We report a simple means to modify an analytic sample holder to perform ambient Hall probe measurements of the sample area inside a transmission electron microscope (TEM). These measurements are important in the case of electron microscopy studies involving magnetic materials. We characterize the magnetic field of the JEOL 2100F-LM, a microscope dedicated in design to perform magnetic imaging, and also of the JEOL 3000F FEG-TEM operated in Lorentz mode. In the case of the 3000F, we measure vertical remnant field about 300 Oe due to the objective lens of the microscope. In the case of the 2100F, design of the objective lens reduces the remnant field to about 4 Oe. We characterize the field along two orthogonal directions, and spatial characteristics of the field profile are made for both microscopes during all stages of specimen entry into the column. In the case of the 2100F, we additionally measure the field conditions as a function of objective lens excitation, which is important for in situ magnetization experiments. Finally, we provide experimental results illustrating the importance of these measurements.