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Spare a thought for the ohm

New Scientist 7 May 1987


Resistance isnít all bad as Antony Anderson explains
THE race for superconductors that  work at room temperature is on, with everyone trying to find that unique combination of rare-earth materials that may conduct electricity without converting it into heat. Spare a thought, then, for resistance, as defined by Georg Ohm in 1827. (At the time, the German minister of education said of the unfortunate Ohm that "A physicist who professed such heresies was unworthy to teach science," and Ohm lost his job.) If the wilder speculations of the newspapers are  to be believed, all that stands between us and a new electric age without resistance is the little matter of finding the right recipe for the proposed conductor. Soon we will  banish resistance from our machines, every electrical contact will be perfect, and the standard ohm will lose pride of place beside the standard volt and ampere.

Resistance, like pain, can be wearisome to the flesh and we would sometimes wish it away. But, as in a world without pain, where there would be no feedback to warn us of danger (or prevent us crushing an apple as we grasped it between our fingers) so, in a world without resistance, there would be no natural means of regulating an electric current. Apply a voltage to a resistance-free circuit and the current would continue to rise, at a rate inversely proportional to the circuit's inductance. There would be no more carefree connection of batteries in circuits and letting resistance take care of things. Careless connection of a coil for too long would transform the energy stored compactly within the battery into an excess of energy stored in the coil's magnetic field. Getting the surplus. energy back into the battery before the coil began to grow under the influence of the magnetic field--or before it developed an irresitible attraction for some other magnetic object--would heavily tax the new technology. Imagine the headlines when it became known that yet another magnetic dipole was on the loose!

Electrical resistance in cables and conductors can lead to burnt varnish, smoke, sudden short circuits and melted metal; but without the benefit of the damping provided by resistance, without even the vestige of Joule heating, any system for, distributing power would be in a state of perpetual oscillation. Our machines might be super-efficient, but they would be afflicted with the mechanical equivalent of Parkinson's disease. Without resistance, there could be no equilibrium, and how the speeds of machines would vary! 

It is a grand plan to transmit power without loss through high-voltage cables: until, that is, the cold weather comes. Then the cables become covered -with layers of ice and they start to gallop in the wind. How will they be rid of their icy burden, if they cannot be warmed by the passage of a double dose of current? Supposing that the tranmission lines do not collapse under the strain and survive till the next thaw, and supposing that power is delivered to our homes without loss, how will we use it? Without resistance, our electric blankets, kettles and incandescent lamp bulbs would be useless. Are the efforts of Swan and Edison to make filaments to illuminate our lives to be set at nought?

If resistance disappeared, we would have to say goodbye to the electric toaster, the hair dryer, the Wheatstone bridge, the potentiometer, the colour-coded resistor, the dry joint and the ubiquitous multimeter. Car manufacturers would miss their robot spot-welding machines and football players their heated pitches. Levitated magnetic trains would glide smoothly through Clapham Junction at 10 times the speed of today's geriatric stock, but we would miss the colourful arcs and sparks caused by highly resistant, iced-up conductor rails.

Superconductivity may usher in a wattless wonderland, but 1 do hope that we leave room for resistance here and there The occasional ohm might come in useful during cold weather, especially if wrapped up in a blanket!


© Antony Anderson 2000


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