Paramagnetic resonance in copper sulphate

Paramagnetic resonance absorption in single crystals of copper sulphate pentahydrate has been investigated at wave-lenghts of 3$\cdot $04, and 1$\cdot $23 and 0$\cdot $85 cm. Only one absorption line is found at wave-lengths of 3$\cdot $04 and 1$\cdot $23 cm., and the position agrees with that calculated from susceptibility data. Two lines would be expected theoretically for many orientations of the crystal in the magnetic field, corresponding to the two inequivalent Cu$^{++}$ ions in unit cell. At 0$\cdot $85 cm. wave-length two lines are found under the appropriate conditions but not at longer wave-lengths. This is interpreted in terms of exchange interaction between dissimilar ions, and the exchange frequency is estimated at 0$\cdot $15 cm.$^{-1}$. Other effects of exchange interaction in this salt are discussed.