Last week, I began riding my new commute bike, a GT Transeo, to work. My new bike allows me to get off the streets with their heavy traffic, and onto the local canal system, which is almost unused. My route to work is along the canal bank on the irrigation canal that runs E-W, between Elliot and Guadalupe, in Mesa, Arizona. I ride almost 4 miles on the canal before I cut South to Elliot for the final mile or so on the road. The canal portion is traffic-free, quiet, and quite pleasant except for one annoying phenomenon. As I ride to and from work I get occasional, fairly sharp electrical shocks, normally to one of my legs at the inside of the thigh, just below my shorts (sometimes the left leg, sometimes the right, sometimes one then soon after, the other). These shocks are sharp enough that the first few times it happened, I thought a bee had stung me, or that I had jabbed a bare bike cable end into my thigh. This happens at least once or twice on each ride, and has had me groping for some kind of explanation. No bees, no bare cable ends, no debris being kicked up by my tires and hitting me in the legs, no residual marks to indicate injury. A couple of times, I’ve reached down right after this has happened and felt an electrical shock to my finger or hand.
This has happened frequently enough that I am certain it’s not my imagination. After a week of this, I finally figured out what was going on. It’s a practical demonstration of the physical laws that govern many of the machines we take for granted around us.
My route on the canal banks runs parallel to, and about 50 feet directly under, the high-voltage transmission lines that share the right-of-way with the canal and distribute power throughout much of the East Valley in Phoenix. These lines produce a sizable electro-magnetic field (EMF), which is one of the reasons they’re in this right-of-way to begin with. It is well known that a conductor moving through an electric field will generate an induced electric current. This is one of the operating principles behind power transformers, motors, and generators.
My bike frame, as it moves through the EMF generated by the power lines, has an induced electric current (stored in the “loop” that the frame makes). The frame is isolated from earth ground by the tires. It is also isolated from me by the rubber-covered pedals, my running shoe soles, the rubber handlebar grips, and the insulated seat. As I move down the canal, a potential difference gradually builds up between me and the frame. The magnitude of the potential diffeence is a function of speed through the field, the strength of the EMF of the lines, and a secondary function of the humidity (high humidity allows charge to leak away more easily). As the potential builds, eventually some body part (usually the inside of one of my legs) gets close enough to some pointy part of the frame, like a nut or something on the down-tube (static field energy dissipates over long, smooth surfaces, and can concentrate at sharp points) , and ZAP! A shock jumps and equalizes the bike frame and me.
So, in order to prevent this phenomenon, I need to make sure that my body and my bike frame keep at the same potential while riding through the EMF. If I do so, no shocks should occur. I can do that by making sure some part of my body has direct contact with the frame always, or at least frequently enough to keep the potential difference less than the “jump” energy. To test this, this morning on the way in I rode with my thumb off the rubber handlebar grip and resting on the handlebar itself. Nary a shock, for the first time in 5 days.
It also turns out that if I simply ride on the side of the canal opposite the power lines, that added distance is enough to reduce the potential difference build-up to the point where the invisible bees that have been plaguing my otherwise enjoyable commute have gone in search of other victims.