Negative Impedances and the Twin 21-Type Repeater

H E N an e.m.f. is applied to the terminals of an ordinary positive resistance a current flows in at the terminal connected to the positive pole of the source and out at the other terminal. This direction of current flow is considered positive and the value of the resistance R, in ohms is given by Ohm's law as R = E/I where E is the applied voltage and I is the current in amperes. Similarly a definite current I may be passed through the resistance and a potential difference or drop E = RI will appear across its terminals. W i t h positive resistances it makes no difference whether we " a p p l y an e.m.f." or "pass a current". The resistance may be a very simple device such as a coil of wire which absorbs energy from the circuit at a rate W = EI = PR watts.