Wednesday, August 24, 2022

gasoline trucks were required by law to carry a dragging chain or electrical discharge strap

On a dry summer day in California, physicist Julius Sumner Miller was driving slowly near the desert when a friend overtook him on the left. The friend’s wife, in the passenger seat, reached out to hand him a package of gum. Their hands were no less than 3 inches apart when “a terrific discharge took place which possessed the classical physiological effects. The shock was momentarily disabling, as a three-inch spark in air can well be.”

Miller published an inquiry about this in the American Journal of Physics and received a reply from R.F. Miller of B.F. Goodrich in Ohio. 

The motion of the cars had built up charges of different amounts; Goodrich had found that the accumulated charges can (or could) increase greatly as the wheel rotates, and “as soon as the tread charges are far enough removed, they will find a lower resistance path through the rim to ground rather than around the tread,” charging the vehicle.

Even at the time the phenomenon was well known; in his original letter Miller noted that gasoline trucks were required by law to carry a dragging chain or strap. But “the question as to how great a charge may accumulate is difficult to answer.”

(Julius Sumner Miller, “Concerning the Electric Charge on a Moving Vehicle,” American Journal of Physics, 21:4 [April 1953], 316.)


I wonder if the generator or magneto on the engine had anything to do with it? 

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1 comment:

  1. Joe7:16 AM

    Nope, the generator (or 'dynamo' for our European friends) and magneto would not be involved as they both have a closed electrical circuit from the charging coils through the 'work' (light bulbs, spark plugs, etc.) through chassis ground and back to the case of the mag or gen.
    The static electricity that builds up and causes problems in cars and trucks is from normal driving motions i.e. tires and bearings rolling, passengers sliding on seats, cargo shifting or sloshing. You may have experienced it yourself on cool dry days if you've caught a slight shock as you touched the door frame of your car on entry or exit due to static electric charge from brushing the upholstery of the car. I have had to explain this to customers from time to time in my garage and reassure them their is no fault in the electrical system of their car.
    Joe

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