machine would feel quite the same.

Plus, he has issues with the fixed rate – tap to give £1 (£2 for Hennessy in Bath, no wonder they are laughing!). Busking, says Simpson, is a “democratic sort of performance, anyone can see it, there’s no standard charge; if you choose to donate, you donate what you want to”.

But for Nick Broad it is adapt or get a job. “There’s no middle ground where you cannot take cashless payments and still earn in a cashless society,” he says. Broad, with his partner Liliana Maz, runs the Busking Project, a non-profit organisation that exists “to promote, celebrate and defend buskers with tech, advocacy, research and opportunities.” Interviewed by Broad for a film, street performers in various countries told him their hats were getting smaller, and some attributed this to a move towards cashless. That was in 2011, since when Broad and Maz have developed various systems to try to make it easier for buskers to take payment via card, Paypal, Apple Pay and Google Pay, via a tip-me button on online performer profiles.

Now they are trialling a new tap-to-tip system, where you tap your phone against something – a process that should take no more than a couple of seconds. Broad’s not saying too much about it, as it hasn’t been shown to work yet.

One thing he and Simpson do agree on is the importance of buskers. Simpson says they can make uninspiring places more engaging, threatening places more hospitable, and can bring people together and make them smile. Broad talks about their place in the music industry, in which live venues are being closed down, and all the money goes to a tiny percentage of artists while everyone else picks over the crumbs. “Street performance, in my opinion, may be the only viable way of making a living as an independent artist,” he says. It is not unheard of for buskers to get spotted and signed. Benjamin Clementine, from Edmonton in north London, was disovered while playing on the Paris Metro, and went on to be shortlisted for the 2015 Mercury music prize. BB King, Tracy Chapman and KT Tunstall are among others who started playing on the streets.

I need to talk to more buskers outside London and Bath, with their mayor-friendly schemes, lotteries for prized slots, rules and licences. So, I head to Glasgow.

Facebook Twitter Pinterest David Burns singing and playing Guitar on Sauchiehall Street, Glasgow. Photograph: Katherine Anne Rose/The Observer

Busking is big in Glasgow, big in its contribution to the atmosphere of the city centre, big in the number of people doing it. If you walk along the eastern half of Sauchiehall Street, then all the way down Buchanan Street, then left on to the pedestrianised part of Argyle Street, you will be accompanied – unless it’s chucking it down – by music. Pink Floyd’s Wish You Were Here fades out and in comes a piper, perhaps, Bob Marley’s Redemption Song or the sound of Oasis being murdered on a beaten-up guitar. It would be surprising if you made the journey without hearing Hallelujah.

David Burns admits that Hallelujah is in his repertoire; I hear him do Blur’s Coffee and TV, and a song by Marshall Chipped, his own indie band. Softly spoken, but with a big, deep singing voice, Burns has been busking in Glasgow for 15 years. When he began, there were just a handful of them, now it has reached saturation point. Today, his usual spot outside Costa was already gone – at 9am on a Thursday in November – so he is further down the street by the Co-op.

He still makes enough to pay the bills, he says. It gets him out of the house, he only sings the songs he wants to sing and no one takes a cut. “Busking is the purest form of this,” he says. “It’s taking your music directly to people. You’re out here on the streets and if somebody likes you they’ll throw a coin in, if they don’t they’ll walk on past.”

Burns has thought about people carrying less cash around – he is guilty of it himself – but thinks it will be more of an issue further down the line. In the meantime, he is keeping an eye on it. He doesn’t rule out having a contactless card reader one day.

I speak to Andy Bargh, who has only been busking for five years. He thinks a card reader would look arrogant, almost as there is an expectation. He reckons that if a busker were photographed with one in Glasgow and it went on social media, they would be torn apart.

Facebook Twitter Pinterest Malachy, who is from Australia, busking in Glasgow. Photograph: Katherine Anne Rose/The Observer

In Buchanan Street, Malachy has bagged the prime spot outside TGI Fridays. As he did yesterday. He gets here early and sets his equipment out (although, as he won’t play for several hours, not everyone appreciates that). Malachy is from Western Australia, and has busked all over. Glasgow’s a fantastic place to do it, he says. “People have time for it here, they will listen and appreciate original music.”

He plays what he describes as “long-hair wanker music”. Malachy isn’t a fan of Hallelujah. “If you walk up and down here, you’ll here Hallelujah about eight million fucking times.” Malachy says he will probably get a card reader next year. I wonder how that will go down with Glasgow’s other buskers. Malachy has busked in the US, where it is already far more common for buskers to take payments by card or phone. In China, it is not just buskers but beggars, too, who are now accepting digital payments.

Here is another familiar tune – Champagne Supernova – played by Connor Stewart on the sitar. Stewart, a bouncer by night, is from the Govan area of Glasgow, but spent four or five years in Varanasi, India, learning the sitar. Indian classical music is his passion, and he plays it on the street as well as his sitar versions of Oasis and the Stone Roses. He hasn’t been doing it long enough to notice any change in hats or cash-carrying habits, but he seems to be doing OK today. And doing something different. I think the music is more important than the money for Connor: he is on a higher plane.

Facebook Twitter Pinterest Nikki Foster, a classically trained opera singer, who busks in Glasgow. Photograph: Katherine Anne Rose/The Observer

Round the corner to Argyle Street, past Bobby Hamilton, “the Bowie and Bolan Busker Bloke”, is segueing seamlessly from The Man Who Sold The World to Hot Love. And here is Hallelujah, again, being sung by Nikki Foster. Foster, originally from County Durham, came to Glasgow to do a master’s in music at the Royal Conservatoire of Scotland. She is a classically trained opera singer, but the operatic voice began to grate, she says. “I felt I wasn’t being my true self, like I was doing it just because I knew people would like it rather than because I wanted to.”

Now she sings whatever the hell she wants to sing – Moon River from Breakfast at Tiffany’s, I Dreamed a Dream from Les Mis, Leonard Cohen’s Hallelujah. And it’s not a cry that you hear at night, It’s not somebody who’s seen the light, It’s a cold and it’s a broken Hallelujah.

Singing on the street has taught Foster a lot. “I feel busking has really helped to reveal who I am,” she says. “I just let go of the bullshit in my mind.”

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She doesn’t do badly from it. If the people of Glasgow aren’t carrying much cash around any more, then most of it seems to be going into Foster’s hat. No one told her about not revealing earnings. It’s really good, she says. “You obviously get your off-days, like sometimes if I’m not feeling that good. The lowest I’d ever make is £20 an hour, if I’ve got a cold or something and I don’t want to be here, I’m forcing myself. But, generally, when I’m in the flow, it’s about 50 quid an hour.” Cash only. Hallelujah indeed.