Despite this limitation, Weiss’s equation provides the best fit for strength-duration data and indicates that rheobase and time constant (chronaxie) can be measured from the charge duration curve with a very small margin of error. Weiss used rectangular, constant-current pulses and found that threshold charge required for stimulation increased linearly with pulse duration. He also found that stimulus charge, the product of stimulus current and stimulus duration is proportional to rheobase, so that only two stimulus durations are necessary to calculate rheobase.