Several embodiments of RF source resistance measuring devices are disclosed. Common to all embodiments is the feature of the inclusion of at least one variable resistor, and a peak readout meter. In one embodiment, two ganged unloaded potentiometers are employed while another embodiment comprises an automatic-nulling RF power bridge circuit with a variable rather than a fixed bridge reference resistance. A third embodiment comprises a calorimeter with a variable rather than a fixed resistor, while in another embodiment attenuator pads with variable resistors are employed.