Abstract
By measuring the natural remanent magnetization of potteries from Bolivia and Ecuador, the intensity of the ancient geomagnetic field was compared with that of the present. These potteries cover the period of about 2000 B. C. to 1200 A. D. It was concluded that the intensities of 2000 B. C. and 0 A. D. were nearly equal to the present intensity, and that of 700 A. D. was about 1.5 times as large as the present one. In this study Thellier's stepwise heating method was used to estimate the intensities of the ancient geomagnetic field. Some samples exhibited anomalous shapes of thermal demagnetization curves of natural remanent magnetization. An experimental examination to clarify the origin of these anomalies indicated that some samples might have been reheated at a temperature somewhat below the highest Curie temperature sometime after the production of the potteries. A criterion to select useful samples was postulated on the basis of these experimental results and only the proper samples were used for the present purpose.
