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I was photographing landscapes in a field under high-tension power lines (see photo) when I discovered my metal tripod was delicately vibrating. I could only feel the vibrations if I rested my fingers lightly on the metal and they were clearly related to the high tension wires (not the gentle breeze). When I moved the tripod away from the lines the vibrations stopped, but returned when I took it back beneath the wires again. What was going on? More importantly, if the lines did this to a metal tripod, what might they have been doing to my body?

• It is not at all surprising that there should be some effect underneath high-tension power lines, when one considers the influence of the magnetic and electric fields. The magnetic field generated by the alternating current running through the wires could induce a current in the person and tripod that would flow back and forth to earth. But in this case, this effect can be ruled out because at 50 hertz (the UK standard alternating-current frequency), the induced current would be very small – especially when one allows for the resistance of the person’s shoes and the probable presence of rubber feet on the tripod.

The culprit is almost certainly the very high electric field under the cables. This field can be detected by holding a long fluorescent tube underneath the cables: lo and behold, it lights up. I tried this under a 400-kilovolt cable near Didcot power station in Oxfordshire, UK, recently.

The vibration the questioner felt is probably the same sensation experienced when you lightly touch the metal case of a piece of electrical equipment from which the earth wire has become disconnected. My bread maker showed this a treat. There is a not unpleasant tingling on the skin which might be interpreted as vibration. It is caused by the capacitive current travelling to earth through the body. The same is happening here between human and tripod.

Brian Burrows,Sutton Courtenay, Oxfordshire, UK

• I suspect your correspondent was sensing a small . If the power lines are operating at, say, 25 kilovolts, the capacitance from line to tripod would allow a small current to flow.

If the tripod has plastic feet or the ground was very dry when your correspondent touched the metal, this current – or a fraction of it – would flow through their body to ground. A similar situation occurs when the earth wire of any appliance is disconnected and the conducting case is touched lightly. It is a somewhat odd sensation that does feel like vibration or rubbing one’s finger over a finely corrugated surface. It is possible that if your questioner gripped the tripod firmly they would not notice anything.

In this example the risk of harm was very low. The most significant instance of danger I have experienced in such circumstances was when a midwife in the hospital where I worked reported this kind of sensation when using a portable ultrasound scanner. I found that the earth connection in the wall socket was faulty and the midwife was sensing the capacitive current between the live and earth conductors in the mains cable. Fortunately no harm resulted, but these faults can be serious in places such as operating theatres where an internal connection might be made to a patient.

John Fleming Glasgow, UK