If electric fields were visible, then even the most barren spot on the earth would provide an awesome sight. Standing on a hilltop, you would see a forest of electric-field lines shooting out of the ground everywhere, stretching up to the ionosphere. You could watch them sweep across the horizon to gather under storms. In fact, the earth's electric field is far more dynamic--and, for me, more interesting--than its magnetic counterpart.

Image: Daniels and Daniels HOMEMADE FIELD MILL measures fluctuations in the earth's electric field. The device relies on two slotted metal disks, only one of which is rotated. For precision, the instrument must be calibrated with additional equipment (red).

This electrical phenomenon is generated by the thousands of thunderstorms that pummel our planet continuously with 100 lightning bolts a second and that also deliver to the ground a tremendous amount of charge on raindrops [see The Amateur Scientist, August 1997]. As a result, we live atop an ocean of negative charge that generates an electric field of approximately 100 volts per meter elevation. In other words, when you are standing, your head is about 200 volts greater than your feet. And when a thunderstorm passes overhead, the electric fields can increase to thousands of volts per meter. Fortunately, there is very little free charge (unattached electrons and positive ions) in the air around us, and so these high voltages cannot create any large currents, which would otherwise surely electrocute us.

To monitor the earth's electric field, I have developed an accurate home-built instrument that can be constructed for under $50. The device is basically an inexpensive incarnation of a field mill, which measures electric fields by using two slotted metal disks mounted coaxially and vertically with their surfaces almost touching. One disk is fixed and grounded to the instrument's case, and the other is rotated at high speed. (Grounding the instrument to the case, and not to the earth's surface, lets the experimenter take measurements anywhere, even from an airplane high in the atmosphere.) When the slots are not aligned, the local electric field reaches the upper plate and drives some of its free charge into ground. But because conductors block electric fields, the lower plate shields the upper plate when the metal sections line up, thereby allowing the banished charges to return. Rotating the lower plate thus causes a current that surges back and forth in the ground wire, and these electrical pulses can be detected with an inexpensive circuit.

I improvised a field mill by taking two steel cake pans and cutting out a dozen equally spaced 15-degree wedges (the slots) from their circular bases. To rotate one of the pans, I used a surplus high-speed electric motor. These motors typically deliver between 1,000 and 7,000 revolutions per minute. At those rates and with the cake pans cut with 15-degree wedges, the earth's field generates nanoampere-size current surges in the ground wire at frequencies between 200 hertz (for 1,000 rpm) and 1,400 hertz (for 7,000 rpm). Such a signal can be observed easily with a circuit containing a transconductance amplifier and a peak detector. In fact, my homemade instrument can readily detect shifts a mere thousandth of the ambient field. Furthermore, a computer analyzing the data will be able to follow the fluctuations with a performance rivaling that of professional instruments.

You can build the device over a weekend. First, use a protractor to lay out the pattern of 15-degree wedges on the inside of one cake pan. Then clamp the two pans firmly against a circle of plywood (to facilitate the cutting) and use a jigsaw to obtain two identical sets of wedges. Select one pan as the stationary sensor disk and the other as the rotating shie

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