Worriers still puzzle over this. What causes variability in the rate at which perpetual motion machines are invented? And, whatever the fundamental reason (if any) may be, and whatever the current rate, to me it seems plain that the followers of Edison* should be perspiring more in pursuit of enduring, environmentally clean, energy than in struggle for the energyless perpetuation of movement. Sad to say, many freshly packaged ideas prove, when unwrapped, to be as old and useless as any ancient’s attempt to do work without enough input energy. However, some principles that are not essentially new are being offered with new twists.

Back-to-naturists keep coming forward with ideas for harnessing the (sometimes only apparently) superfluous movements of people as they go about their business. Past successes include the wrist-watches that tap their wearers’ byproduct kinetic energy to keep going without batteries or wound clockwork to help. Research workers in North American universities have improved on the principle of the regenerative braking systems used in hybrid automobiles. Employing kneebrace-like devices the academics are said to procure about 5W per pedestrian step, without distressing walkers equipped with the devices.

This is generating capacity not to be sneezed at, ** if one does not need to power more than an expedition’s mobile telephones or laptop computers. By the way, I have not yet encountered any proposal to employ sneeze-energy for generation. This may be because of some objection like that which certain sceptics still raise to wind power: the energy source can be intermittent or seasonal.

More surprising, perhaps, may be the absence of enthusiasm for any restoration of muscle-powered drives, as for example by (literally) horse-powered treadmills for electrical generation. Such recourse would probably be frowned upon by animal rights activists. I feel confident, however, that human rights activists will not be overly exercised by the enterprise of one, Adam Boesel, reported in The New York Times. Boesel exploits in the USA an idea from Hong Kong. A club in that city has power-generating machines connected to useful microloads. The machines get their inputs from fitness-seekers pedalling away in microgymnasia.

Nothing so low-tech underlies certain schemes to apply ionised gas or ‘plasma’*** to the incineration of waste matter, thus to obtain heat, power or CHP. The technology and its economics do not always make happy bedfellows, it is known. However, a Norwegian firm, Energos, has been building a 2.3MW power station on the Isle of Wight, UK, for start-up this year.

The renaissance of wind and water power is well known to be just another rebirth of ancient Chinese know-how. A rather different revival with historical roots is to be found in one of many attempts to resurrect electric automobilism. Here, however, the roots are traceable to muscle-powered transport. Electric-battery cars date back to the beginning of motoring but have not shared in the phenomenal worldwide growth of the latter, which was brought about by the internal-combustion engine rather than the electrochemical cell.

Now enthusiasts want to swing the pendulum hack with the aid of advanced cells and other embellishments. But perhaps the cutest back-to-batteries idea has come from carmakers Renault and Nissan, and some movers and shakers in Israel. Their inspiration is the nineteenth-century stagecoach, which relied upon an equine infrastructure. Their proposal is to institute a high-speed battery-changing service at road-vehicle refuelling stations. So travellers could swap batteries en route, just as in the old days they could swap horses at roadside inns. The proposal sounds almost romantic, but I fear that it would be much harder than it sounds to put into practice.