What instrument records the electrical activity of the heart over time?

The instrument that records the electrical activity of the heart over time is the electrocardiograph. This device measures the electrical impulses generated by the heart as it beats, producing a visual representation of the heart's electrical activity known as an electrocardiogram (ECG or EKG). This information is crucial for diagnosing various cardiac conditions, monitoring heart health, and assessing the overall function of the heart.

In contrast, a stethoscope is primarily used for auscultation, which allows a clinician to listen to internal sounds such as heartbeats and lung sounds, but it does not record electrical activity. A pulsometer is a device used to measure pulse rate, but it does not provide the detailed electrical information that an electrocardiograph does. An electroencephalogram, on the other hand, records the electrical activity of the brain rather than the heart, placing it outside the context of heart monitoring. Thus, the electrocardiograph stands out as the correct choice for the purpose of assessing cardiac electrical activity.