Two experiments to measure the electric field at the Earth's surface and the variation of potential difference with height

The Earth is always, somewhere, being struck by lightning flashes, so that it is usually both negatively charged and surrounded by an electric field directed downwards. With increasing height, therefore, the potentials become less negative, i.e., more positive with respect to the Earth's surface. McKenzie (1956) gives a short account of this electric field and its variation and Schonland (1950) and Feynmann et al. (1974) give descriptions of lightning and its causes. There is no 'correct value' for the Earth's electric field at its surface, but the first experiment described measures the field strength at a particular moment to an accuracy with a level of error of less than 2% (this estimate is based on the results of the first preliminary experiment using known artificial fields). The second experiment, which can be set up to give continuous readings of potentials in the field, is much less accurate (error of perhaps 10%) but it does give a good picture of the diurnal variation.