  The Hall constant R_H in a tight-binding model of correlated electrons on a
ladder at T=0 is expressed in terms of derivatives of the ground state energy
with respect to external magnetic and electric fields. This novel method is
used for the analysis of the t-J model on finite size ladders. It is found that
for a single hole R_H is hole-like and close to the semiclassical value, while
for two holes it can vary with ladder geometry. In odd-leg ladders, R_H behaves
quite regularly changing sign as a function of doping, the variation being
quantitatively close to experimental results in cuprates.
