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

More technology but it's getting more distant

We moved to Hereford ten years ago and this is our fourth phone system since then. Each has offered a - time for a cant techno-phrase! - step-function improvement over its predecessor. This one is part of a wi-fi trio and was acquired for two reasons. It stores thirty names and numbers and a small screen allows me to check the name of the person I'm dialling rather than rely on my vagrant memory and just the number.

More important I now have the luxury of a phone on my bedside table and can sleepily tell those who phone with a rising note of panic in their voice when it's still dark that, no, I am not Hereford Gas Services and why not dial 272329.

Parenthetically, the relationship with Hereford Gas Services has endured the full ten years. It reached its peak eight years ago when my shared-line fax (now surely as relevant as an illuminated manuscript) started cranking out an invoice for some gas parts one afternoon. I phoned HGS, not in anger - I enjoyed the illusion of a wider social life from these misdirected numbers - but to ask what they wanted me to do with the fax. After I'd explained things the telephonist wondered whether I might consider becoming one of their paid agents.

Back to the phones. They cost a mere £40 and have all the bells and whistles I need for the moment. What they don't provide are the extra memory cells whereby I can recall the procedures for activating these bells and whistles. I still haven't successfully transferred a call to one of the other phones.