The electrical resistivity of coal appears to decrease with ash content. The laterlog measures what is virtually the true resistivity of the coal and may ultimately provide information on seam quality. The laterlog uses a sheet of current that is focused on each formation in succession and so measures the resistivity of that formation only. The mud column or a salty mud has no effect on the measured resistivity. The laterlog may be measured by a seven- or three-electrode arrangement but the former is preferred.