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Practical Electronics Editorial offices Electron Publishing Tel +61 2 9939 3295 (Australia) Email pe<at>pemag.au Web www.electronpublishing.com Address mail to: Electron Publishing (Australia) PO Box 194, Matraville NSW 2036 Australia Advertising enquiries +61 2 9939 3295 pe<at>pemag.au Editor Nicholas Vinen Publisher Nicholas Vinen Digital subscriptions Stewart Kearn Tel 07918 614662 Online Editor Alan Winstanley Web Systems Kris Thain Production Bao Smith Technical staff Tim Blythman, John Clarke Print subscriptions Practical Electronics Subscriptions PO Box 6337 Bournemouth BH1 9EH Tel 01202 087631 United Kingdom Email pesubs<at>selectps.com Technical enquiries We regret that technical enquiries cannot be answered over the telephone. We are unable to offer any advice on the use, purchase, repair or modification of commercial equipment or the incorporation or modification of designs published in the magazine. 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The laws vary from country to country; readers should check local laws. 2 Volume 55. No. 03 March 2026 ISSN 2632 573X Editorial Quantity kinds, tagging and units I’ve previously mentioned the importance of units and why it’s a bad idea to leave them off numbers. For example, if I’m calculating the power in a resistor, I’d write 5V × 2A = 10W, not 5 × 2 = 10W, because you don’t get a value in watts by multiplying two plain numbers. The result is in watts because you multiplied a voltage by a current. This concept is well-established across the sciences. However, something that’s talked about much less is the idea of quantity kinds – that is, the type of thing a number represents, even when it has no physical units. Take pH, a measure of acidity or alkalinity. It’s a dimensionless quantity, because it’s derived from a ratio of ion concentrations. So we might say that pure water has a pH of 7, but not “7 pH” because pH isn’t a unit. Similarly, angles are technically dimensionless (although radians are treated as an SI unit), and decibels are a logarithmic ratio without units. Other examples of unitless quantities include refractive index, relative humidity and coefficients of friction. Despite all these being unitless, it would be nonsense to add 90° to the pH of water, or to add the number of decibels of a CD’s dynamic range to the refractive index of the laser lens that reads it, and then to the coefficient of friction of the spindle motor. These values are all unitless numbers but they represent very different things. One of the great advantages of having units is that they prevent this sort of mistake. You can’t blindly compute 9.81m/s + 25°C ÷ 3kg without realising something’s off – unless you have a meaningful way to convert or transform the units involved (you probably don’t). But with unitless numbers, there’s no such safety net. This is where quantity kinds become useful – a way to tag a number with what it represents, even when there’s no physical unit. It makes me wonder whether scientists should adopt quasi-units or semantic tags for every distinct kind of dimensionless quantity, to make it clearer how they can (or can’t) be combined. We already do something like this in electronics. Consider decibels: we often write dBV for a measurement relative to 1V, dBu for 775mV RMS, dBm for 1mW into a reference load, dB SPL for sound pressure relative to 20µPa, and dBi for antenna gain relative to an isotropic radiator. These aren’t units in the strict sense, but they tag the number with its meaning and help prevent confusion. I think this is a good idea, and it might be worth extending it to all dimensionless quantities more broadly. I’m not entirely sure how that could be implemented, but it seems like a fruitful idea. Nicholas Vinen, Electron Publishing (Australia)* * a division of Silicon Chip Publications Pty Ltd. Practical Electronics | March | 2026