There is disclosed apparatus for measuring precisely, to 0.01 C*, difference in temperature including thermistors, which are subject to the temperatures whose difference is being measured. The logarithm of electrical resistance of each thermistor is proportional to its temperature and the thermistors are connected in a ratiometric circuit for measuring their relative resistances. Each thermistor is a ceramic of ferroelectric material (for example, barium-strontium titanate) formed by mixing powders of different compositions, each composition having a different ferroelectric resistance anomaly, and then firing and oxidizing bodies pressed from the mixture of powders. The mixture of powders is heterogeneous so that the grains of the ceramic include a plurality of particles of powder which microscopically would manifest distinct ferroelectric resistance anomalies if fired alone but the macroscopic response of the ceramic is loglinear over a substantial temperature range.