of the Thelliers [@Thellier1959], or some of its more common variants [e.g. the Thellier-Coe approach; @Coe1967]. In these experiments, samples are exposed to a series of paired heating steps (Figure 2). For example, in the Thellier-Coe approach, a sample might first be heating to 100 $^{o}$C in field free-space, during which a natural remanent magnetization (NRM) is removed. Next, the sample is reheated to the same temperature, but this time in the presence of a known applied field (this is called the “field-on" step). After cooling, the sample magnetization is measured and the thermoremanent magnetization acquired can be calculated. Because one knows the magnetization lost, the magnetization gained and the applied field ($H_{\rm lab}$), one can solve for the ancient field ($H_{\rm paleo}$) recorded by the 100 $^{o}$C temperature range:

$$H_{\rm paleo} =  \frac{M_{\rm NRM}}{M_{\rm TRM}} H_{\rm lab}$$

In practice, one conducts this experiment at many steps up to the maximum Curie temperature of the magnetic minerals in a given specimen.\

Chemical change
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The need for the second, or “field-on" step makes paleointensity experiments far less successful than a study of magnetic directions alone. The main problem is that heating can induce changes in magnetic minerals, changing the TRM-capacity; 