Why electricity and water don't mix

When you write for the Web you're allowed second thoughts. Typically, I've tweaked the blog explanation on my home page at least half a dozen times.

My piece about Ohm's Law ("Introducing two mega-stars", May 6) deserves a coda. The law itself - current equals voltage divided by resistance - is easily understood. But it's harder to grasp the nature of current and voltage. And, alas, I'm not about to define them. To do so within the confines of this post would be to run up against a barrier in teaching any technical subject where the elements are, and must be, invisible.

Electricity is good and invisible so the instructor uses analogies. "Think of electricity as water flowing through a tap. Voltage is the amount of water flowing, current is the pressure the water is subject to." Are we discussing electricity or hydraulics? And that's as nothing when the instructor must find analogies for coils (inductance) and capacitors (capacitance) which have no useful parallels outside electrical circuits.

It's going to sound like a cop-out but the answer's what you suspected all along. As soon as is humanly possible the instructor junks the analogy approach and starts attaching numerical values to these phenomena. Then he invokes a relationship like Ohm's Law and plugs in the appropriate values. Finally we have a cool clear sentence - as it were - that makes sense. And the language it uses is, of course, mathematics. Not terribly hard maths to begin with. But by the time it's got rather harder the initial maths has been digested.

This sneaky revelation doesn't invalidate my piece about Ohm (and John Donne). His law remains neat and concise, its effects are easily understood and I love it to bits. The next post will be about hammers and nails