Abstract: An apparatus for measuring electrical activity in the heart, such as an ECG apparatus, has an input amplifier with multiple inputs to which the respective poles of electrodes introduced into a patient are intended to be connected. The input amplifier includes a sensing stage for sensing the impedance across the amplifier inputs after connection of the electrodes, in order to identify the poles which are in contact with the patient's heart. A display is connected to the input amplifier to graphically indicate which electrode poles are connected, and which are in contact with the heart.

Abstract: A calibrating impulse generator which is connectable to the input of a signal-shaping element, the latter of which reshapes the input signal in comformance with the function ##EQU1## The calibrating impulse generator is constructed, whereby a calibrating impulse proceeds over a period of time from an initial value in accordance with an e-function to a final value, and after reaching the final value return again to its initial value. Calibrating impulses with the described sequence will produce rectangular or square-wave impulses of known amplitude at the output of the signal-shaping element.