His latest acquisition is the tiny, shirt-pocket-sized Sony CM-R111 mobile phone complete with typical Nineties-style non-toxic biodegradable packaging. For all its diminutive size, it looks solid enough. Yet it is but an eye-blink in technological history. 'It didn't exist last year and it may not exist next year,' Mr Bridgman says. 'Consumer technology is evolving at an unbelievable rate.'
The Sony is the smallest of three mobile phones standing on his desk like monoliths already half-way into antiquity. The biggest and oldest, the Motorola 8800X, nicknamed 'the brick', has been assigned a display case with a printed card: 'Hand-portable mobile telephones from Motorola, 1992'. One day it will be joined by the original 'brick', the Motorola DynaTak 8000X, which went on sale in Year One: 1985, when Cellnet and Vodaphone opened up. They are Britain's two Tacs (Total Access Communications Systems) networks. The 8000X came and went so quickly that the museum never acquired one.
Three mobile phones diminishing in size: is this a lesson in technology's drive towards miniaturisation? Hardly. There will be a lot more big bricks, but people, not technology, will decide that, Mr Bridgman believes. Mobile phones are seen by some as conferring status - and bulky ones are irresistibly conspicuous. The outrageously colourful Swatch Twinphone (1993) - designed by Enrico Quell of Galli, Milan, made by Nokia of Finland and redesigned annually - is deliberately big and eye-catching.
By contrast, the hardly visible Sony seems to have shrunk beyond fashion. Mr Bridgman tips the third of his trio, the medium-sized 1993 Ericsson EH 237 (from Sweden, land of mobile-phone freaks), as the next bestseller. He was among eight judges who voted it Mobile Phone of the Year at Cellnet's annual Caesar awards last month.
'When the cheaper networks cover the country,' he predicts, 'the mobile phone will become just as important as the plug-in phone. It will become normal for whole families to carry them. In hypermarkets, a lost child will receive a call on his or her mobile and report back - 'I'm behind this stack of paint, silly.'
'It's the M25 effect. That was intended as a London bypass, but planners did not anticipate that it would become choked by Londoners using it to drive from suburb to suburb. It goes to show that the interaction between people and technology is quite unpredictable. People will use technology as it suits them.
'Another example is the way video recorders, intended to record television programmes, have come to rival not only the cinema as a source of entertainment but also television itself. That was not predictable.'
Having watched the present tumbling into the past ever more rapidly, Mr Bridgman believes more familiar predictions will be realised ahead of schedule. 'We are on the verge of seeing the death of the reference book. The computer is pushing it aside,' he says. He has been chuckling over a chronicle of changing times, Cellnet's Mobile Manners: A Guide to Mobile Etiquette in the Nineties (1994), published to reduce consumer resistance.
It warns against suddenly blocking the pavement when making a call, says that old people should be warned that a car-phone might ring, and advises when to switch to call-back - for example, when meeting a client or watching a play at the theatre.
However, it defends their use on trains ('As the train cruises effortlessly through the countryside, receivers chirrup gently') and tries to answer complaints about their use in restaurants ('How many people can honestly admit to having heard a mobile phone go off in a restaurant?').
One of the oldest problems of adapting to electronic broadcasting still afflicts mobile phone users. Cellnet's book says: 'There is no need to yell or shout.'
But such comedies of manners are simply an echo of Alexander Graham Bell, 1876 and all that, Mr Bridgman says. In those days, the rich agonised over whether they should handle a telephone receiver or if it was the servants' job to relay messages. If you find this hard to imagine, consider today's technophobic parents begging their children to programme the video recorder.
People power, or at least the purchasing power of the technologically adaptable, means that Mr Bridgman finds big retailers such as Rumbelows or Dixons more accurate than manufacturers in predicting trends in consumer technology. He wonders if it is the retailers who are dictating trends, rather than the inexorable march of technology.
Sales figures for the tiny Sony and other evanescent products will tell. That is why the Sony is a prized acquisition. 'Other products that keep selling for years may not reflect what was going on at the time they were introduced.' Unlike private collectors, who can collect according to whim, Mr Bridgman feels he should emulate the British Library in amassing a representative selection of new products. 'I want future historians to start from the best possible base,' he says.
To keep up to date, he scours newspapers and the trade press, and attends ceremonies such as the Caesar awards, where this year he had no difficulty in getting Ericsson and Sony executives to part with specimens of their award-winning EH 237 and CM-R111. Makers are usually disappointed to be told that their product is heading for the archive instead of display. 'They see it as advertising, but I want to change that perception. Acquisition may not be a seal of approval, but it is certainly a seal of significance. There is kudos in having a product archived.'
The trouble with display is that technological objects, whether finished products or prototypes, do not improve with age and are often boring to look at. 'They don't acquire a patina like artworks. Marconi's prototype of the first tuned transmitter is just four bits of wood with wire wrapped round. Prototype CD-players, before being reduced to chip size, are just grey boxes with plugs on the front. No visual appeal. Unlike an artwork, they would not draw anyone to them without a label explaining their significance.'
Four bits of wood with wire? Sounds daringly conceptual to me. And with Swatch's designers let loose on the mobile phone, there could be a collecting genre in the making. Maybe the few surviving copies of that tiny Sony CM-R111 will end up like the rarest and most valuable of limited-edition Swatch watch 'specials' stored, in their original packaging and unused, in rich people's display cabinets.
Meanwhile, if you have a Motorola DynaTak 8000X in original packaging, Mr Bridgman would be delighted to hear from you.