Detecting the Earth's Electricity

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.

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 (Details of the circuit). 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 shield.

VERTICAL MOUNTING of various parts, including two cake pans with wedges cut from their bases, is required to construct a homemade field mill.

Because an electric motor invariably generates intolerable amounts of electromagnetic clutter, you must take several countermeasures. Even the metal shaft gives off such deleterious energy, and if the rotating shield pan were connected directly to the shaft, this radiation would spew out between the two pans, where the instrument is most vulnerable to noise. To avoid such degradation, add to the shaft a nonconducting extension, such as a wooden dowel 2.5 centimeters (one inch) in diameter. Use a drill press to bore a precision hole along the exact centerline of the dowel and then epoxy the shaft inside it. Next, electrically isolate the motor by mounting it coaxially, with Teflon screws and washers, to the inside of an upside-down metal trash can like the ones found in offices and schools. You can capture most of the radiated gunk with two layers of aluminum window screen.

Next, cut off part of a second metal trash can so that it will fit within the first can with the motor attached. Then drill a hole through the centers of the sensor pan and a third cake pan (for calibrating the instrument) that can amply accommodate the dowel. During the calibration process, you will need to charge the third pan, so affix a wire to its outside surface by using a conducting metallized epoxy. Three small rubber gasket spacers wedged between the sides of the sensor and calibration pans will hold them close together without their touching. Bolt them as a unit to the inner trash can by using insulating standoffs and drill a hole in the can to take the wire out.

The circuit must be attached to the inner trash can at a location directly below the sensor pan. To reduce electrical interference, shorten as much as possible the wires that connect the sensor pan to the circuit and the circuit to the inner trash can. Also, completely enclose the circuit with a patch of aluminum screen. Remember that you need to force all the signal current to pass through the amplifier, so make sure that the ground wires are the only electrical link between the sensor pan and the trash can.

Install the entire assembly inside the outer trash can by again using insulating standoffs. Then, from the end of a PVC pipe of 2.5-centimeter inner diameter, cut a spacer that is precisely one centimeter long. Thread the spacer over the wooden dowel and rest it against the sensor pan; epoxy the spacer to only the dowel. (If you bond it to the sensor pan, the shaft will not rotate.) Finally, cut a hole in the center of the shield pan so that you can secure it onto the dowel between the spacer and a second piece of PVC pipe.

To calibrate the instrument, you first need to glue a fourth cake pan coaxially to one end of a dowel and a large pizza pan (for screening out the earth's electric field) to the other end. Insert this implement within the trash-can assembly. A voltage placed between the top and bottom cake pans creates a field that approximates the earth's. With a two-centimeter spacing between those pans, a two-volt difference creates a 100-volts-per-meter field inside the instrument. You can calibrate the device at the low end of its scale with nine-volt batteries and a rheostat. Simulating the field generated by a powerful thunderstorm requires a 200-volt power supply, which you can find at most electronics surplus shops for less than $100. But keep in mind that these devices can deliver enough current to kill, so use extreme care.

Use your homemade field mill outdoors, far away from buildings, either rigidly suspended from a pole or resting level on an insulated ring. Either way, make sure its opening has an unobstructed view of the ground. If you run the signal into your home through a coaxial cable, you will be able to monitor the field comfortably in all kinds of weather.