The clever gadget doesn't need any batteries that run out or wires that get tangled. It can be stuck in hard-to-reach places, like inside a ship's engine, and spy on how much power it uses and how well it works.

The boffins made a device that can sense temperature by pinching energy from the magnetic field around a wire. You need to clip the sensor around a wire that has electricity in it- maybe the one that runs a motor- and it will help itself to some juice and keep an eye on the motor's heat.

MIT top boffin Steve Leeb said this was free power- energy that does not need any fiddly connections to get. And that makes this sensor a piece of cake to install.

In the lead article in the racy January issue of the IEEE Sensors Journal, the researchers provide a handy guide for an energy-nicking sensor that lets engineers work out how much power they can get from the environment and how much they need for their sensing job.

The paper shows the critical bits of a device that can continually sense and control the energy flow.

The nifty design can be used for sensors that nick energy from other sources, like shaking or sunlight. It could  be used to make cheap and easy-to-set-up networks of sensors for factories, warehouses, and shops.

Daniel Monagle, a student who helped write the paper said: "We have shown an example of a sensor that doesn't need batteries, does something useful, and is possible to make. Now, we hope others will use our guide to make their sensors."