We have compared the low‐frequency capacitance of glass thermometers of various shapes, made of very pure SiO2doped with ≊1200 ppm OH−, in the temperature range of 4–100 mK in a magnetic field of 9 T. The construction of the electrodes, the way the contacts are made, and the connection of the leads or cables influence both the voltage and the field dependence of the capacitance and of the loss. We have been able to construct capacitive glass thermometers of which the capacitance is not shifted in a field of 9 T at temperatures down to 4 mK, within our temperature accuracy of ≊5%. This has been determined by comparison with a cerium magnesium nitrate (CMN) thermometer in a field‐free region, and by comparing ultrasound measurements versus the thermometer capacitance with and without a magnetic field.