A simple, sensitive circuit to measure Boltzmann's constant from Johnson's noise

A simple, easy to build electronic circuit consisting of an amplifying chain followed by a state-variable bandpass filter has been used to measure the thermal noise produced by a resistor. Using Johnson's theory for thermal noise, Boltzmann's constant could be derived with an accuracy of 1.4% of its accepted value. This simple instrument can be very useful for modern physics laboratories due to its low cost and use of standard instrumentation.