1. Furthermore the complex a.c. resistance of two contrary current-carrying strip lines is given.

2. Under certain conditions, the a.c. resistance can become zero or even negative giving rise to thermal instability.

3. The a.c. resistance is found to be in general different from the isothermal or the steady-state differential resistance.

4. Non-linearity is less marked using 20 c/s alternating currents, and a.c. does not affect the d.c. resistance, but d.c. lowers the a.c. resistance.